B7 fuse keeps getting blown

Hello Everyone, I have 1999 996 Tiptronic Cabriolet. I've come across a problem recently and would like your collective wisdom. The problem is that B7 (15A - Stoplight, Cruise Control) fuse keeps getting blown. This prevents the car being able to shift into gear. This problem recently started happening. I did have a the car wrapped recently and the service tech had to remove the brake lights to finish the wrap. I originally assumed maybe he didn't put them back correctly or maybe the wires were being pinched in some manner. I had my local mechanic look at brake lights and he mentioned he didn't see anything out of the ordinary. The pattern I was able to observe is that, this issue occurs whenever the lights are on (or at night). This might have been why the mechanic didn't see the issue. I just happened to notice this pattern recently Any guidance on where I should look. Cheers.

There is a short in the harness. No easy way to find it but to meter the wire and start pealing back the wrap. My guess would be an issue near the tail lights. Unplug one side and then the other to see if u can isolate which side is the issue. Also pull the housings and inspect all the bulb sockets for chaffed wires.

I had a problem with that fuse, and it turned out to be the contact at the bottom of one of the bulb sockets had broken off causing the short.

Is it B7 fuse's birthday?

Rear center brakelight switch. The question I have now is.. what kind of monster leaves electrical wire like that.

wow.

A mouse or some other type of rodent. They love to eat the wire insulation. I would check your entire car and engine bay for more damage.

Follow up electrical question, Does anyone know for sure.. If the rear break light or any light is disconnected.. is there still voltage that is sent to the wire. In other words.. if i've disconnected the read break light but there is still a short.. will it blow a fuse only when the completes the circuit with the break light attached or regardless of light attached if there is a short it will blow a fuse.

Quotes source post: 17728217

Possibly. It depends on how it's shorted. No electrons would flow if the circuit is not completed, but they could be shorted out to each other, or the chassis, Which means you could disconnect the light, but still have the same problem. the portion of the harness that I can see in the pic above needs repaired. I do electronics so I would be cutting out the exposed copper and replacing it with new wires of a similar color. Electrical tape could be used to try to repair it. Not a great fix, but could help you solve the problem. You could also try to purchase a part of the wiring harness, cut it out, and splice in the used one. Hang in there!

In my experience with Porsche wiring (and other cars) the 12V is always hot (meaning it is always provided a 12V supply) and the circuit is completed by providing a ground. So for a brake light, the 12V positive is always there, and the brake light switch simply provides a ground. Does that make any sense? Well, it is the way it works.

Quotes source post: 17728369

Yep, and would be true for any circuit

Have you checked the brake light switch itself - by the pedal?

I agree with the statements above in that they generally hold true but in case of the brake light switch I looked it up in the manual and found the opposite. Unfortunately I don't have a wiring diagram only below schematisch of the ABS (TC) 5.3 system and that shows the brake light switch switches +12V. What you observe is proof (ETA if all brake light bulbs are removed!) that the wire that connects the brake light switch to any of the bulb holders (in each rear light and both center brake lights) itself shorts to ground somewhere along its length as soon as you step on the brake pedal. Do check the wiring to the center brake light in the retractable spoiler as well as in both rear light clusters. Brake light switch connects brake lights to +12V, not to ground. Found the wiring diagram. Search the internet for the file name and you get to download the full set. Diagram confirms the above as '15' denotes terminal 15 which is ' +12V when ignition is on' in (allmost) all German make passenger cars.

Would anyone know the part # for the frayed wire mentioned above?

It's not available separately therefore has no separate part number. Don't think you want to replace the whole loom where the shorted wire is part of. You mentioned 'whenever the lights are on'. Which lights do you mean? Any other than the brake lights means your mechanic has to check out the wiring to those lights as well. Going out on a limb here but it sounds like with lights switched on (turning the light switch) and the applying the brakes that the brake light circuit is also powering some other (rear) lights and the added current drawn blows the fuse. Do all (rear) lights work when you turn the light switch to all its respective positions? Since this fuse also protects the cruise control circuit then that should be checked as well (on the premise your car has it)

For posterity: Part# 996-612-507-02

@Snyper Thanks for the thread, helped me diagnose my center brakelight issue. Question - was the part number you listed above the center brakelight "side" of the harness? in my situation, the insulation has stripped from the wires right next to the plug, causing a short. fixed with some electrical tape for now but looking to splice in a more permanent fix