911 Brake Switch failure mode
 

Just wondering if any one had diagnosed what is the failure mode of the brake switches used on power brake system 911's. I just had my two, German purchased, Porsche switches fail, seemingly at the same time.

There has been a lot of posts about this topic over time, but none that I could find which offered a solution as to why they are failing.

I took one of the old (failed) switches, cleaned it with laquer thinner and hooked it up to a test circuit. 90 psi of air pressure was not enough to get the test light to come on (even though you could hear/feel an audible click as the actuator apparently moved), but when I pressed on through the brake fluid orifice with a wire, the light came on with only moderate pressure....moderate finger pressure.

I am going to put this "fixed" switch back into service tomorrow, just wondering if anyone else had tried this and determined what in fact is the thing that causes the failures.

BTW, these are the 7 th and 8th failures in 20,000 km. I am using silicone brake fluid.

Thanks

Dennis

Dennis,

Were you using silicone brake fluid in your system for all of the eight switch failures???

With traditional glycol brake fluid use ... my assumption has always been that rubber wear particles and old, moisture-contaminated fluid were the gremlins that attacked switch internals.

If your failures were all in systems with silicone ... there must be something else happening!

I don't know why they fail. Theoretically the fluid is totally separated form the pressure switch contacts. Apparantly not. It seems that there are bad batches, if one switch fails quickly they all will, A different source will be needed, but even that won't guarantee longevity.

I put 9 brake switches in the '77 Targa I owned in about the same amount of years. Funny, it didn't start doing that until I put silicone fluid in. Finally, I went back to regular BF. Didn't help much, so I never did find out what the source of the problem was.

It was always the same switch, too. The other one seemed to be good for the life of the car. Anyway, putting the one in got the ligts working each time. So, the other must have been OK.

Maybe there's something I don't understand about the switches. That's entirely possible. :D Had the car 15 years and got 180,000 mi. out of it. No complaints.

Just an update, put the "old" switch back in the car, result....NFG....no brake lights. Big drag.

I can't imagine that the brake system pressure isn't high enough to move the actuating part.....considering I could get connectivity with moderate finger pressure. Very mysterious.

My guess is that the brake switches are like the window switches. The originals were good quality, up to Porsche standards, but the new ones are cheap parts. It seems the original brake switches and window switches last a long time, but as soon as you replace them the life expectancy goes from 10-20 years to a year tops.

Bill, you bring up a good point. I have been researching this lately too as I had my brake lights go out due to switch failure. What I have read indicates the fluid is separate from the switch, though I haven't removed and inspected one yet. I assume there is some sort of diaphragm that translates the fluid pressure to mechanical pressure on a switch. The failure mode then must be mechanical. Dirty fluid and particles should have no effect, unless perhaps it is the diaphragm itself that leaks and allows high pressure fluid to enter the mechanical portion of the switch. Maybe there is a way to rebuild the mechanical switch??

It seems to happen commonly after track days, so something concerning the high pressures seen during threshold braking or possibly the inflated temps helps the process along.