Headlights make taillights go nuts

[Jrlovvorn]

All lights work as they should on my 79.

No interior light and no trunk light, but no biggie.

Turn signals all work correctly UNTIL I turn on the headlights.

With low or high beams on, the front lights are fine. Right blinks right, left blinks left.

The light show at the rear is very interesting.

Left turn signal lever position, I get both red taillights flashing together, and the right amber turn indicator flashing (in the offbeat from the red flashing), along with the rear right side marker.

Right turn signal lever position, I get both red taillights flashing together, and the left amber turn indicator flashing (in the offbeat from the red flashing), along with the rear left side marker.

Car was recently painted, and all lights removed.

Checked my rear connections and main connect to left unit looks good, connect from left unit to right looks good, and both connectors to the side markers look good.

Something is crossed, with interesting results.

Why it works fine without headlights, and why the front lights and signal markers are unaffected by the headlights being engaged, is a total mystery.

Don't know if I should start in the trunk, under the dash, the steering wheel, or just go with it and keep people guessing as to my next move

Wondering if anyone has ever seen this particular light show?

[DieselSpider]

Check the ground star on the right front fender. They may have left one lead unplugged or got too much paint on it. When I first got my 78 a few weeks ago I went though all sorts of weird light variations of the tailights until I noticed one lead had fallen off that star up front. Cleaned it, tightened the spade plugging it back in afterwards and that solved most of the issues. Note also that one of the nuts holding each of the tail light housings should have a ground lead on it that goes to that star up front so ensure that connection is clean and consider using a toothed washer on it. Before that turning on the headlights prevented the turn signals from working only on the passenger side. Still am only getting 11.5 volts to the tail lights even though up front the main line is reading 12.8.

Mine is going to be a bit of an enigma since it has an Isuzu Diesel engine and charging system along with some creative wiring. Will be chasing down and capping all the bare ended live wires they left in the engine bay as one of the first priorities.

[spider2081]

I recently resolved a similar problem. The cause was the front light socket was damaged and both the parking light and signal light terminals were touching in the socket. The operation of the front light appeared good because its hard to tell if both filaments in a dual filament bulb are working. The problem appeared to be in the rear circuit because there are separate bulbs for parking light and signal light.
Remove the front signal light bulbs one at a time. When I removed the right bulb all the lights worked correctly. Test them to be sure they work correctly. Inspect the sockets to be sure the internal bulb contacts can't short together when the bub is installed. It is possible for a filament in a dual filament bulb to break and short internally in the bulb and cause this problem also.

[Jrlovvorn]

Thanks! I will start at the right front ground star, as I recently had a dim right front headlight that was solved by cleaning the ground. Might have loosened something. Then will dig into bulbs. Thx.

[Jrlovvorn]

Diesel Spider takes the prize here.

I dug around under the left tailight, and found an unattached ground wire down in the well of the car.

Attached it to one of the left taillight bolts, and everything is resolved.

Did NOT find a similar wire for the right side, but I assume this year didnt have one.

Thanks everyone! Issue resolved, and thread is recorded now for anyone with disco taillight problem in the future....

[spider2081]

Great to hear its fixed.