84 911 Euro Brake light issues

[cjczootv]

I just had my 911 in for a ton of service a couple months ago, almost 4k worth (trans, brakes, etc..). Car has been driving like a dream - except today, I was in the front trunk and popped off the fuse cover and took a look - and sure enough my brake light / cruse control fuse #7 was blown (ah, the reason why cruse stopped working - and I suppose my brake lights have not been working since then!). I replaced it, and the second I turned the car on, it blew again...

I started trying to isolate things by unplugging the lights, and checking the wiring for any lose connections or tape that could have fallen off, but so far I have gone through 10 fuses without any success. I also removed the cruse control from the equation by disconnecting it from the fuse panel and also removed the brake lights and tested the cruise which worked. So I am 80% sure it is somewhere in the brake light system, but that really does not explain why the fuse blows as soon as I turn the key...

Can anyone give me some pointers? I know two sets of eyes are better than one... My dad (who passed away, and who would have this fixed already) is laughing at me right now.

The good thing so far is that I have been able to clean the tail light housings - its was amazing the dirt in and around them...

Thanks,

-chris

[MattAlpha]

Chris,

It sounds like you've been able to count out the cruise section entirely.

From the symptoms, I'd say that you have a ground between your brake lights' wiring to the fuse box and your electrical ignition system. If I remember correctly, there is a wiring bundle that runs down the driver's side to the engine compartment. If the people who did the work had to remove this bundle (or messed with it) they may have cracked or cut some of the insulation off of the wires allowing them to ground directly to each other.

One way to check this is to remove all of the brake light bulbs (including your thrid brake light if you have one) and try to start your car with only one in. If that works, add another bulb (leaving the first one in place. Continue with this until you find out which wiring set has a ground on it. Once this has been identified, trace the wires to the fuse box being very careful to check for any exposed or frayed wires. If you trace the wires (major pain in the butt) you will eventually find the ground.

On a side note, if you do not have the drain tubes in the brake light housings, then the crud will build up a lot quicker.

Good luck,
Matt

[cjczootv]

Thanks Matt,

I spend some additional time yesterday trying to trace some wires - major pain in the butt is right. So far I have found a couple with broken insulation, but wrapping them in tape has not given me a problem yet.

Is there any other way to continue testing this without using up fuses?

-chris

[MattAlpha]

The other way is to use an ohmmeter to check the resistance of the wires. The only problem is that you have to check them from lead to lead.
So in other words, you'd have to have one probe for the volt meter in your brake light box (on that contact) and have the other end in the (proper) fuse terminal at the same time. A value of a few tenths of an ohm is normal for a wire that long, is the reading is infinity, then you know for sure that it is grounded out. The problem lays with the ability to have probe wires long enough to allow you to do such a thing. If you do have wires that long you would most likely need a helper that understood exactly what you were doing.
It sounds like you're attacking it the right way though. Repairing the cracks in the insulation is generally the best bet (even if it currently not the culprit). One place to check will be any areas where the wires pass through small opening in which an exposed wire can ground out to the chassis. Another is to do a trace of the wires running off of the fuse box. There are a lot of wires there in close proximity to each other and chaffing can be a problem.
Chasing these grounds are a bit daunting, but you'll learn a lot about your car along the way.
-Matt