73-87 Chevy _ GMC Trucks => Electrical => Topic started by: 1985GMCSierra on May 17, 2019, 01:58:22 PM

Title: Brake light issues
Post by: 1985GMCSierra on May 17, 2019, 01:58:22 PM
I have a 1985 GMC Sierra that has been having some brake light issues. At first, just the right taillight went out and I went to replace the bulb, but that wasn't the problem. I tested the assembly with a test light and everything was working, but then I had to put it in storage for a while and go to school so I didn't mess with it for a few months. Looking at it today, the left taillight won't work either, but when I put a bulb in the right taillight and put on the right turn signal, both taillights and my license plate light flash as if I have my hazards on. When I flip the left taillight, it doesn't flash. It seems like there are some wiring/ground issues, but I'm not sure and don't really know where to start. Any ideas?
Title: Re: Brake light issues
Post by: Irish_Alley on May 17, 2019, 02:23:26 PM
check the ground behind the left tail light assembly. make sure its good
Title: Re: Brake light issues
Post by: bd on May 17, 2019, 04:07:26 PM
The truck probably has compromised grounds for the rear lamps.  There should be ground wire connections to the bed sheet metal hidden behind both tail lamp assemblies.  There may be an additional ground connection to the rearmost cross member, adjacent to the square 4-wire harness plug for the rear lamps.  Physically remove the tail lamps from the bed and then remove the ground wire attaching screws and burnish the sheet metal and wire terminal ends until they are shiny.  Coat the freshly cleaned metal with some antioxidant paste available from the electrical departments of most hardware stores.  Verify that the black ground wires are not corroded or severed.
Title: Re: Brake light issues
Post by: MIKE S on May 17, 2019, 05:00:16 PM
You could also have a bad bulb. Look at bulb filaments real close if they are touching you will get feedback to the the other side. 
Title: Re: Brake light issues
Post by: JohnnyPopper on May 17, 2019, 09:57:22 PM
I agree with Irish and bd!

JP's rule of thumb: When electrical goes wonky, look for ground problems.
Title: Re: Brake light issues
Post by: 75gmck25 on June 09, 2019, 05:30:50 AM
Also remember that the stock GM ground wires may be white on the rear wiring harness.  At least they are on my '75.  No idea why GM did not use a black wire.