Help! No brake lights!

[Kemo]

I was at TWS this weekend for a DE in my 1978 911SC and got black flagged on day 2 because I lost my brake lights. It was a real bummer. I checked the fuse, and it was good. The turn signals, hazard lights and running lights all work. Even the running light filaments in the brake bulbs work, just not the brakes...

I did some trouble shooting and came up with the following.

I have 3 wires in a redundant round connector that goes onto the brake master cylinder:
Black
Black/Yellow stripe
Green/Red stripe

With Key off and the plug pulled out of master cyl:
Black - no voltage and not ground continuity
Black/Yellow - ground continuity
Green/Red - no voltage no ground continuity

With Key ON and plugged still pulled out:
Black - no voltage no ground continuity
Black/Yellow - ground continuity
Green/Red - 12V hot

With the key on, I jumpered the Green/Red with the Black/Yellow and my brake lights came on.

Then I went to the switch on the master cylinder:

With the brake pressed I checked for continuity between the connector pins:
Black to Green/Red - no continuity
Black to Black/Yellow - no continuity
Green/Red to Black/Yellow shows continuity

so if the switch shows continuity between the pins, then the switch is connecting these two wires right? Im a bit confused as to why the lights wont come on and I'm confused as to why the black wire is even in there.

Any help here is greatly appreciated!

[Kemo]

ok...i have a plan. I'll be replacing the switches on the master cyl. one question tho, do I need to drain the brake system prior to installing the switches or can this be done as is?

thanks

[jpnovak]

Steve, I was going to suggest your brake MC switch. you should be able to change this without bleeding the system.

btw, the brake lights should only work in key-on position.

[JeremyD]

No you do not have to drain. partially pushing in the brake pedal prior to removing the switched will help keep the brake fluid in the lines.

"prime" the switches with a little bit of fluid.

common issue - not one of Porsche's greatest moments.

I keep spare switches and take to all DE (or used to) my new ride has a mechanical switch... woo woo.

[juanbenae]

Originally Posted by jpnovak

btw, the brake lights should only work in key-on position.

that one has gotten me at tech more than once.

[aston@ultrasw.c]

I had a prob with brake lights quitting on the track. I rebuilt the light housing 2 or 3 times carefully cleaning all connections etc. The brakes would light at home in the garage but as soon as I got to the track nada.

TCALSS, it was the bulb bayonet pins making intermittent contact. The retaining springs were old and tired allowing the bulb to move into an unhappy position. All I did was slide an o ring over the bulb which just became compressed when the bulb is installed. This holds the pins firmly against the ground detents.

[Kemo]

thanks for the rapid response. II guess I can wedge a 2x4 in there to hold the pedal down and make the switch swap. I think ill try Aston's o-ring idea JIC. The last time i replaced the bulbs, i noticed they were not super snug in there. If these switches dont last, i might go with something mechanical...i just replaced the master cylinder last summer.

[dsmithwc04]

Originally Posted by aston@ultrasw.c

I had a prob with brake lights quitting on the track. I rebuilt the light housing 2 or 3 times carefully cleaning all connections etc. The brakes would light at home in the garage but as soon as I got to the track nada.

TCALSS, it was the bulb bayonet pins making intermittent contact. The retaining springs were old and tired allowing the bulb to move into an unhappy position. All I did was slide an o ring over the bulb which just became compressed when the bulb is installed. This holds the pins firmly against the ground detents.

Thanks for this! My right brake/running light wasn't working but the turn signal worked fine. I had my wife hit the brake pedal while I pushed the bulb inward and it lit right up. Saved me a bunch of time for sure as I would probably have skipped something so easy as my issues are usually complicated ones

[Baby]

So was Kemo's solution to replace the switches?

My brake lights went out about 5 years ago. The dash light lit; that's how I was first alerted. I replaced the switches. My brake lights are out again, but this time no dash light. I tested the fuse and switches and bulbs. When I jump the green and black/yellow wires, the brake lights work. Does that mean the switches are stuck open?