Rear lighting issues

[palmer6strings]

I hope this is in the right place, kinda new and haven't learned exactly the site layout. So if its not please feel free to do as needed.

I'm working on my dads 94 k2500 5.7 4l80 and am having some issues with the rear lighting.

Checked all fuses and they are ok. The wiring is all good from the bulk head to the very back harness when ohmed.
There is no trailer harness wired in.

Tested old circuit boards and new boards with no difference. Both sets are push in style bulbs.
Used all new bulbs.

The rear parking lights work but seem pretty dim.
Turn signals work.
Backup light stays on in all gear positions but turn off when actually in reverse.
No brake lights at all. Switched the old switch out for a new switch and still no brake lights. When I push the switch by hand the third brake light will turn on but not the rear lights. Its like the shaft the switch sits on will not activate the switch when fully installed correctly.
I'm at a major loss on what to do. Im reading over wiring diagrams and everything seems right.

 

[Schurkey]

94 k2500

Is there a trailer-hitch wire harness connector?

Checked all fuses and they are ok. The wiring is all good from the bulk head to the very back harness when ohmed.

I'd be more interested in voltage-drop testing than resistance testing, but at least the wires are not broken and conduct SOME amount of amperage.

The rear parking lights work but seem pretty dim.

They're not supposed to be as bright as the brake lights. Verify the reflectors and lenses are clean. Assure the ground is good.
A 12-volt test light having an old-style incandescent bulb (NOT a new-style test light using LEDs) will give you a better idea of available voltage under load.

Backup light stays on in all gear positions but turn off when actually in reverse.

That's a puzzler. Apparently the switch works, but is somehow screwed-up in terms of supplying power.

The gear position (backup light) switch in my '88 was worn and defective, but it wasn't working "backwards" from how it should.

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Replacement switch, for comparison. Has an extra circuit that my truck doesn't use.

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Let us know when you figure this out.

No brake lights at all. Switched the old switch out for a new switch and still no brake lights. When I push the switch by hand the third brake light will turn on but not the rear lights.

1. Verify the bulbs. Some have two filaments, one filament can burn-out so the bulb still lights up "sometimes". If the third brake light works like it should...the brake switch is not the problem.

2. These vehicles have a known, common problem with the circuit board that holds the rear bulbs. Earliest trucks were worst--they used the old-style, push-and-turn bulb sockets. The later (updated) circuit boards use push-in bulbs but they still have problems with corrosion.

3. "Weird" lighting problems are sometimes due to a faulty turn signal switch. Look on your schematic to see which lights have the turn signal switch in common.

 

[palmer6strings]

I will edit my main post since I forgot to add them in there but will answer so of what you stated.
No trailer light harness or anything wired in.
We got the old and new circuit boards. Both sets do the same thing. They are both the newer style push in bulbs.
We also got all new light bulbs. I do know that running lights will be dimmer than the brake light side, they just seemed dimmer than normal. And they are the 2 filament bulbs.
I did do a voltage check and everything that was working was getting up to around 12v

I will check into the neutral safety switch and the signal switch.
I also had some guys say that they had similar problems and a new headlight switch and dimmer rheostat fixed their problem so I will look into that too.

 

[Das Hatt]

Check to be sure you have a clean ground at the inside rear of the frame on the driver side. You want shiny metal.

 

[palmer6strings]

Check to be sure you have a clean ground at the inside rear of the frame on the driver side. You want shiny metal.

Thank you, that was one of the first things I checked.

 

[palmer6strings]

Ok so I finally got all the lights fixed EXCEPT the brake lights.
It is 100% the switch not becoming engaged. There is no play on the brake boosted rod that goes to the shaft on the brake pedal so the shaft will not engage the points mechanism to turn on the brakes. This is a terrible design compared to the old style that you can actually adjust.
I have tried everything that I could find on this site, from bending the prong to make less play room on the switch and shimming the switch.

The only thing I havent tried, (which I'm gonna try tomorrow) is swapping in a different brake booster to see if the rod going to the brake pedal shaft will give it enough play to fix it. If it doesn't I am completely out of ideas.

 

[Schurkey]

How did you fix the backup lights?

 

[palmer6strings]

How did you fix the backup lights?

Sorry my bad thought I said what it was. It was the neutral safety switch.
I saw in someone else's thread the fixed their brake light issue I'm having with the new booster so I'm crossing my fingers.

 

[palmer6strings]

Ok last update. Everything is fixed.
I dont know when, and dad said he doesn't remember ever having it changed out, but someone replaced the brake booster with the wrong one. So yes there is a shaft difference that changed in 93 when they went from the manually adjusted brake light switch to the non adjustable one.
I also don't know how whoever did the change got the brake lights to even work. Maybe they didn't ever check the lights worked and just assumed they did.
Luckily I had a 94 parts truck so I was able to just steal the booster off of it so it didn't cost anything.
Anyway glad its fixed.

 

[1990Z71Swede]

So... decided to fix the backup lights on our truck.
They are a bit different for us Swedes in that red turning lighs are not alowed since way back in maybe early Seventies. The usual fix for imports is to put yellow bulbs in the backuplight position and reroute the turning light cables to them. The backup light is then handled by a separate light mounted wherever suitable.

Anyways simple fix, because the separate backuplight was totaly missing.
I installed a new tiny LED backuplight and could even use part of the old cables for it. BUT... as expected it was not going be that easy.

The backuplight turns on alright, bright as day, but only between P and R. When the shift lever goes fully into Reverse the light goes out.

I can adjust the switch so that it works in Reverse, that part was surprisingly easy. But as soon as I put it in Park the Switch is forced back to its original position again.

Soo, my question is this.
Will a new switch fix this or is there something else going on here?