No brake lights. Brake light circuit diagram needed.

someotherguy said:

I'm not the one to discourage troubleshooting, but if your hazards and signals work and the brake lights don't, the #1 suspect is the brake light switch. They are cheaply made (even originals) and don't last forever. They are cheap to replace. Replace it, and on 1988-1993 there is an adjustment procedure before it will work correctly.

Here's a thread about adjusting the 1988-1993 GMT400 brake light switch: https://www.chevytalk.org/fusionbb/showtopic.php?tid/180723/

Once you've got the new one installed, adjusted, and working - pop your old one open and discover the set of broken leaf switches/contacts in there.

Richard

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I just noticed that they look like a sloppy design. My brake arm hits on one side of this switch and has it cocked to one side. I can see that eventually giving trouble.

ben2go said:

That's correct. When we noticed no brake lights, we parked the truck. I checked the lights to see if signals lights and flashers worked, they did. Then my son had to drive it. When I got a chance to start looking into it deeper, none of the signals worked, no hazards, and no brake lights. All the lights we have are running/park lights and head lights. There's no power at the brake light fuse. Whatever caused the brake lights to fail I think caused the rest of the failures. The rain keeps coming and I can't work on the truck. I would like to figure this out in case someone else comes up with this problem.

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The body grounds can cause a bunch of problems and another is the ground under the dash above the obd port. those dash frames get rusty too.

I'm having a similar problem with my '98. No brake lights or turn signals in the back. However the center light on the cab works when I apply the brakes. To beat all its an intermittent problem. When I'm ready to fix it they start working again! Sometimes I think my truck has a sick sense of humor. I replaced the brake light switch after checking fuses to no avail.

Here's a link to manuals with wiring diagrams "hatzie" posted years ago. https://www.gmt400.com/threads/88-95-service-manuals.43575/ I didn't check the links to see if they still work, but I can send you screenshots from the ones I downloaded if need be.

Queso Del Rio said:

Here's a link to manuals with wiring diagrams "hatzie" posted years ago. https://www.gmt400.com/threads/88-95-service-manuals.43575/ I didn't check the links to see if they still work, but I can send you screenshots from the ones I downloaded if need be.

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Thanks. I downloaded it. I hope it's the right color wiring.

Thanks to Ben2go, Hatzie and all that are so helpful. I was about to start tracking down my problem but when I stepped on the brake pedal to move my truck the lights were working again ‍ . I hate intermittent issues. Just a bad connection somewhere.

GMTMark said:

Thanks to Ben2go, Hatzie and all that are so helpful. I was about to start tracking down my problem but when I stepped on the brake pedal to move my truck the lights were working again ‍ . I hate intermittent issues. Just a bad connection somewhere.

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If you do have to go upside down and sideways under your dash again, you should take a look at the Neutral Safety Switch (plastic) too. Mine disintegrated when troubleshooting my Reverse Lights not working. I just looked at it wrong! It was the problem for not getting power back there. Oh well, pretty good for 30+ years.

I will do that. Thanks.

OK, here is what I learned from experience - actually it was taught to me by an experienced mechanic the first time I started having intermittent brake light failure about 10 years ago or more.

The multi-function switch WILL fail on our trucks and a myriad of other GM vehicles. In our case - and universally - the first failure mode is the intermittent failure of the main brake lights - high mount is always OK. It will get worse until they just fail. Makes sense to me that if a bad multi-function switch stays in service long term it could get bad enough to take out the hazard and turn signals.

I know they can cost over $100 to buy (Rock Auto) and replace - if you pay someone to replace it. My mechanic charged me $40 and did NOT remove the steering wheel. Considering the likelihood that it WILL fail, just change it and then tell us if your problems are solved. That will be way faster than days of digging through the electrical system and far less intrusive.

This is really showing my weakness in electrical. Thanks again for the tip. I'll buy one very soon. Maybe after I receive the part I can figure the installation without removing the steering wheel.