Stop lights not working

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mongoose2231

mongoose2231

10 Year
Joined
Dec 23, 2011
Threads
111
Messages
3,601
Location
Orange County, NY
I have done a bunch of searching and FSM studying but want to pick your brains. I have a 1986 pickup and the brake lights stopped working. I have running lights and backup lights and turn signal lights, just not no brake lights. I have replaced the switch, since i realized the stop lights were out at work, 70 miles from home and its the first thing that came to mind. that still did not solve the problem. I now had some time to do some testing and of course, the fuse is good, switch is new, I have all other lights working, could it be a ground somewhere? Thew fsm points to a ground if everything else checks out. My question is where is the tail light switch ground located? Also, The voltmeter only reads about 6.5v at the switch. Is that the right voltage?
Any help is appreciated.
Thanks.
 
Thats pretty low volt....check the main input connector at the bottom of the fusebox in the engine compartment, it gets metal fatigue, and will snap off easily.

In the wagons, there is a connector on the back pillar, there may be one behind the driver side brake light in trucks.
 
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I forgot to check for voltage at the fuse. I will try that tomorrow. I will also check the connectors you mentioned above. I did trace the wires from the back all the way into the cab, under the passenger seat connector. Everything is kosher there. I may have to just clean all the grounds on the truck to make sure all is well.
 
ok, I checked voltage at the fuse and I only have 4.5v. this must be a weak ground somewhere, right?
 
no, no trailer harness, no accessories, nothing. Harness is stock, unaltered. I will go back in the FSM to see location of body grounds
 
Excellent, will try that. BRB, thanks
 
so I couldn't get an ewd for an 86, this is for a 91 but it should still be useful in diag. wire colors may be different but hopefully the ground location will be the same
Screenshot (12).webp
 
Just grounded the fuse to another location and it reads 12.45v. That would indeed mean that my tail light ground is ineffective. I have to find it and see where it is located. Thank you for the diagram. I think it is clear that I have a bad ground somewhere. I do have water seeping in sometimes so I suspect corrosion
 
so you don't have a power issue, your stop light switch is good so I would start form the ground and work backwards to the switch. there could be corrosion in a connector, issue with the bulbs or just a bad ground. if you have dieletric grease I would give a little blob in the connectors as you check them
 
That is what I usually do, a bit of dielectric grease in the connectors. I initially thought the switch was bad but after that checked out ok i figured maybe ground. Couldn't have been relay because i have all other lights in the tails working.
 
I dug up the ground locations from FSM. I will go inspect, dismantle, clean and reinstall ground points to see where that takes me. Will report shortly.
 
Ok, so I have gone back to square one and found both bulb filaments were bad. Initially I checked the bulbs and they were good. Regardless, I cleaned all grounds checked fuses, replaced bulbs and all is good. Thanks all for the help.
 
Sometimes, it's just stupid, easy, schit like this that drives us nuts. Nice job.
 
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