Unique tail light problem?

[shadow94]

Hello all;

I am totally stumped with my problem that makes my truck Very unsafe to drive.(light issues) I have only 1 brake light responding, both have parking lights and no matter which way you signal, one tail light blinks. (I have switched bulbs and no difference)

Here’s the messy details.
This is a 94 GMC 1 ton (K3500) dually 4x4 automatic 4L80E transmission 454 engine and a service body (flatbed). A plan truck with NO daytime running lights, 3rd brake light, airbag or power windows/locks. The basic you can get. Not even any A/C. The rear tail lights do not have the circuit boards that others talk about. These look like an aftermarket type that have rubber sockets for the bulbs that can stretch/move around so I am assuming they are for ruff service.

The seller of the truck has a mess of wires for a trailer hookup. I have had problems with the lights so I decided to straighten everything out as I was relocating the trailer hitch to a higher position. The flatbed has marker lights that are powered off the running light wire. I removed the large round trailer plug as it was damaged and my new light kit (for trailer) uses the small 4 blade connector.

I was able to find the 2 original plugs and connect them. The brown wire (running light) was snipped from the connector. I used a jumper wire to go from the harness (coming along frame rail by gas tank) to the brown wire for the marker lights. The yellow wire is for Left tail and the Dark green is for Right tail. The Light green is for reverse and has power as soon as key is turned. I never had reverse lights and found the seller un-plugged the bulbs and left them in the housing. I saw online that I have a bad back-up switch if power is on all the time.

I plugged the back-up blubs in and snipped the wire at the harness to connect later when I replace the that bad switch. I have brown running to both lights and bed lights for marker light operation. I have the yellow to the left tail and the dark green to the right tail. I ran new grounds and connected them into the existing grounds to make sure there is no lost ground somewhere.

The tail lights now light up with the parking lights on. The brake pedal only lights on the drivers’ side. Turning the turn signal left or right lights up the driver’s side only. The front lights do respond to the correct direction. The lights have always blinked fast, and then slow, then fast. I tried new bulbs and no difference.

I finally probed into that harness right where it comes out of the fuse box/firewall connection on the engine compartment side. (near brake reservoir ) I show voltage on both yellow and dark green wires when activating the turn signals. That tells me the harness is fine from there to the back of the truck. Or am I wrong about that?

Someone said to get a new turn signal switch. I have a new one and to test, I plugged it into the long connecter and flipped the cam on it to test the signals. I probed the wires at the connector. I did notice that the original one shows voltage at both yellow and dark green wires, the new one shows them correctly (on voltage on connector wire when correctly signaled.) I haven’t removed the steering wheel to swap the parts but even though it shows the correct voltage at its plug, the tail lights do the same thing.

I have noticed the brake lights (when they were working last week) go on and off. Someone said it is a bad switch that is above the brake pedal. Could that brake switch cause the tail light issues?

As it stands now, I have running lights, 1 brake light, and the one brake light flashes with either signal. I don’t know what to do next. I also just noticed that with the horn wire dis-connected at the steering wheel, when I flipped the turn signals on with key on, there is a buzzing noise and the dash gauges go crazy. (speedo flips back and forth and other gauges peg to max) I put the wire back on and it stops that.

I have no idea what to check next. Any ideas would be greatly appreciated. I have nothing else to drive and am at risk of getting hit because of the light issues.

[ChevLoRay]

My first thought is that you have a "floating" ground. You've done a lot of research and testing, but saying that your rear harness is good because you have the correct indications at the firewall, is not entirely correct. You have the signals you need at the firewall, but what you need to do is what the military called "half-split-half" in troubleshooting your problem. You simply find the next connector in that harness and check for the signals you need. If they are there, fine. Move further on towards the rear. If not, then go back half-way and check.

Just split the distance from the firewall to the rear of the vehicle, in half, and check to see what you have. Having another person to assist, would make this an easier task.

Hoppy makes a plug that inserts into the harness on the drivers' side, that enables trailer light hookup without cutting/splicing anything. If you had one of those, and found that connector on the rear portion of the drivers' side frame rail (inside), and hooked it up.....you'd also have a ready reference point...as well as a trouble-free connection.

Good luck.