Brake warning light mechanism?

For the second time in the year and a half I've owned my '80 SC, I've got a problem with my car which results in my brake warning light being permanently on. Last year about this time, my master cylinder failed while driving at about 110 MPH on the track. Out of the corner of my eye, I saw the pretty red brake warning light come on, and in an instant, my day at the track became very exciting, to say the least, but all turned out OK. I replaced the master cylinder, and things have been fine ever since.

A couple of weeks ago, about two days before I dropped my engine/tranny, the light came back on. I couldn't find any obvious reason for it, as the brakes felt normal. I reset the light by disconnecting the battery, but it came back. If I reset the light, and start the car, the light goes out as it should. But the instant I touch the brake pedal, it comes back on. I've bled the brakes very, very thoroughly a couple of times, and they feel OK. I replaced my oil pressure switch while the engine was out of the car. I can't believe the master cylinder is bad already (it was a new, not rebuilt, part). So I'm investigating the pressure switches in the side of the MC. Does anyone know what the exact mechanism is that causes them to indicate a failure? Is it a lack of pressure? A difference in pressure between the two? Something else? I've studied the wiring diagram a bit, but there's no way to know exactly how the logic works for latching the light. I'm about to head out into the garage to poke around, but I'd appreciate any insights anyone might have.

Thanks

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Greg Slater
1980 SC
gslater@uswest.net

[This message has been edited by gslater (edited 09-12-2000).]

I've got the same problem on my 83 but haven't had chance to investigate. However, I thought that the light only indicates a drop in fluid level, nothing more.

I also cannot figure out what's up with my break light. Mine just came on when I was heading heading home on Sunday with the gren OEL light at the same time.

I was only 1/2 mile from home, so I figured I would look into it then. But, on Monday morning, all was fine until about half way to work when both came on again?

Any ideas?

If it came on in conjunction with the green oil light, then the problem is almost certainly related to the oil pressure switch. The one that you can just barely reach, behind the engine, with a huge amount of patience. I've replaced mine a couple of times, and I'm pretty good at it now. $8.95 at my local Porsche dealership.

For some reason, the brake warning light circuitry is tied to this switch. You can see this in the wiring diagrams in the Haynes manual (911SC, 1981 models, diagram V).

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Greg Slater
1980 SC
gslater@uswest.net

Thanks Greg

I'll see if I can dig up an answer to your question.

Ted


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'80 911-SC Targa

One of the switches on the MC is pressure switch and is used for the rear brake lights, I think the other switch is activated by piston position. When master piston overtravel occurs because of low fluid level, or system leak the switch is activated. I don't think it's a fluid level switch.

[This message has been edited by leon (edited 09-13-2000).]

The switch on my car is in the resevior lid and is definitely for fluid level.

Cheers

Definitely no fluid level switch on my SC, just the two switches that screw right into the side of the body of the MC.

I can't picture the specifics of the MC internals, but it seems possible that the two switches might be for the two independent diagonal braking circuits, and that one of them has a problem of some sort. All I have to do is barely touch my foot to the brake pedal to trip the light. It definitely isn't caused by overtravel or a low fluid level.

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Greg Slater
1980 SC
gslater@uswest.net

Umm, maybe it's different on Euro cars?

Hello Greg , have you had any luck in fixing your problem ? This seems to affect SC's ( as mine is also an 80 911SC ) . I've rebuilt all calipers , new master and new parking brake contact to no avail still had to pull the bulb . Actually I rebuilt the whole car but it irks me to have to pull a warning light bulb . I see where the circuit is ( in the wiring diagrams ) connected to the oil press sending unit but don't understand how it would affect the brake system ? This a good thread but haven't seen an answer to our problem yet ??
Mike Rooney
80 911SC

Actually, guys, if you look at the following electrical diagrams, you will see that the two VW-style brake light switches are single-pole double-throw switches that operate the brake lights redundantly, but also provide a 12 Volt signal to the brake warning flasher when either one of the switches fails to operate ... which can happen even if the master cylinder is working properly, if water-contaminated brake fluid causes one of the VW-switches to rust internally and fail!


http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part3-1.jpg

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part5-2.jpg




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Warren Hall
1973 911S Targa

I think Greg hit the nail right on the head.
I bet if you replace your $5 oil pressure sender switch, you will have your problem solved. The thing is probably leaking, anyways! And, its an easy job.
(after you replace the switch, disconnect the battery to reset the system, too).

Yup, it's fixed now. In my case, it was replacing the two MC switches that fixed the problem (I had already replaced the oil pressure switch, but yes, that had caused the light about a year ago).

I mentioned my warning light problem to my local mechanic, and he asked if I had replaced the switches when I replaced the MC. I said I had not, and he suggested I should, since all the rust and such that I saw on the MC surfaces meant that the switches would likely be suspect as well. So I picked up two switches from him for $6 each. Went home, removed the old switches, and found that they were manufactured in late 1979. Put the new switches in, reset the light, and it has been fine since then.

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Greg Slater
1980 SC
gslater@uswest.net