nick49

Bad ground possible. You mention you are getting voltage, but possibly not enought current to light the bulb. Check the lamp sockets for corrosion, also make a test light by soldering a wire to the ground and another to the power lug on an 1141 or other single filament bulb. You can use this tool to check current for all kings of automotive applications. Voltage only tells part of the story, often current is more important.

countrysquire

OK, attaching a ground wire to the bulb housing gives me a bright light. Now to find the bad ground...

countrysquire

The bad grounds have been found and the problem is solved. The base of the bulbs were a little loose in the socket and would only pass enough current to light the small tail light filament, but not enough for the brake light filament. I replaced the GE 1077 bulbs with Sylvania 7528s, which are about 0.050" larger diameter. I had already cleaned the contacts, which didn't show any corrosion, so this had me trying to find another problem that didn't exist. Naturally, I discovered the source of the problem after pulling the interior quarter panel and carpet to check all of the connections there...

Thanks again for the advice.

redridge

Good Job.... you are now our guru for grounding brake lights issues.

You think that this would trigger the brake warning light.

countrysquire

Guru / dumb *** for not checking more thoroughly...

ntsc443

Great, that I have found this topic, I have exact the same problem with my 966C4S. Is there any chance you remember what exact location was this issue on your car? Is there grounding point under some panel in interior?
Thank you in advance, hope you still having fun with Porsche's

pdxmotorhead

I have had the 1157 style bulbs blow the filament at one end, then the filament drops and fuses to the tail element of the bulb, still has continuity, but no light..

in some cases it will cause the marker lights to flasher so faintly...

ntsc443

Thank you for suggestion. In my case, I have already checked bulbs. In addition I get ABS/PSM error and sometimes my center brake light is glowing when brake is not pressed, so it will be definitely bad grounding somewhere, and I hope someone can help me to point directly, where brake light grounding is located at interior.
I also replaced few brake switches hoping to find problem there, but it was not my luck.
And this is 996 Carrera 4S, but I believe other 996 has similar grounding point layout.

pdxmotorhead

Yep I was just sharing tiny issues that will have you screaming in the garage.. LOL

I find the bulbs with the oval contacts on the bottom , seem to play nicest with Bosche/German sockets..

I have not found a ground lug diagram for the 996, it used to be common on the old bug manuals since they were considered a annual service item.

Gixxed_Spyder

Hey there, were you able to find your grounding issue? I'm having a similar issue. Also, if measuring for Voltage and Current, what should be expected to see when measuring current? Thanks!