Tail Light Wiring Issues on '95 Jetta TD

[Kneale Brownson]

Chasing down the whatever causing my weird taillight performance:



I've cleaned up all the trunk grounds that a couple years ago caused a similar symptom with the tail lights without getting the solution I got then. 



I've determined there is continuity between ground and the contacts of the bulb holders where the power goes to the bulb, as well as between the ground on the bulb holder and the power contacts.



What happens:



With the brake pedal depressed, both filaments of the tail light bulb light up, along with the bulb in the yellow turnsignal section at the rear and the yellow lights in the front of the bumper.



With the pedal still depressed and the light switch activated,  both tail/brake bulb filaments stay on, but the rear yellow goes off and the front side marker comes on.



With the pedal still depressed and the key and lite switch on, when using either flasher activated, both tail/brake light filaments work and the yellows flash front and rear.  I get a faint glowing of the green arrows on the instrument cluster between flasher uses.



With the brake pedal released, and key and lite switch on, rear brake/tail lights have just one filament lighted and rear yellows go off.   Dash arrows go off too.



With the pedal reseased, the key off and the lite switch on,  still one filament in the brake/tail bulb and front yellow lites.



Any suggestions to where the continuity between the grounds and the power contacts of the bulb holders comes from?



I do have a trailer lights setup with its own power supply installed.  It was installed after solving the grounds issue a couple years ago and worked fine with the trailer until the current issue developed.

[Doug]

You have a high resistance ground connection in the rear lamps or ground return. Where, I cannot begin to tell. Brake lamp current is finding its way to ground through the other lamp circuits all the way back to the instrument panel lighting up the turn direction indicators. You can waste alot of time tracking down the poor connection or rotted wire. I would suggest that you make up a separate ground bus for all the rear lamps. Then attach that to your battery ground point usually common to the engine with a piece of number 10 copper wire with crimped connections at the ends running it through the cabin or underneath the body against the frame.

[Quantum TD]

MK3 rear bulb-holders are notoriously crap. Replace them both, especially the driver's side one. They simply unplug from the harness. I'm banking that the ground section of your bulb-holder is burned out at the plug.

[Kneale Brownson]

So, Doug, you're saying I have a "weak" ground and possibly could solve this issue with a "stronger" direct ground wire, like a 10-gauge, directly from the ground of the battery?

Mk3, I have continuity between ground and the wires in the power feed line to the left bulb housing.  I have continuity while testing each of the wires with the connector unplugged from the bulb holder except for the red/black wire to the #2 position in the connector.  So the issue is in the wiring, not the bulb holder itself.

[Doug]

Kneale, If you are checking your continuity with a low current device like a VOM then you may not see a high resistance connection. You need to use a higher current device like a test lamp with its own power supply. Relying on the chassis to provide an adequate ground may be fine when new but after years of rust and corrosion especially if the ground wire back to the instrument panel has begun to fail then you may be in for problems. That is why I suggest an independent ground wire if you have identified the ground wire as the problem area. Chasing through corroded wire can chew up a lot of time.

[Vincent Waldon]

And again, just so we're clear and because you don't say so specifically... you've removed every single turn signal bulb, on both sides, before starting your continuity tests on the wiring, right?

[Kneale Brownson]

I had not pulled the front bulbs, just the wiring harness connector out of the driver tail light.

When I pulled the four front bulbs (turn and marker), depressing the brake pedal gave me a rear brake lite.  Turn on the lights or turn on the key and depressing the brake pedal gave me no lights at the rear.  With light switch on and pedal released, I get tail lights.

Green arrow lights show up when depressing the brake pedal.  Turn signals don't work with no front bulbs.  Hazard yellows flash the brake lites rapidly with no front bulbs, do not flash with the brake pedal depressed.

I tried connecting a 10gauge wire directly from the battery ground to the tail light fixture, with no difference in results.

[Vincent Waldon]

I just noticed your first post mentions a trailer light conversion kit... most definately I'd recommend temporarily disconnecting it as well while you troubleshoot... when they go bad they often cause weird communication between lighting circuits... in a way, that's their job. 

Just fixed my bro's van because of this exact issue.

[Kneale Brownson]

I have eliminated the trailer hookup with no change.

[Kneale Brownson]

Further problem:

I started taking the connectors out of the back of the fuse panel thinking I'd take it out of the car and examine for signs of a short, and realized half way through that I might not be able to get them back in properly, so I reversed direction. 

I have all the multiple wire connectors back in place, but I'm left with several single-wire connectors I have no clue as to where they go.

I have a really heavy (maybe metric 6) red wire with a red connector, a smaller red wire with a red connector, a heavy (maybe metric 2.5) red/white wire with yellow connector, a smaller red/white wire with red connector and a fine red wire with red connector.
 
These don't seem to relate connector-color-wise to the schematic (Fig. 5) on page 97-4 and list of connections on 97-5 of  the Bentley.  Can anyone tell me which wire goes where on that schematic?