Fuse keeps blowing, lose turn signals, brake lights.

[dougcl]

Suddenly I'm finding an intermittant problem that causes fuse S11 to blow. I lose turn signals, brake lights, and parking brake light. I can't make the problem happen, but it seems to happen once per driving session. It seems to happen near the beginning of the drive. Once the fuse is replaced, everything is fine. Looking on the wiring diagram, the only device of significance on this circuit, besides switches and lights, is the turn signal flasher. Just wondering before I dig into this... is there a common failure mode here that I am missing? Does the flasher begin shorting out when it's due for replacement?

Thanks,
Doug
75 911S

[Jim Sims]

Fuse 11 also feeds the backup lights and the brake lights. Has the engine been out lately or has the pedal cluster been worked on? When the engine is pulled leaving the transmission behind the wires connecting to the back-up light switch in the front of the transmission are often forgotten and pulled apart (the circuit for this switch goes through the 14 pin connector at the back of the engine). The wires are often fixed after such a mishap but can develop a short at the repair splice. I know of one instance where there was just enough current leakage at such a splice that all would be fine backing up and then as soon as the brakes were applied the number 11 fuse would blow. The extra current going to the brake lights exceeded the capacity of the fuse. Disconnecting the green and yellow wire (at fuse 11) going to the backup light circuit eliminated the fuse blowing until the wire to the back-up switch could be repaired. Also, the backup light switch could be bad or the connector to it loose. Also, check the brake switch/light circuit (the green and blue wire) attached to fuse 11. One can remove these wires by loosening the little screws that clamp them in place. If the problem goes away after disconnecting the circuit you know the short is there. Cheers, Jim

[dougcl]

Hi Jim, the engine has not been out recently, nor has the pedal cluster been worked on. I didn't see the backup lights on the same circuit, but i could have missed them. I did think to test the backup lights though to cause the fuse to blow, but they work fine. You mention a good idea: disconnecting portions of the circuit to isolate the problem.

Doug
75 911S

[Jim Sims]

Doug, In the instance I'm recounting the backup lights also worked fine and did not blow the fuse by themselves when tested in the garage. However when used with the brake lights such as when backing up and applying the brakes the no. 11 fuse would blow. Disconnecting the green and yellow wire to the back-up light circuit at the fuse junction ended the fuse blowing. Cheers, Jim

[tmctguer]

i'm not an electrical expert by any means, but i've read some threads that suggest that you should remove bulbs on a circuit, then try to get the fuse to blow to that circuit. if it does not, then the bulbs or the connections they make are probably the culprit. if it does blow even without bulbs, wiring and/or relays are probably the culprit. at least it might help point you in the right direction.

next, as suggested by another poster, i'd try to isolate parts of the circuit and testing for which section causes the fuse to blow. perhaps removing the bulbs in a common circuit, and testing, then disconnecting that same circuit would help track down where the fault lies. also, try checking whether operating your hazard flashers causes the fuse to blow.

i had a case in my '73 911 E in which the backup lights were inoperable. repeated testing (and many blown fuses later) proved that there was a grounding problem with the small plastic housings and the metal of the rear engine cover. i makeshifted something to isolate the housings from the engine cover, and now they work fine.

i had another instance where i had disassembled the passenger side turn signal to fabricate an extension of the wires to the turn signal, and i re-installed it with the wires crossed. i looked for that one for a LONG time before i figured it out.

my comments are more toward the fact that pesky electrical problems are often caused by very minor things that are well hidden. time and patience is what have helped me find mine, even with a limited amount of electrical knowledge.

i don't know if these will apply in your situation, but i thought i would offer them up as a suggestions.

[dougcl]

Removing bulbs is an excellent idea. Also, checking the hazard flasher is a good idea that I have yet to try.

Doug
75 911S

[autobonrun]

Although the problem could be in any of the circuits that are fed from the fuse. The fact that it happens at the start of a driving session could mean it's your brake circuit. If you're talking about a track driving session, I'm assuming you did not necessarily use your turn signal or parking brake.

If that is the case, look for a frayed wire or one with torn insulation on the hot wire that feeds your brake lights. Check particularly where it passes by a screw or other metal piece that could cause an intermittent short. There is a brake light switch in that circuit that also could be a problem.

It is unusual, if it is a load problem, that once you replace the fuse it doesn't happen again immediately. This leads me to think intermittent short on the hot circuit.

Unfortunately, there is a lot on this circuit so as mentioned earlier, start by isolating loads. Start with isolating the loads you were not using during your driving session then see if applying brakes blows the fuse. I'd next check the turnsignal circuit. Get someone to activate the brake while you wiggle the wire to see if the short is there. I've usually found the problem near a termination point rather than along the wire so also look for corrosion near the bulb or anwhere the hot wire is terminated in a water environment.

Tracing intermittent problems sucks but just take it one circuit at a time. With my luck, its always the last circuit I pick to check.