Lighting/ turn signals

This is one of the classic symptoms of a bad ground wire. The first clue is if you look closely at the two brake lights, you'll see the left one isn't as bright as the right one. The best clue is to leave the running lights off, turn on the left turn signal, then, while it's flashing, remove the right brake / signal bulb from its socket. If the left one stops flashing, it has a bad ground wire. Current for the left signal light can't find its path to ground, but it finds an alternate path by doubling back through the tail light filament in the same bulb, then over to the interconnected right tail light, then to ground on the right side. Removing the right bulb removes that alternate ground path, so the left bulb goes off.

To test Caradiodoc's theory.

Remove your brake light red lenses.

Attach a wire (or long wire) to the frame or even better, to the negative battery terminal. Maybe with vise-grip pliers.

In a pinch, I have chopped up a cheap extension cord to make test wires. Many times in the past to figure out bad trailer wiring!

Next, activate the signal and your lights.

While it is "screwing up", touch the other end of the wire from the negative battery terminal to "body" or metal "bulb holder", thus giving the holder an uninterrupted ground. This test may show that you need to check/ repair the original grounds to the light fixture.

Keep us informed!

The Medic

Good luck!

Waiting for good news!

The Medic

What I'd try next, since this isn't a factory harness, is to remove the left rear bulb from its socket, then use a digital voltmeter to take three readings on the socket. Turn on just the running lights, then read the voltages on the two terminals in the socket, then on the brass housing. One terminal should have full 12 volts. There must be 0 volts on the other terminal and on the housing.

Next, turn the running lights off, and use a stick from the seat to the brake pedal, or a helper, to hold the brake pedal down. Now you should find 0 volts where you had 12 volts in the last step. There must be 12 volts on the other terminal where you had 0 volts before, and there must still be 0 volts on the housing.

If you ever find something other than 0 volts on the housing, at least two wires are switched somewhere. If you find 12 volts on one terminal, and something other than 0 volts on the other one at the same time, we have to figure out where that second voltage is coming from. Typically that would be from a bad ground on the other side, and the voltage is back-feeding from the turned-on circuit onto the turned-off circuit.

So you did try the temporary ground touched to the bulb holder other end to a good ground?

Here's the only thing I see that could be wrong:

The front turn signal/ park lights and the rear turn signals and rear lights all share a one each bulb.

The turn signals and brake light all use the "brighter" filament within the bulb. Thus the brighter filament is controlled by the turn signal switch and the brake switch only/ half of the bulb I used this way.

Same bulb, when you pull the light switch to on, the dimmer element is now on (running lights and park lights) this is the other half of the bulb.

The only thing common to these four bulbs is the ground.

We have one more light issue to consider, that would be the side markers. They are a one filament bulb. They work with the light switch as well as blink with the turn signals.

I think that it's not a ground, somebody crossed up two of the wires.

I'm going to chop up a diagram to only show the lighting. I will also provide you the whole diagram as well.

I scanned and colored this diagram the best I could.

Tracer stripe colors on wires were not labeled with a color, so I used the base color that was listed. I "dashed" these tracer wires to denote they were tracers. You will be able to pick out the real wire and it's stripe color as you dig into your wires. Sorry! The only way for me to have gotten the tracers colors would have been to take my whole Jeep apart and look! White wires are drawn with black lines!

Here they are. I may have missed something. You can double check me with the original diagrams. Even though this is a 1979 diagram I think the lighting is the same through the nearby years.

The Medic

Dandy. Happy to hear you solved it. This kind of problem is hard to diagnose when the wiring is not original.

Glad you figured it out!

I get all wound up and tunnel visioned with confusing issues.

Gets me all flustered and I can't "see past" what's in front of me.

Best to keep an open mind, approach the issue from the other end (whatever it is), and walk away for a while to clear your mind.

If you get caught talking to yourself, explain that you are listening to expert advice.

The Medic