74 GMC Tailight problems

I have a problem with my taillights on my 74 GMC truck. The LH taillight turn signal and brake light don't function properly. With the lights on the taillight is dime, when I put on the turn signal it goes out or blinks even weaker.

When I apply the brake the whole light goes out. The right taillight works just fine.

What could be causing this? Any info on where to start would help.


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Check the ground wire from the socket to the sheet metal of the box.

I tried the ground near the LH taillight and it seemed to make my turn signal work better, but my running lights in the rear don't seem to work. Brake lights work. If I tap the taillights they come on very dim. What could this be?

Check the brown wire first.
It could be corroded.

I would find the harness connector under the truck about center.
I've seen corrosion and loose connections there.
After you open the harness you can check for continuity in the wires from your lamp socket to the harness connection point.

You can also probe with a test light to see if power is getting that far.

pugsy is right you more than likely have corrosion, go to radio shack, or a hardware store and get some electrical cleaner, or electronic cleaner. Spray down that connector, You can spray any connection with it. I always have a can with me up in this country, they put salt down by the ton. every time I hook my boat trailer up, I spray both connectors good.

Bob

PS After posting I see where you all are from, I'm like in Texas to you guys.

The copper wires start to turn all cruddy sometimes even inside the plastic insulation on the wire.
The salt gets everywhere.

Brown wire? Would that be in the rear about center under bumper? There is a random brown wire that is cut off going nowhere. It sticks out of the harness about 8-10 inches away from the harness connector. What is it for? could that be causing my problems?

Let me try to explain.

There are 4 wires coming from the firewall connector to the back of the truck.

I believe the brown wire is for the running (tail lights), the light green is for the reverse lights, the dk green and yellow are for left and right signal lights.(forget which is which).

Do you have a test light?
You can't do any electrical diagnosis w/o one. They're cheap and worth their weight in gold.

You take the ground from the test light and hook it to ground.
Take off all the tape from the harness connector at center and seperate the connection.
Turn on the running lights and check the pins in the connector (coming from the front of the truck side) to see if you have power at least to this point. I would do it for signal lights also. (just turn on the 4 way flashers for this test)
You can also check reverse light power with the engine off and wheels blocked).

This will tell you where your problems lie.
I've had only one truck that had issues forward of this connector. All others have been due to corroded wires or the lamp bulb sockets rotted out.

I always use dialectric grease on my bulbs when installing them to help the sockets from corroding.

For the loose hanging wire, follow it, untape where it joins the rest of the wires and you may find a loose connection there. This is common when hacks hook up trailer lights and stuff. They connect the wires by twisting them together and taping them up. The connection will always fail eventually.

Whoops, first check if that loose wire is supposed to go to your licence plate light. Is that working now?

Also one more thing, do you have a test light?

There is one ground at the taillight, and there is another ground in the center of the wiring harness that runs across under the back of the bed. It should be a fairly large gauge white wire, and its probably grounded to the frame rail on the left side. You need to clean up the frame area and make sure it is grounded well.

There is also a junction in that rear wiring harness that pops apart. In my truck it was used to insert a Hoppy wiring adapter for my trailer lights. Just pop it apart and make sure all the wires are making a good connection.

You also need to check the instrument cluster ground. There is a black wire that is grounded to a post on the upper part of the emergency brake mechanism. If this ground is off you usually have a multitude of problems with gauges and lights; however it seems to affect turn signals more than running lights. It is very important to ensure that ground is connected before you spend time with other troubleshooting of the taillights.

Bruce

Go here: http://www.crankshaftcoalition.com/wiki/Wiring_diagrams

this should help you find your solution.

Thanks for all the info guys. Should help alot