esseight

So I've been trying to sort some issues on my '92 coupe, and have come to electrical, specifically lighting.

My brake lights work fine when all other exterior lights are off, but as soon as I turn on the parking lights or headlights, the brake lights illuminate as well (including the 3rd light under the spoiler), and at full brightness so there's no way to tell the car is braking. This does not happen if the turn signals are used or when the reverse lights are on.

I've been trying to make sense of the wiring diagram, but am no electrical engineer and can't seem to figure out how to trace the problem. Anyone had this happen, know what the fix is, or can point me in the right direction to troubleshoot?

Thanks!

968workaholic

Unplug each rear tail light one at a time to see if one of them is causing the problem

esseight

Thanks, already tried that, but with no luck. Center-mounted brake light wouldn't come on unless there was a wiring issue. Going to do a deeper dive tomorrow.

968workaholic

How about an aftermarket alarm system? Do you have one installed, or is there any evidence of one ever being installed?

thomasmryan

Probably a bad dual filament bulb or some dissimilar metal action going on in the outboard brake/parking socket. Generally, the sockets are marked but most German cars use a 4/21 watt bulb.

esseight

No aftermarket alarm, stereo or led's. Going to change out all rear bulbs tomorrow. Then it's on to checking the main lights circuit against the brake lights circuit. Seems somehow the brake lights are no longer wired independently.

Eric_k

I picked up a 968 that had this issue. For my car, thomasmryan was correct. One dual filament bulb in the passenger taillight had been incorrectly replaced by a single filament bulb. With the lights on, power was being back fed into the brake circuit so the brake lights lit (including the 3rd light) when the headlights were turned on. I put the correct bulb in and every thing was back to normal.

culture

bad/incorrect bulb was going to be my guess as well