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Discussion Starter #1
Has anyone ever heard of the following scenario?

I have a '94 Spider and 10-20% of the time everythings great, the other 80-90% this is what happens:
1. WHEN HEADLIGHTS ARE OFF - turn signals work only when brakes are NOT being applied;as soon as brake lights are activated turn signal's don't flash. Brake lights work fine when headlights are off.
2. WHEN HEADLIGHTS ARE ON - turn signals don't work (brakes or not) and, when brakes are appled tail-lights turn off completely and only the high center brake light activates.

In either scenario there isn't any shorting or dimming of the lights - they either work fine or most of the time don't. When they are working it's for a stretch of time - like hours. The bulbs, fuses and relays seem to be in order. The car has never seen rain or snow so there doesn't seem to be any corrosion of electrical components anywhere - it only has 18,000 miles.

Any ideas would be greatly appreciated

Thanks!

Glen
 

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Trained (ex)Professional, , 1953-2018 RIP,
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Welcome aboard, Glen!
Sounds like the classic 'bad ground'. I'd clean every ground connection you can find starting in the trunk.
 

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Discussion Starter #3
Thanks Jim

I was afraid it was that simple (or difficult depending on how you look at it). Now, if only I could figure out how to use the multimeter :)

BTW, are there any specific grounds on the late model series 4 (I guess the '94 is just a retitled '93 production) that I should look for as being typical to go bad or is everything fair game?

Thanks

Glen
 

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Not being at all familiar with late model Spiders, I can't say it's this connector or that connection. But I'm sure someone else can.
But since both tail lights are affected, I'd look for something common to both. Like a multi-pin connector or a bunch of ground wires screwed to the body somewhere in the trunk. For the connectors, I use a run-of-the-mill electrical contact cleaner. Then use enough dielectric grease in the plug so some squishes out when assembled. The grease prevents moisture penetration virtually eliminating future corrosion. The ring terminals at the ground connections can be dressed with steel wool or sandpaper.
 

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Did you recently change bulbs? If so, are you sure that you put the correct type bulb back in?
 

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Discussion Starter #6
Thanks for the responses and tips so far guys.
It's hard to imagine it being a bulb problem though because the problem appeared before I tried changing a few bulbs. Also, the brake lights ALWAYS work when the headlights/parking lights are off and the turn signals ALWAYS work when the brakes are not being applied. No difference between left or right and when the turn signals aren't working the relay isn't clicking. These problems don't appear at separate times - either everything is working fine or all the problems are exhibited - so I guess it makes sense that it must be a common problem like a ground. The thing that freaks me out (though I'm terrible at electrical diagnosis) is why the driving tailights would turn OFF when the brake is applied and turn back on when the brake is released.

I'm gonna try the methodical examination and cleaning of the grounds. I hope there's no chance that a hidden wire could be worn through somewhere and grounding to the body or a component.
I wish I had more time to putz around in the garage with my car - this 4 hours of daily Chicago area commutting is a killer.

I'll let you know if I have any luck

Thanks,
Glen

Oh I forgot to mention the battery has gone dead if the car sits for 2 weeks or more - even a new battery - that spells "short" somewhere, doesn't it?
 

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You've got a short somewhere. Put an sensitive ammeter tester on the battery and see if you're getting a low grade drain. To trace, check your main battery cable's integrity. If that look good, and the drain is still there, start taking out fuses one at a time and see if you can isolate the drain to a particular circuit.

As far as the tail light problems go, I think you have no choice but to chase wires. Start by removing the rubber covers in the trunk and visually check the integrity of the wiring.
 

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When I purchased my 93 S4 I had a similar problem with the tail lights.

Before I broke out the surgical gear, I took a couple of cans of electrical contact cleaner and starting with the tail lights, I dismantled every light assembly, sprayed, cleaned, and re-fitted the assemblies.

Also, bad bulbs ... as in all 4 trailing lights ... were replaced. Every bulb, ground, & connector that I could find I did this. Including the engine compartment connections.

After that the back-ups lights began to work as did the 3rd brake light. I can say that every light now works correctly. A long afternoon of work so one might need good music, a selection of cold beverages, and patience.
 

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Discussion Starter #9
My thanks to everyone who took the time to evaluate my problem. I was just about ready to go after the combination switch unit on the steering column, thinking that's all that's left when I suddenly noticed the interior trunk light flashing on and off with the hazard lights. The problem has to be in back! Even though I thought everything was OK back there I removed the trunk liner again and upon further inspection noticed that bolt which grounds the tailight harness to the left rear fender, even though tight, was separated enough from the fender to allow the tailights to occasionally lose their ground. All I had to do was tighten the bolt further and all is well.
 
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