How I adjusted the mechanical brake light switch on '74 - 911

[Larrybrook]

I was always concerned that my brake light didn't come on until fully dressed. So when I rebuilt my pedal cluster I installed a new mechanical brake switch hoping that it would solve the problem (40 year old switch?) I completed the rebuild put it all back together and there was no real change to the brake lights. So I too everything out and tried to find if I was missing something, or needed to Mcgyver a solution.

Here's what the pedal cluster should look like:


When I adjust the nut for the brake switch, the large washer becomes very floppy. So I added an extra nut in order to 'sandwich' the larger washer along the threaded shaft

Here's what I did:


I reinstalled everything and made some final adjustments, now the brake lights come on with minimal effort. I don't know if all mechanical switches take such long travel to close the circuit, but this was my scenario/solution and I am very happy with the results.

[ossiblue]

If you look on the body of the switch, you will see a small slot head screw with a jamb nut. This is the adjustment for the internal contact of the switch. If you loosen the jamb nut and turn the screw in, the switch will make contact with a shorter travel of the pedal. If you look at the screw on your new switch, you can see how far out it is set compared to the one in the top picture.

What you did was to move the washer that holds the switch arm, a bit forward which is basically like pushing the pedal forward. In your case, in worked because the adjusted length of the MC pushrod left a gap that fit the extra nut.

[Larrybrook]

I guess my method worked, I'll leave it for now because it does the job. Next time I get to the pedal cluster I'll take your advice and adjust things as Porsche meant it to be. I wish I knew this before I spent hours trying to figure out the problem. Thanks for the info, anyone else faced with this same problem can now use this information to make their lives easier. I wonder if the previous owner meddled back there to cause the brake lights to take so long to come on.

Oh well, live and learn,
Cheers!

[Jay Laifman]

Ha. Thank you Larry for saving me the time!

I just noticed when looking in my rearview mirror at a stop light at a chrome grille of the truck behind me, that my brake lights were not on unless I pressed a bit firmer that I do at rest. I guess I don't know for sure where that is in the driving braking pressure. I always want the lights on as soon as possible - of course to warn the idiots behind me. So I think I'll be doing some adjusting.

[Koizumi]

Interesting, many years ago I did a similar modification. It didn’t really work that well. I guess I didn’t pay attention to the slotted screw possibly being an adjustment screw. I will revisit this since most cars on the road these days are behemoths and visibility is an issue. Having a tiny black car doesn’t help either.

[blucille]

Best practice is to remove the small slotted screw completely and clean the tip. There will likely be some oxidation on the tip of the screw and the contact inside the switch. I’ve also seen some dirt that kept the switch from functioning properly.

This is a super easy and quick process. I’ve done it for a few folks during rallies when they had no functioning brake lights.

[Hi_Fi_Guy]

All you need to know about the brake light switch but were afraid to ask
https://forums.pelicanparts.com/porsche-911-technical-forum/1061085-stupid-brake-light-question.html#post10867986