1989 C1500 Standard cab dome light wont turn off

[Scooterwrench]

This is the little button I'm referring to;

You must be registered for see images attach

It's a large head plastic rivet and it pushes the door switch plunger when you close the door.
Also the interior lights are turned on by the instrument panel dimmer knob. Rolling the knob all the way up turns on the lights and overrides the door switches.

 

[tppower]

This is the little button I'm referring to;

You must be registered for see images attach

It's a large head plastic rivet and it pushes the door switch plunger when you close the door.
Also the interior lights are turned on by the instrument panel dimmer knob. Rolling the knob all the way up turns on the lights and overrides the door switches.

I know this which is why i put in an entirely brand new dimmer switch (it was barely working anyway and now it does). rolling up the knob all the way up or down does change or overrride the switch. The light just stays on. And both plastic rivets are intact. I also tested both switches with them out of the socket and manually pushing them in .

 

[H2814D]

I know this which is why i put in an entirely brand new dimmer switch (it was barely working anyway and now it does). rolling up the knob all the way up or down does change or overrride the switch. The light just stays on. And both plastic rivets are intact. I also tested both switches with them out of the socket and manually pushing them in .

How did you test the door plunger switches?

The circuit/dome lamp is operated with what are called "normally open (when the door is shut)" door plunger switches. Normally open meaning- when the door is closed the circuit is open and not sending power to the dome lamp. When the door is open, and the pin is out, there should be continuity across the wires. At this point the circuit is considered closed and the current to the dome lamp flows. When the door is shut, and the pin is pushed in, there should be no continuity and the circuit path is disrupted.

You can test the wires to the plungers with a multimeter set on the continuity setting. If you don't know which one, the setting on the meter looks like a sideways wifi signal. Door open- circuit closed- multimeter should beep or have a steady tone. Door closed, or button pushed in, the tone should go away.

Once again, though, that dome lamp is operated by more than one switch/plunger, so in order for the light to be off, the circuit to that dome light must be open at every switch. This means both door plungers on both doors and the rolling dimmer switch on your panel have to have an open circuit in all three places. That is why that white wire runs to all of them. One single orange (+) to the lamp, then that return white to the convenience center/fuse block.

I didn't see where you tried removing the ground from the dome light as discussed several comments ago. Did you try that yet?

 

[tppower]

How did you test the door plunger switches?

The circuit/dome lamp is operated with what are called "normally open (when the door is shut)" door plunger switches. Normally open meaning- when the door is closed the circuit is open and not sending power to the dome lamp. When the door is open, and the pin is out, there should be continuity across the wires. At this point the circuit is considered closed and the current to the dome lamp flows. When the door is shut, and the pin is pushed in, there should be no continuity and the circuit path is disrupted.

You can test the wires to the plungers with a multimeter set on the continuity setting. If you don't know which one, the setting on the meter looks like a sideways wifi signal. Door open- circuit closed- multimeter should beep or have a steady tone. Door closed, or button pushed in, the tone should go away.

Once again, though, that dome lamp is operated by more than one switch/plunger, so in order for the light to be off, the circuit to that dome light must be open at every switch. This means both door plungers on both doors and the rolling dimmer switch on your panel have to have an open circuit in all three places. That is why that white wire runs to all of them. One single orange (+) to the lamp, then that return white to the convenience center/fuse block.

I didn't see where you tried removing the ground from the dome light as discussed several comments ago. Did you try that yet?

I appreciate the help everyone by the way

When i say i tested the plunger, to turn off the light i pushed in, simulating the door closing. Now granted while it could be the passenger side door, the wires were perfect, no cuts were seen when i removed the radio, and the plunger is new. The radio is aftermarket and kenwood, i dont know if the previous owner tapped that into the system which could cause it or if the exhisting radio wiring has a 12v hot wire like most cars.

I tried using a voltmeter on the wires bare a year ago but i dont remember the readings. Ill try it again tomorrow. How do you recomend testing the wires while it is connected to the switch, there will be no bare wires after.

I didnt try removing the ground yet, its been raining really bad here for the past 4 hours since right after i got home from work. Will have to be a tomorrow adventure. Also as i said before the roller dimmer is brand new and its functions work perfectly besides obviously the dome light issue. The "ground" on the dome light itself is connected to the white wire by the way.

EDIT : I removed the "ground" that wasnt a ground, it’s just the wire that goes to the map lights in the dome light board. screw needs to be there to hold everything up. The wires to the dome light look really bent and bad. I didnt bust out the multimeter cause i should have been asleep an hour ago but, Im going to do what the manual says and test continuity at the dash switch as well as making sure my new door jamb switches work. If not then im going to tie fishing line to the old orange and white wire and then cut it and pull it through, and then run new wire to the dome lights and re splice it.

The 1988 Wiring diagram shows the passenger side only having one white wire and the driver side having a white and TAN ONLY like my truck does. (pictures on page two of the bare wires on the drivers side. However the passenger side on my care has black and white wires like the 1989 diagram.

My last theory according to the 1989 wiring diagram: is that the tan wire dangling is the dash wire for the key buzzer, the white is the hot wire, and then im missing the black wire which is the ground. My question is why is the light staying on, if the driver side switch was cut off completely and the wires were bare, wouldnt that make the circuit stay off? or would the lack of the dome light switch cutting the circuit make it stay on?

 

[Road Trip]

Greetings tppower,

Sometimes the electrical circuits we take for granted (like the courtesy, dome, & cargo lights)
can be the most counter-intuitive ones to troubleshoot. Normally when this happens, it's because
the circuit was implemented using a form of switch logic.

So when I read your questions I realized that I was unfamiliar with the actual layout of this circuit, so
it felt like a good opportunity to bust out the '89 GM CK Light Truck Wiring Manual and see what the
tech writers had to share.

What does the the left hand door jamb switch tan wire go to, what does it do, where does it go under the dash near the column and why do i not see a blank wire. No bare wires are cut and every thing on the rest of the truck works. I want to solve that before i run wire.

The tan wire you refer to is connected to the driver's door jamb switch, but at the same time it is *not* related to your
Dome light dilemma? So let's answer your legitimate question about this wire and get that off the troubleshooting table.

The short answer is that the TAN wire from the Left Door Jamb switch is for electrical circuit #159. But why was it added?

Well, here's the electrical chain of events that turns an opened driver's door + a key in the ignition = an audible Key-In Warning buzzer:

1) When the driver's door is opened, the spring loaded jamb switch plunger physically extends, and the internal switch
electrically closes, connecting the black ground wire (circuit #150) on one side to the TAN wire on the other side. (circuit #159)

2) This circuit #159/TAN wire leaves the Left Jamb switch and is routed to/thru connector C306 & up the steering column.
IF the key is still in the ignition when the door is opened, the associated key in switch will be closed, allowing the
ground to continue back down the steering column and turning into circuit #80. (LT GRN wire)

3) This ground carrying LT GRN wire is routed to the "Key-In Warning Buzzer" that is plugged into the front of the Convenience Center.
Since the multi-function warning buzzer has "Hot at all Times" power fed to it, ground being applied by this LT GRN
wire will allow current to flow, giving voice to the buzzer most readers are now hearing in their mind's ear while reading this. :0)

And as we all know, either pulling the key out -or- shutting the driver's door will open this circuit, silencing the buzzer.

So in order to wed the theory above to the diagram below, do yourself a favor and trace this circuit on the
wiring diagram below, starting at the Left Jamb switch on the bottom right corner of page 92:

You must be registered for see images attach

****

Now that we've tied the TAN & LT GRN wires to the Key-In Warning buzzer, let's move on to the Dome light circuit & how to best troubleshoot it?

Well, to be perfectly honest I found what the '89 Wiring Manual had on page 92 (above) was complete, but at the same time
it had that 'wallpaper' feel that seems to make diagrams like this a bit stand-offish to normal DIYers?

When this happens, most folks think that automotive wiring just isn't for them. But I would instead ask you to first
try to redraw the circuit in such a way so that you can better see the functional flow. And I also happen to find that putting the
+12v for the circuit at the top of the page, and the ground at the bottom of the page helps me visualize the circuit work more easily.

So with that in mind I redrew the Dome light circuit, and came up with the following.
When you look at it, you will see that it's broken into 3 sections, from top to bottom:

* The Always On +12v power section. (Circuit #40/Orange wire)
* The ON/OFF control section. (Circuit #156/White wire -- switched grounds.)
* The Always grounded section. (Circuit #150/Black wire)

You must be registered for see images attach

Here are some troubleshooting hints to keep in the back of your mind while looking at this Functional Flow:

* I simplified the Always On & Always grounded sections of the diagram. This was done because we aren't
concerned with missing/flaky power or ground. Instead, our focus is on the reported symptom, which
is the inability to control the current flow through the Dome light. (It's always on, instead of us being
able to switch it on & off as desired.)

* Even when the truck is shut off, whatever voltage is on the positive terminal of the battery will
also be sitting on the filament on the bulb. Even when the bulb is dark. In order to illuminate
the bulb, the 3 switches that are supposed to control this buib are going to connect a ground
to the other side of the bulb, allowing that stationary +12v power to now flow through the
filament. Remember voltage is just how much potential there is to do work...but it's the
actual current flow that does real work. (In this case, lighting the interior of the truck.)

* For those raised on binary logic, this circuit is the equivalent of a 3-input OR gate.
Left door open OR switch on dash closed OR Right door open = active circuit.
I share this perspective, because as the troubleshooter it has to be kept in mind that
any combination of one, two, or all three switches closed will light the light.

* If we were having a problem with one or more switches NOT working, then we would be
all over the Orange or Black wires. But for your symptom nearly all of our focus needs to
be on the 'switched ground' circuit #156/White wires in this circuit. ANY ground (circuit #150/Black)
wire directly connected to *any* of the white wires is an illegal condition...and will cause the
Always On symptom you are experiencing.

* I've attached the truck wiring diagram showing the physical layout of the wiring harness.
Here in the rust belt others have reported wires in the vicinity of the driver's door opening
where they have chafed to each other or to the chassis/frame.

In English, in order to be able to deliver a switch-controlled grounding in this circuit, the
entire white wire circuit needs to be isolated from ground at the beginning, middle, & end
of this circuit. No chafes or splicing a ground wire directly to a white wire, so that there is
zero current flow unless any one of the 3 switches closes, and *then* a solid ground should
be forwarded to the Dome light bulb so that it can shine.

* Speaking of troubleshooting this circuit as quickly as possible, pay close attention to the
Convenience Center on page 92 above. It looks like you should be able to easily unplug
all 3 switches individually, and see which circuit is causing the 'Always On' symptom?

Or if you unplug all three switches, and the light is still on? This would tell me that there
is an illegal ground (on purpose or by chafe) somewhere on the white wire section of this
circuit, and that needs attention first.

****

So there you have it. All the FSM diagrams I shared are from the '89 Wiring Diagram, so if
there are still color code mismatches then I don't know whether that's a documentation error
in the '89 FSM? Or possibly some undocumented mod made to your '89 truck prior to your
purchasing it?

And for what it's worth, you have been getting super solid guidance from the other contributors to
this thread. My motivation to redraw the circuit was simply to clarify exactly which color wires in
this circuit would most likely lead to the ON / OFF control symptoms you described..

****

Apologies for the length, but if the PO modified the wiring in this circuit without understanding
the switch logic nature of all this, it's easy to cover up the proper functioning of this circuit
with a well-intended but incorrect extra ground connection here or there.

But armed with an understanding of how this circuit is supposed to work, I am confident that
you will be able to fix this long-standing issue. Don't hesitate if the stuff above
created additional questions on your end of this long distance team troubleshooting effort.

Best of luck --

 

[H2814D]

Or if you unplug all three switches, and the light is still on? This would tell me that there
is an illegal ground (on purpose or by chafe) somewhere on the white wire section of this
circuit, and that needs attention first.

The OP mentioned in his first comment that the dome light has an actual ground connected to it-"The dome light is properly grounded to the cab,". That would be that "illegal ground" you are talking about. If that is the case, that ground should be removed before trying any of the other troubleshooting steps you have mentioned.

 

[Road Trip]

-The dome light is properly grounded to the cab, and the wiring to the dome light harness isnt corroded

(Original post)

The OP mentioned in his first comment that the dome light has an actual ground connected to it-"The dome light is properly grounded to the cab,". That would be that "illegal ground" you are talking about. If that is the case, that ground should be removed before trying any of the other troubleshooting steps you have mentioned.

Yes, I also saw the OP's comment above. It would be interesting to see what kind of continuity check
you would get if you were to check the ground he referenced to the grounded side of the dome light?
(A photo of his setup would also be helpful for those of us working remotely. I'm at a disadvantage when I
can only 'see' what's in the service manuals versus how the truck is actually wired?)

Obviously anything different between the truck's actual physical wiring vs the factory wiring diagrams is the most
suspect for changing the circuit's behavior?

****

When I'm trying to get a more complicated circuit to work as advertised (see attached for the expected Dome light operation)
sometimes I find it helpful to divide the circuit up into the simplest possible subset, make that work first, and then step by step
add back in all the more complicated bits?

So, if this was my truck, I would try to get the Dome light to go on & off using just the jamb switches? That is, Dome
light goes out when the door is closed, and turns on when the door is opened? To illustrate my point, I took the p. 92 out
of the '89 wiring manual, and pruned out as much extraneous stuff as I could, leaving only the 2 jamb nut switches & the
wiring to the Dome light. The result is a lot easier to follow while you are measuring the circuit:

(Page 92 of the '89 FSM, Dome light circuit pruned back to just the jamb switches & Dome lamp)

You must be registered for see images attach

This is more like it. Given the way that this circuit is drawn, I'm thinking that I could have the dome light
temporarily laying in the middle of the bench seat, and by opening or closing the jamb switches I could have the Dome
light turn on and off as you would expect the circuit to operate? Sometimes a Divide & Conquer approach pays big
dividends when working on a new to you circuit for the first time.

****

By the way, it felt a bit weird to write about 'illegal grounds', for normally we think that the more
grounds the better? But when I was studying this circuit, I was struck by the fact that a full
ground (black wire circuit #150) is not connected directly to the Dome light bulb?

Instead, there *is* a ground connected to the bulb, but it is a switched ground. And thanks to the
General's rule that every time a wire in a circuit changes it's function, it gets a new circuit number.

So if we start at the ground (G302) the black circuit #150 wire is right there, just like I have learned
to expect. But then the constant ground gets run through a jamb switch, and now we end up with a
conditional (switched) ground. Per the wiring diagram documentation rules, this switched ground is
now visually identified with a white insulation, and is referred to as circuit #156.

****

So hopefully this better explains my underlying thought process. And I'll keep simplifying the circuit until
I can get it work. If I end up with a single jamb switch feeding the single Dome lamp bulb (and the dome light
is resting in the middle of the bench seat 100% isolated from the rest of the truck) ...then that's exactly
what I will do.

And once I get that light working, I'll put the Dome light back where it belongs, and ensure
that I can still turn it On & OFF with the jamb switch. (ie: I'm not getting a 100% of the time ground
superimposed onto the desired switched ground circuit when the Dome light is physically connected to
the roof?)

And then add back in the other jamb switch? Followed by reconnecting the switch in the dash? I assure
you, I don't disassemble a circuit every single time I troubleshoot one. Sometimes, it's just that the bulb
is burned out. That's a 20 second check. Fix it & move on.

Or I just take a handful of voltage/resistance/continuity checks with everything in place. Common sense
dictates the approach.

But once in a great while a tough, intractable, long term problem like this crops up, and the factory
troubleshooting flows only cover 100% stock, unmodified circuits? Only then, after everything else fails,
I reserve the right to redraw the circuit until it makes sense to me, and then (temporarily) prune back the
circuit until it's too simple to not fix. And then start adding the complicated bits back into the circuit,
until it's all working as advertised once again. (!)

****

Apologies to the OP if this explanation feels like I'm somehow talking down to you. (Or anyone else reading along.)

My goal is simply to share how we use the divide & conquer approach to electrical circuits that have a lot of fan out
or use switch logic techniques. Some neat tricks were used by the original designers to give us the circuit behavior
that makes the Dome light work automagically/intuitively to our needs when we use our GMT400s.

As always, let us know what you discover.

Best of luck with the hunt for the fix --

 

[tppower]

Greetings tppower,

Sometimes the electrical circuits we take for granted (like the courtesy, dome, & cargo lights)
can be the most counter-intuitive ones to troubleshoot. Normally when this happens, it's because
the circuit was implemented using a form of switch logic.

So when I read your questions I realized that I was unfamiliar with the actual layout of this circuit, so
it felt like a good opportunity to bust out the '89 GM CK Light Truck Wiring Manual and see what the
tech writers had to share.



The tan wire you refer to is connected to the driver's door jamb switch, but at the same time it is *not* related to your
Dome light dilemma? So let's answer your legitimate question about this wire and get that off the troubleshooting table.

The short answer is that the TAN wire from the Left Door Jamb switch is for electrical circuit #159. But why was it added?

Well, here's the electrical chain of events that turns an opened driver's door + a key in the ignition = an audible Key-In Warning buzzer:

1) When the driver's door is opened, the spring loaded jamb switch plunger physically extends, and the internal switch
electrically closes, connecting the black ground wire (circuit #150) on one side to the TAN wire on the other side. (circuit #159)

2) This circuit #159/TAN wire leaves the Left Jamb switch and is routed to/thru connector C306 & up the steering column.
IF the key is still in the ignition when the door is opened, the associated key in switch will be closed, allowing the
ground to continue back down the steering column and turning into circuit #80. (LT GRN wire)

3) This ground carrying LT GRN wire is routed to the "Key-In Warning Buzzer" that is plugged into the front of the Convenience Center.
Since the multi-function warning buzzer has "Hot at all Times" power fed to it, ground being applied by this LT GRN
wire will allow current to flow, giving voice to the buzzer most readers are now hearing in their mind's ear while reading this. :0)

And as we all know, either pulling the key out -or- shutting the driver's door will open this circuit, silencing the buzzer.

So in order to wed the theory above to the diagram below, do yourself a favor and trace this circuit on the
wiring diagram below, starting at the Left Jamb switch on the bottom right corner of page 92:

You must be registered for see images attach

****

Now that we've tied the TAN & LT GRN wires to the Key-In Warning buzzer, let's move on to the Dome light circuit & how to best troubleshoot it?

Well, to be perfectly honest I found what the '89 Wiring Manual had on page 92 (above) was complete, but at the same time
it had that 'wallpaper' feel that seems to make diagrams like this a bit stand-offish to normal DIYers?

When this happens, most folks think that automotive wiring just isn't for them. But I would instead ask you to first
try to redraw the circuit in such a way so that you can better see the functional flow. And I also happen to find that putting the
+12v for the circuit at the top of the page, and the ground at the bottom of the page helps me visualize the circuit work more easily.

So with that in mind I redrew the Dome light circuit, and came up with the following.
When you look at it, you will see that it's broken into 3 sections, from top to bottom:

* The Always On +12v power section. (Circuit #40/Orange wire)
* The ON/OFF control section. (Circuit #156/White wire -- switched grounds.)
* The Always grounded section. (Circuit #150/Black wire)

You must be registered for see images attach

Here are some troubleshooting hints to keep in the back of your mind while looking at this Functional Flow:

* I simplified the Always On & Always grounded sections of the diagram. This was done because we aren't
concerned with missing/flaky power or ground. Instead, our focus is on the reported symptom, which
is the inability to control the current flow through the Dome light. (It's always on, instead of us being
able to switch it on & off as desired.)

* Even when the truck is shut off, whatever voltage is on the positive terminal of the battery will
also be sitting on the filament on the bulb. Even when the bulb is dark. In order to illuminate
the bulb, the 3 switches that are supposed to control this buib are going to connect a ground
to the other side of the bulb, allowing that stationary +12v power to now flow through the
filament. Remember voltage is just how much potential there is to do work...but it's the
actual current flow that does real work. (In this case, lighting the interior of the truck.)

* For those raised on binary logic, this circuit is the equivalent of a 3-input OR gate.
Left door open OR switch on dash closed OR Right door open = active circuit.
I share this perspective, because as the troubleshooter it has to be kept in mind that
any combination of one, two, or all three switches closed will light the light.

* If we were having a problem with one or more switches NOT working, then we would be
all over the Orange or Black wires. But for your symptom nearly all of our focus needs to
be on the 'switched ground' circuit #156/White wires in this circuit. ANY ground (circuit #150/Black)
wire directly connected to *any* of the white wires is an illegal condition...and will cause the
Always On symptom you are experiencing.

* I've attached the truck wiring diagram showing the physical layout of the wiring harness.
Here in the rust belt others have reported wires in the vicinity of the driver's door opening
where they have chafed to each other or to the chassis/frame.

In English, in order to be able to deliver a switch-controlled grounding in this circuit, the
entire white wire circuit needs to be isolated from ground at the beginning, middle, & end
of this circuit. No chafes or splicing a ground wire directly to a white wire, so that there is
zero current flow unless any one of the 3 switches closes, and *then* a solid ground should
be forwarded to the Dome light bulb so that it can shine.

* Speaking of troubleshooting this circuit as quickly as possible, pay close attention to the
Convenience Center on page 92 above. It looks like you should be able to easily unplug
all 3 switches individually, and see which circuit is causing the 'Always On' symptom?

Or if you unplug all three switches, and the light is still on? This would tell me that there
is an illegal ground (on purpose or by chafe) somewhere on the white wire section of this
circuit, and that needs attention first.

****

So there you have it. All the FSM diagrams I shared are from the '89 Wiring Diagram, so if
there are still color code mismatches then I don't know whether that's a documentation error
in the '89 FSM? Or possibly some undocumented mod made to your '89 truck prior to your
purchasing it?

And for what it's worth, you have been getting super solid guidance from the other contributors to
this thread. My motivation to redraw the circuit was simply to clarify exactly which color wires in
this circuit would most likely lead to the ON / OFF control symptoms you described..

****

Apologies for the length, but if the PO modified the wiring in this circuit without understanding
the switch logic nature of all this, it's easy to cover up the proper functioning of this circuit
with a well-intended but incorrect extra ground connection here or there.

But armed with an understanding of how this circuit is supposed to work, I am confident that
you will be able to fix this long-standing issue. Don't hesitate if the stuff above
created additional questions on your end of this long distance team troubleshooting effort.

Best of luck --

I have tested the functions of the door jam switches using an amprobe , both work as they should, as well as the dome light switch on the dash .I took out the old cargo lamp switch which also worked properly using the meter, and noticed that the wire is orange and , orange with a white stripe. I the only wires going through the sill plate and connect to the dome light are a white wire, and a orange and white wire . Could it be that the previous owner wired the cargo lamp wire to the dome light and it’s ruining the circuit? Or should it still work .

I disconnected all the switches and it stayed on, so how do I test for continuity or a shorting the wires? I’ve never tried before. The driver side switch was also properly wired according to the chart, and I made a black ground to the factory ground near the OBd port

 

[tppower]

(Original post)



Yes, I also saw the OP's comment above. It would be interesting to see what kind of continuity check
you would get if you were to check the ground he referenced to the grounded side of the dome light?
(A photo of his setup would also be helpful for those of us working remotely. I'm at a disadvantage when I
can only 'see' what's in the service manuals versus how the truck is actually wired?)

Obviously anything different between the truck's actual physical wiring vs the factory wiring diagrams is the most
suspect for changing the circuit's behavior?

****

When I'm trying to get a more complicated circuit to work as advertised (see attached for the expected Dome light operation)
sometimes I find it helpful to divide the circuit up into the simplest possible subset, make that work first, and then step by step
add back in all the more complicated bits?

So, if this was my truck, I would try to get the Dome light to go on & off using just the jamb switches? That is, Dome
light goes out when the door is closed, and turns on when the door is opened? To illustrate my point, I took the p. 92 out
of the '89 wiring manual, and pruned out as much extraneous stuff as I could, leaving only the 2 jamb nut switches & the
wiring to the Dome light. The result is a lot easier to follow while you are measuring the circuit:

(Page 92 of the '89 FSM, Dome light circuit pruned back to just the jamb switches & Dome lamp)

You must be registered for see images attach

This is more like it. Given the way that this circuit is drawn, I'm thinking that I could have the dome light
temporarily laying in the middle of the bench seat, and by opening or closing the jamb switches I could have the Dome
light turn on and off as you would expect the circuit to operate? Sometimes a Divide & Conquer approach pays big
dividends when working on a new to you circuit for the first time.

****

By the way, it felt a bit weird to write about 'illegal grounds', for normally we think that the more
grounds the better? But when I was studying this circuit, I was struck by the fact that a full
ground (black wire circuit #150) is not connected directly to the Dome light bulb?

Instead, there *is* a ground connected to the bulb, but it is a switched ground. And thanks to the
General's rule that every time a wire in a circuit changes it's function, it gets a new circuit number.

So if we start at the ground (G302) the black circuit #150 wire is right there, just like I have learned
to expect. But then the constant ground gets run through a jamb switch, and now we end up with a
conditional (switched) ground. Per the wiring diagram documentation rules, this switched ground is
now visually identified with a white insulation, and is referred to as circuit #156.

****

So hopefully this better explains my underlying thought process. And I'll keep simplifying the circuit until
I can get it work. If I end up with a single jamb switch feeding the single Dome lamp bulb (and the dome light
is resting in the middle of the bench seat 100% isolated from the rest of the truck) ...then that's exactly
what I will do.

And once I get that light working, I'll put the Dome light back where it belongs, and ensure
that I can still turn it On & OFF with the jamb switch. (ie: I'm not getting a 100% of the time ground
superimposed onto the circuit when the Dome light is connected to the roof?)

And then add back in the other jamb switch? Followed by reconnecting the switch in the dash? I assure
you, I don't disassemble a circuit every single time I troubleshoot one. Sometimes, it's just that the bulb
is burned out. That's a 20 second check. Fix it & move on.

Or I just take a handful of voltage/resistance/continuity checks with everything in place. Common sense
dictates the approach.

But once in a great while a tough, intractable, long term problem like this crops up, and the factory
troubleshooting flows only cover 100% stock, unmodified circuits? Only then, after everything else fails,
I reserve the right to redraw the circuit until it makes sense to me, and then (temporarily) prune back the
circuit until it's too simple to not fix. And then start adding the complicated bits back into the circuit,
until it's all working as advertised once again. (!)

****

Apologies to the OP if this explanation feels like I'm somehow talking down to you. (Or anyone else reading along.)

My goal is simply to share how we use the divide & conquer approach to electrical circuits that have a lot of fan out
or use switch logic techniques. Some neat tricks were used by the original designers to give us the circuit behavior
that makes the Dome light work automagically/intuitively to our needs when we use our GMT400s.

As always, let us know what you discover.

Best of luck with the hunt for the fix --

The illegal ground was false per my edit last night, it was never grounded and was simply the bolt that attaches the fixture to the roof and also conncts the map lights to the circuit, which reminder work fine

 

[tppower]

Greetings tppower,

Sometimes the electrical circuits we take for granted (like the courtesy, dome, & cargo lights)
can be the most counter-intuitive ones to troubleshoot. Normally when this happens, it's because
the circuit was implemented using a form of switch logic.

So when I read your questions I realized that I was unfamiliar with the actual layout of this circuit, so
it felt like a good opportunity to bust out the '89 GM CK Light Truck Wiring Manual and see what the
tech writers had to share.



The tan wire you refer to is connected to the driver's door jamb switch, but at the same time it is *not* related to your
Dome light dilemma? So let's answer your legitimate question about this wire and get that off the troubleshooting table.

The short answer is that the TAN wire from the Left Door Jamb switch is for electrical circuit #159. But why was it added?

Well, here's the electrical chain of events that turns an opened driver's door + a key in the ignition = an audible Key-In Warning buzzer:

1) When the driver's door is opened, the spring loaded jamb switch plunger physically extends, and the internal switch
electrically closes, connecting the black ground wire (circuit #150) on one side to the TAN wire on the other side. (circuit #159)

2) This circuit #159/TAN wire leaves the Left Jamb switch and is routed to/thru connector C306 & up the steering column.
IF the key is still in the ignition when the door is opened, the associated key in switch will be closed, allowing the
ground to continue back down the steering column and turning into circuit #80. (LT GRN wire)

3) This ground carrying LT GRN wire is routed to the "Key-In Warning Buzzer" that is plugged into the front of the Convenience Center.
Since the multi-function warning buzzer has "Hot at all Times" power fed to it, ground being applied by this LT GRN
wire will allow current to flow, giving voice to the buzzer most readers are now hearing in their mind's ear while reading this. :0)

And as we all know, either pulling the key out -or- shutting the driver's door will open this circuit, silencing the buzzer.

So in order to wed the theory above to the diagram below, do yourself a favor and trace this circuit on the
wiring diagram below, starting at the Left Jamb switch on the bottom right corner of page 92:

You must be registered for see images attach

****

Now that we've tied the TAN & LT GRN wires to the Key-In Warning buzzer, let's move on to the Dome light circuit & how to best troubleshoot it?

Well, to be perfectly honest I found what the '89 Wiring Manual had on page 92 (above) was complete, but at the same time
it had that 'wallpaper' feel that seems to make diagrams like this a bit stand-offish to normal DIYers?

When this happens, most folks think that automotive wiring just isn't for them. But I would instead ask you to first
try to redraw the circuit in such a way so that you can better see the functional flow. And I also happen to find that putting the
+12v for the circuit at the top of the page, and the ground at the bottom of the page helps me visualize the circuit work more easily.

So with that in mind I redrew the Dome light circuit, and came up with the following.
When you look at it, you will see that it's broken into 3 sections, from top to bottom:

* The Always On +12v power section. (Circuit #40/Orange wire)
* The ON/OFF control section. (Circuit #156/White wire -- switched grounds.)
* The Always grounded section. (Circuit #150/Black wire)

You must be registered for see images attach

Here are some troubleshooting hints to keep in the back of your mind while looking at this Functional Flow:

* I simplified the Always On & Always grounded sections of the diagram. This was done because we aren't
concerned with missing/flaky power or ground. Instead, our focus is on the reported symptom, which
is the inability to control the current flow through the Dome light. (It's always on, instead of us being
able to switch it on & off as desired.)

* Even when the truck is shut off, whatever voltage is on the positive terminal of the battery will
also be sitting on the filament on the bulb. Even when the bulb is dark. In order to illuminate
the bulb, the 3 switches that are supposed to control this buib are going to connect a ground
to the other side of the bulb, allowing that stationary +12v power to now flow through the
filament. Remember voltage is just how much potential there is to do work...but it's the
actual current flow that does real work. (In this case, lighting the interior of the truck.)

* For those raised on binary logic, this circuit is the equivalent of a 3-input OR gate.
Left door open OR switch on dash closed OR Right door open = active circuit.
I share this perspective, because as the troubleshooter it has to be kept in mind that
any combination of one, two, or all three switches closed will light the light.

* If we were having a problem with one or more switches NOT working, then we would be
all over the Orange or Black wires. But for your symptom nearly all of our focus needs to
be on the 'switched ground' circuit #156/White wires in this circuit. ANY ground (circuit #150/Black)
wire directly connected to *any* of the white wires is an illegal condition...and will cause the
Always On symptom you are experiencing.

* I've attached the truck wiring diagram showing the physical layout of the wiring harness.
Here in the rust belt others have reported wires in the vicinity of the driver's door opening
where they have chafed to each other or to the chassis/frame.

In English, in order to be able to deliver a switch-controlled grounding in this circuit, the
entire white wire circuit needs to be isolated from ground at the beginning, middle, & end
of this circuit. No chafes or splicing a ground wire directly to a white wire, so that there is
zero current flow unless any one of the 3 switches closes, and *then* a solid ground should
be forwarded to the Dome light bulb so that it can shine.

* Speaking of troubleshooting this circuit as quickly as possible, pay close attention to the
Convenience Center on page 92 above. It looks like you should be able to easily unplug
all 3 switches individually, and see which circuit is causing the 'Always On' symptom?

Or if you unplug all three switches, and the light is still on? This would tell me that there
is an illegal ground (on purpose or by chafe) somewhere on the white wire section of this
circuit, and that needs attention first.

****

So there you have it. All the FSM diagrams I shared are from the '89 Wiring Diagram, so if
there are still color code mismatches then I don't know whether that's a documentation error
in the '89 FSM? Or possibly some undocumented mod made to your '89 truck prior to your
purchasing it?

And for what it's worth, you have been getting super solid guidance from the other contributors to
this thread. My motivation to redraw the circuit was simply to clarify exactly which color wires in
this circuit would most likely lead to the ON / OFF control symptoms you described..

****

Apologies for the length, but if the PO modified the wiring in this circuit without understanding
the switch logic nature of all this, it's easy to cover up the proper functioning of this circuit
with a well-intended but incorrect extra ground connection here or there.

But armed with an understanding of how this circuit is supposed to work, I am confident that
you will be able to fix this long-standing issue. Don't hesitate if the stuff above
created additional questions on your end of this long distance team troubleshooting effort.

Best of luck --

I also cannot unplug anything from the convience center. None of the wires will come out . I can’t access what I think is a side tab and when I did they won’t come out . The only clue I can give is that the top three have a orange and white wire combo into each one . The far left two are routed under the sill plate. I only thought there was one when I looked in the B pillar . I’m wondering if the second is remains of the cargo light and is grounding the whole circuit . I will be removing my interior panels next Tuesday on my day off to check this.

Every wire including the ones cut on the driver side have continuity when I ground them . The white and orange wire at the dome light also have continuity . The dome light and the convience floor lights all read 12.54 volts hot at all times . As I said previously the switches were tested and all function normally. I tested my grounds and they are not hot. Using the cut driver side white wire I found that it was not hot .

I am completely lost. I don’t have a hot white wire and all my switches work

One last thing, when I pulled wires from the sill plate to check , i got tags number the following :

281 3075 3076. Don’t know what wires those are

Also glass in the sill which explains the salvage title