1993 GMC Sonoma Brake/Tail/Back-up Lights not working

My tail lights have recently stopped working. When I use blinkers, they are slow, and don’t blink if brakes are pressed. The brakes show as a very faint light that cannot be seen in day time. I turned on the hazards to look myself, and they started beeping. I called GMC support and it’s too old for them to still know anything about it. I checked the fuses for the rear end and they are both working.

I just checked the lights’ ground and it’s fine.

How did you check the ground?

I also replaced the brakelight switch, and I still don’t have full taillights or back-ups.

Yes, Rod Knox, as posted 12 minutes before your question.

It would also help to know if the rear running lights are working ok. If not then it seems there is a grounding issue, perhaps to the bed of the truck. I suggest you check the voltage getting to the brake light switch using a meter and tieing the common lead of the meter to a known good ground reference point. You should see 12 volts getting through the switch and on to the lights. If that is good then check the voltage at one of the brake lights using that same ground connection. If you still see 12 volts at the light then the grounding to the bed is most likely bad. Try running a temporary ground to the bed to see if that changes things.

I agree with Cougar and Rod Knox that this sounds like a grounding issue. Rod Knox was asking what method you used to determine that the ground was “good”. It sounds like pretty much all of your bulbs have poor to no function, and that usually points to a bad ground because it would be odd for all of your sockets, circuits, and switches to be bad all at once. Find your ground wire or wires and run it/them to a new grounding point on the body of the truck. Be sure to remove all paint and rust so you are connecting to nothing but bare metal. If doing this brings function back to your lights, the ground was bad. If that doesn’t help, your best bet would be to remove the entire rear harness (it unplugs somewhere under the bed) and give it a good look, then replace anything that looks bad or marginal. I had to do this on a truck I had once to restore full function to all the taillights and brake lights, but that truck was 35 years old at the time. Hopefully your problem isn’t that bad because that project for me was pretty time-consuming.

There is a connector between the rear light harness and the main wiring harness on the rear drivers side with a ground wire connected to the frame. I would check that connection, it will disconnect the two wiring harnesses from each other, and the ground. Clean that ground and clean the connectors and see if that works. That is also where you install a pig tail for a trailer.

Maybe I shouldn’t have asked a question?

Some people want an answer that doesn;t conflict with their preconceptions. I too would like to know how the ground was checked.

The ground was checked by disconnecting and cleaning. I’ll try moving it. I have also been advised to try starting at the fuse block and working backward. I do have a pig tail installed by U-Haul in 1998, which could also be a problem, as they use the method of clipping through just the insulation. I checked the one light I had working (blinker/hazard) and it was getting between 1-2 volts, so I’m assuming the problem is a dirty connection. I’ve just replacing my radiator so I can start checking lighting now.

I can’t find any information on the hazard/beeping. When the hazards are turned on they beep, which doesn’t happen normally. Does anyone know what code this signals so I can skip to that step?

The U-Haul guys have the option of doing it right (as in plugging into an existing wiring system as installed on some trucks), or clipping right into the wires. Typically, they’ll use those sexy blue clips and destroy the wiring. While the hitches and stuff they have may be of decent quality (I personally have no equipment complaints), the way some of it gets installed is not necessarily what I would call the right way.

Most likely, I would guess the rear wiring harness is gone at that point. Also, most likely, you’ll need to replace about a foot of wiring around those clips, on each wire.

The ground test you were being asked about…since they all stopped at the same time, it is still very likely a ground problem. You could try something as simple as running a wire from the ground on the light fixture, all the way to the battery ground, and see if it works.

What you did was clean a connection…not test it. Kudo’s for starting there, but it still needs to be tested.

I changed the location of the ground and it increased to 5-6 volts in the taillights. I changed the bulbs, and one of them was the cause of the beeping, but I moved it to the other side and after I wiggled some wires it stopped beeping and I got 11 volts in each of my top lights but my middle lights dropped to .5-.6. When I put the unit back onto my truck they must have wiggled the wrong way because I lost them again. I’m going back through the end of the harness to find where the short is.

Ever seen a car that when they hit the brakes, the other lights would dim? Or the blinker would cause others to dim? That’s a ground problem.

Everything you’ve described so far still tells me you have a ground problem, not a short.

I’m running out of holes to test my ground on. It worked fine when I switched it for a few minutes, but now I’m at a loss. I seem to have a leak between my pigtail and ground, but all of the connection bits look good. Should I just clip the end and start over with a new ring? I’m not sure how much extra wire I can get from the original set-up, and I’m trying to avoid rewiring it all, as that will be way beyond my electrical abilities.

I also don’t have any idea where on my light fixture to attach a wire from ground to fixture. And I’m assuming the battery ground is just negative, although I’ve been trying to find out and am just seeing “the black wire”, so I may be wrong.

The negative post of the battery is the chassis ground or return side of the battery. Make sure the connection to the chassis is clean and making a solid connection to the chassis.

In order to confirm this is a ground problem I suggest you run a temporary wire from the negative battery post to a good ground point near the rear lights. If the lights work ok after doing that then you need to find the ground wire between the chassis and the bed of the truck and look for a bad connection. If the lights still are dim then there is a problem in the power wire side to the lights. From what you state the trouble appears to be with the ground side but there may be a problem with both sides of the connection. Do the running lights work ok?

Apparently you have learned to cut to the chase on such problems, Cougar. It really does save a great deal of wasted time and frustration. I have found some success with a digital LED test light and eliminated some of the hard wiring tests but sometimes nothing else will get to the bottom of the problem. I hate to admit it but I once replaced a fuel pump only to find the intermittent problem was due to a poor ground.

Rod Knox, fuel pump was on my replacement list last summer, but it turned out to be a cross-firing distributor camp and ridiculously hot summer, plus a leaky radiator which didn’t help the heat any. Back to lights- I checked the ground for the pig tail, and moved it, and tested again. I disconnected it and used the bypass, and we had current to the lights, but still couldn’t find a bad ground. Then the lights came back on again, so I went through and reconnected the pigtail with no problems. We pulled apart the light casing to test directly with a multimeter, and there seemed to be no problem, but no light either. Turns out the connection between the bulb and the socket is wearing out, so if the bulb gets bumped out of its placement the lights go. I redistributed the yellow paste in the casing and around the bulbs, so hopefully they will last until I find replacements for the sockets and surrounding case. Thanks for the help.

It sounds like there may also be a connection problem with the power side of the wiring to the lights. I find it hard to believe all the bulb sockets are wearing out. You have at least proven there is a intermittent ground problem to the truck bed.