Fuel pump issues

[Jobey]

I meant to say I did find out for sure that the relay is not getting the signal from the vehicle control module when I turn the key on. I had my multimeter on it and was watching it when I turned on the key and it didn’t turn on for the normal 2 seconds.

Greetings Jobey,

Here's the wiring diagram for your fuel pump / fuel pump relay. After this diagram we'll walk through
the circuit and try to figure out what has to happen in order to get the fuel pump to operate properly:

You must be registered for see images attach

EDIT: (p. 6-1677 of the '98 FSM. Decluttered of dual tank & #87a test circuitry in order to make single gas tank fuel pump troubleshooting easier.)

Things we have to have / has to happen for success:

* The 20A ECM B fuse in the Underhood Fuse/Relay Block has to be not blown.
* When the Igntion Switch is turned to RUN, the VCM must energize the Control side of the fuel pump relay for an audible 2 second prime.
IMPORTANT: As soon as the VCM sees the engine turning over (ie: Reference pulses coming from the CKP (CranKshaft Position) sensor, it
will continue to power the control side of the fuel pump relay for as long as it continues to see those Reference pulses from the CKP.
(seconds, minutes, hours, etc)

But as soon as the crankshaft quits spinning, the VCM will remove power from the control side of this relay. The good reason for this to
happen is that the crankshaft actually stops spinning. The bad reason is that the crankshaft is spinning but the CKP sensor isn't doing it's job.

Troubleshooting: Hot wiring +12v to Pin 30 of the fuel pump relay socket and having the fuel pump turn on means that we have a good pump,
good wiring, and a good ground. In this scenario, the 4 possible causes of failure would be:

* Missing power from the ECM B fuse to Pin #87
* Missing control signal from the VCM at Pin #85
* Missing ground (not allowing current flow from VCM input signal) at Pin #86
* Power/grounds at #85 / #86 / #87 all good, fuel pump relay itself bad. NOTE: All 4 square relays in Underhood Fuse Block are identical, so it's easy to
temporarily swap one of the other relays into the circuit and prove/disprove that the original relay is good or bad. (!)

Here's a bird's eye view of the Underhood Fuel/Relay block with the fuel pump relay socket marked up:

You must be registered for see images attach

(NOTE: The Fuel Pump Relay is the Forward Inboard position, with the smaller rectangular Horn relay next to it. See attached for big picture of Underhood Fuse Block.)

In case there is a failure of this fuel pump relay circuit, there is a backup source of power for the fuel pump.
If you look at the wiring diagram, once there is ~4 psi of oil pressure at the Oil Pressure Switch, +12v will
be switched from Pin D to Pin C, and this will connect that power to the fuel pump.

So if you look at the gray wire, at splice S100 there will be +12v power from either the Fuel Pump Relay,
-or- the Oil Pressure Switch, or both. The problem with the Oil Pressure Switch is that it can take a lot
more cranking before the oil pressure switch closes, so especially in cold weather for the quickest starts
possible you really want the fuel pump relay to be working properly.

Another timing consideration to keep in mind: The VCM will give you a 2 second prime when the key
is first turned to the RUN position. It will also give you another 2 second timed run when you turn
the key from RUN to OFF. However, if ~10 seconds hasn't elapsed, don't expect another 2 second prime
when you turn the Ignition Switch back to RUN. (But you will always get the 2-second prime when you
go back to OFF.)

So don't get all tangled up in that when you are troubleshooting. Working methodically, make sure that
you are getting your 2-second prime. This confirms that you have VCM > Fuel Pump Relay connectivity.

If need be, using a trouble (test) light you can confirm the 2 second prime as a 2 second light being lit. And
when you crank the engine, the light should come back on, because the crank is spinning. If it doesn't,
then you have to find out why the VCM isn't seeing the Reference pulses from the CKP sensor.
(Note: In a pinch a voltmeter can be used to monitor the power being sent to the fuel pump relay
socket connections, but a real load in the form of a trouble/test light is preferable.)

If we never get the 2-second prime, then is the fuse for the VCM blown? Etc.

****

I am going to send you this now. In my next post I will discuss the 87a input a little.

Let me know if you have any questions about what I just wrote.

Best of luck --

thank you very much for your very informative reply. I’m working on my truck now and I’m gonna run through it quickly with what you wrote in mind and I’ll let you know as soon as I got this figured out. The only thing that I’m really starting to wonder is why I’m not getting the initial prime from the fuel pump. VCM fuse? VCM maybe? I’m gonna look into that now.

 

[Road Trip]

I meant to say I did find out for sure that the relay is not getting the signal from the vehicle control module when I turn the key on. I had my multimeter on it and was watching it when I turned on the key and it didn’t turn on for the normal 2 seconds.

thank you very much for your very informative reply. I’m working on my truck now and I’m gonna run through it quickly with what you wrote in mind and I’ll let you know as soon as I got this figured out. The only thing that I’m really starting to wonder is why I’m not getting the initial prime from the fuel pump. VCM fuse? VCM maybe? I’m gonna look into that now.

Your primary focus on not getting the 2 second prime is correct. You don't need *any*
CKP Reference pulses in order to get that 2 second prime. The VCM (of course) needs
Power, but it isn't connected to a "Hot at All Times" circuit, for a constantly powered VCM
would drain the battery too fast while your GMT400 is parked.

IF I were to assume a good VCM, then I would be making sure that the power sent to
the VCM from the Ignition Switch is making it from there to the VCM. A good VCM being
sent power from the Ignition Switch should be powering up the fuel pump (via the relay)
for 2 seconds as part of the initialization routine.

In other words, the whole CKP Reference pulse stuff is not a concern at this time.
We want that Key On 2-second prime. (!) Either good VCM not getting the hint
from the Ignition Switch, or bad VCM getting the hint & ignoring it?

NOTE: A bad VCM *is* possible, but not as probable as a loss of power due to
some wiring harness fatigue/failure.

Get that 2 second Prime, and hopefully everything else will fall into place.

Best of luck with the discovery process --

 

[Jobey]

Greetings Jobey,

Here's the wiring diagram for your fuel pump / fuel pump relay. After this diagram we'll walk through
the circuit and try to figure out what has to happen in order to get the fuel pump to operate properly:

You must be registered for see images attach

EDIT: (p. 6-1677 of the '98 FSM. Decluttered of dual tank & #87a test circuitry in order to make single gas tank fuel pump troubleshooting easier.)

Things we have to have / has to happen for success:

* The 20A ECM B fuse in the Underhood Fuse/Relay Block has to be not blown.
* When the Igntion Switch is turned to RUN, the VCM must energize the Control side of the fuel pump relay for an audible 2 second prime.
IMPORTANT: As soon as the VCM sees the engine turning over (ie: Reference pulses coming from the CKP (CranKshaft Position) sensor, it
will continue to power the control side of the fuel pump relay for as long as it continues to see those Reference pulses from the CKP.
(seconds, minutes, hours, etc)

But as soon as the crankshaft quits spinning, the VCM will remove power from the control side of this relay. The good reason for this to
happen is that the crankshaft actually stops spinning. The bad reason is that the crankshaft is spinning but the CKP sensor isn't doing it's job.

Troubleshooting: Hot wiring +12v to Pin 30 of the fuel pump relay socket and having the fuel pump turn on means that we have a good pump,
good wiring, and a good ground. In this scenario, the 4 possible causes of failure would be:

* Missing power from the ECM B fuse to Pin #87
* Missing control signal from the VCM at Pin #85
* Missing ground (not allowing current flow from VCM input signal) at Pin #86
* Power/grounds at #85 / #86 / #87 all good, fuel pump relay itself bad. NOTE: All 4 square relays in Underhood Fuse Block are identical, so it's easy to
temporarily swap one of the other relays into the circuit and prove/disprove that the original relay is good or bad. (!)

Here's a bird's eye view of the Underhood Fuel/Relay block with the fuel pump relay socket marked up:

You must be registered for see images attach

(NOTE: The Fuel Pump Relay is the Forward Inboard position, with the smaller rectangular Horn relay next to it. See attached for big picture of Underhood Fuse Block.)

In case there is a failure of this fuel pump relay circuit, there is a backup source of power for the fuel pump.
If you look at the wiring diagram, once there is ~4 psi of oil pressure at the Oil Pressure Switch, +12v will
be switched from Pin D to Pin C, and this will connect that power to the fuel pump.

So if you look at the gray wire, at splice S100 there will be +12v power from either the Fuel Pump Relay,
-or- the Oil Pressure Switch, or both. The problem with the Oil Pressure Switch is that it can take a lot
more cranking before the oil pressure switch closes, so especially in cold weather for the quickest starts
possible you really want the fuel pump relay to be working properly.

Another timing consideration to keep in mind: The VCM will give you a 2 second prime when the key
is first turned to the RUN position. It will also give you another 2 second timed run when you turn
the key from RUN to OFF. However, if ~10 seconds hasn't elapsed, don't expect another 2 second prime
when you turn the Ignition Switch back to RUN. (But you will always get the 2-second prime when you
go back to OFF.)

So don't get all tangled up in that when you are troubleshooting. Working methodically, make sure that
you are getting your 2-second prime. This confirms that you have VCM > Fuel Pump Relay connectivity.

If need be, using a trouble (test) light you can confirm the 2 second prime as a 2 second light being lit. And
when you crank the engine, the light should come back on, because the crank is spinning. If it doesn't,
then you have to find out why the VCM isn't seeing the Reference pulses from the CKP sensor.
(Note: In a pinch a voltmeter can be used to monitor the power being sent to the fuel pump relay
socket connections, but a real load in the form of a trouble/test light is preferable.)

If we never get the 2-second prime, then is the fuse for the VCM blown? Etc.

****

I am going to send you this now. In my next post I will discuss the 87a input a little.

Let me know if you have any questions about what I just wrote.

Best of luck --

Essentially I could prime the engine with a switch and as soon as it is running I could shut the switch off and the oil pressure switch should take over correct? Even if my relay is bad?

 

[Road Trip]

Essentially I could prime the engine with a switch and as soon as it is running I could shut the switch off and the oil pressure switch should take over correct? Even if my relay is bad?

That is a possible workaround. As a matter of fact, there is another forum member running
a similar workaround, and we're waiting to hear back from him that he has the final fix in place.

This is assuming of course that your Oil Pressure Switch is working as advertised. It's a backup
system, and sometimes backup systems that have never been needed will fail on the down low,
and nobody is the wiser...until you try to make use of it.

Not trying to be a naysayer. Just trying to set reasonable expectations in a vintage engine bay.

****

But before anyone raises a Safety concern (like accidentally leaving the pump on all day while you
are at work, and now the truck won't start when you want to go home) ...if you have a 'Momentary On'
switch that you can use to implement the work-around, you should be good to go.

I'm inferring from your thought process that once you get the fuel pump running that your engine
is also running? If this is the case, then it sounds more like a wiring issue and less like a dead VCM?

Let us know what you end up with in the short term.

Standing by...

 

[Road Trip]

I went into the '96 FSM and collected the 4.3L troubleshooting section for
the problem you are experiencing. (See attached.)

As far as I can tell, we're on the right track per the FSM fault trees. But just in
case I overlooked a step here's the whole enchilada.

Best of luck getting your old truck back as part of the solution instead of
part of the problem. (!)

 

[Road Trip]

If you continue to have trouble getting that VCM to give you a 2 second fuel pump prime,
be sure to check *all* the fuses that it feeds from. Take note that the VCM has more than
1 finger in the power pie:

You must be registered for see images attach

Let us know what you discover.

Best of luck. Too bad vehicles delight in making the humans work on them in this kind of weather...

 

[Jobey]

Well, update on my truck. Got my little experiment done only to find that had no spark as well. So no not quite back to square 1 but maybe square 1.5? I’m getting fuel to the rail. I’m guessing I got adequate pressure since it’s a brand new pump and after this whole fiasco I’m really getting tired of the process so I’m gonna just trust it’s good.

 

[Road Trip]

Well, update on my truck. Got my little experiment done only to find that had no spark as well. So no not quite back to square 1 but maybe square 1.5? I’m getting fuel to the rail. I’m guessing I got adequate pressure since it’s a brand new pump and after this whole fiasco I’m really getting tired of the process so I’m gonna just trust it’s good.

Let's see -- we need Spark, Fuel, & Compression for an engine to run. So you are missing 2 out of the 3.
(No spark + no 2 second prime.) We've already covered what it takes to get the Fuel Pump to run.

The spark generation path for the Vortec engine is:

ECM 1 fuse > CKP > VCM > ICM > Ignition Coil > Distributor > Spark Plug Wires > Spark plugs.
(See below)

('96 FSM - 4.3L - Engine Cranks but Does Not Run - p.1)

You must be registered for see images attach

('96 FSM - 4.3L - Engine Cranks but Does Not Run, p.2)

You must be registered for see images attach

(Step #6 above = NO -- Go to Enhanced Ignition System (System Check)

You must be registered for see images attach

From here the FSM has 3 more pages (27 steps) of checks that work you through the Ignition system.
I tried to attach those 3 pages as a single .pdf file to this reply, but it was rejected as 'too big'.

In the next reply I'll attach those 3 Ignition system troubleshooting pages. But just in case I get hit by the
Ring Ding truck, you should download the '96 FSM and have all this info at your fingertips. We have a
thread where all the links for all years of the GMT400 manuals are located. The '96 FSMs were among the
last to be uploaded, so they ended up way down in reply #84. Follow this link to that reply with the
'96 FSM download info: (LINK)

More to follow shortly.

 

[Road Trip]

And here are the 3 pages of Ignition checks in the '96 FSM. (See attached.)

If you were getting the 2 second prime then I'd tell you that the ICM is known
to flake out & fail. Especially if thermal compound wasn't used during a
previous repair. Or the ICM gets taken out by the coil secondary winding arcing
back into the primary winding > toasting the ICM.

So it looks like you need to regroup and troubleshoot on why there's no spark.

But don't forget about the missing 2 second fuel pump prime at Key On.
And whether both issues are from 2 separate failures, or is it from a common cause?

No SES light? Stored DTCs? We may be at the point where you have no
choice but to get a live data scan tool involved?

Keep us in the loop. Possibly someone else reading this who's worked
through a similar failure can jump in with what worked for them.

Best of luck --

 

[Jobey]

I’m guessing that the computer is bad. Or ecm. That’s the only thing I can think of that would stop both the fuel and the spark at the same time. I was just being stupid and didn’t check one of the simplest things due to my just being sure that was it.‍

And here are the 3 pages of Ignition checks in the '96 FSM. (See attached.)

If you were getting the 2 second prime then I'd tell you that the ICM is known
to flake out & fail. Especially if thermal compound wasn't used during a
previous repair. Or the ICM gets taken out by the coil secondary winding arcing
back into the primary winding > toasting the ICM.

So it looks like you need to regroup and troubleshoot on why there's no spark.

But don't forget about the missing 2 second fuel pump prime at Key On.
And whether both issues are from 2 separate failures, or is it from a common cause?

No SES light? Stored DTCs? We may be at the point where you have no
choice but to get a live data scan tool involved?

Keep us in the loop. Possibly someone else reading this who's worked
through a similar failure can jump in with what worked for them.

Best of luck --

I just replaced the ignition control module a couple months ago. And I made sure to use the heat sink paste on it. I can’t remember how to test it but I’m sure I can figure it out again and check it out. I’m gonna go test the crankshaft position sensor now. I read up on how to do that. As far as I can tell, if it ain’t the crankshaft sensor the ecm is bad. Or in the unlikelihood that the icm is bad I’ll just replace that. But it looks like I got enough info from you amazing people that I think I can get it back on the road.