brake warning light not cooperating

[john walker's workshop]

usually it resets by turning the key on, disconnecting and reconnecting the + battery clamp and then starting the engine.
did that many times, along with the - clamp.

sometimes the brake light switches need replacing. did two sets.

sometimes it's a bad connection at the warning light plug, but very rarely. checked that.

82 does not have pad sensors or reservoir sensor switch.

it's not over-charging.

the brake lights work and are not corroded.

the plugs on the switches are clean and tight.

one wierd thing i noticed, i pressed the pedal down with a rod, key on, battery clamp on/off, started car and the light went out, but when the pedal was released, it came back on. wierd.

so i've had a few over the years that wouldn't reset regardless of any amount of hair pulled from my head. i have an 82 right now that's being difficult.

the circuit board on the light has wires to the oil light switch and the seat belt buzzer, which is gone.

has anyone found some obscure thing that finally turned the light off? maybe the circuit board in the light is wacky. nope, tried another one. about the only thing left is the master cylinder, although the braking feels fine.

[MichaelN]

I have an '81, my first Porsche. After more than a year of ownership the brake warning light came on and stayed on. Being new to Porsche, and given the age of the car, I just presumed it was a simple problem with the switch on the hand brake. Well...no. Then I checked the owner's manual and discovered the function of the light was quite a bit more sophisticated than I had anticipated so I started digging deeper. At this point it occurred to me that just after I got the car I was surprised and somewhat disappointed that the front brakes seemed to lock up somewhat quicker than the rears during "panic" stops. I had always read how Porsches had such great brakes (I even posted on this board asking those more experienced if this was the normal bias for all SCs). I finally decided that it was most likely the over-sized rear tires the P.O. had installed. To get to the point, after remembering my earlier puzzle over the brakes I changed out the master cylinder and the warning light went out immediately. And of course the front/rear bias evened out as well.

I realize this is not the most sophisticated analysis, but I offer it up because during this whole period of time the brakes felt (pedal hardness, travel, etc) completely normal; it was only during extremely hard stops (and I DO mean extreme, like in autocrossing) was I aware that the front brakes locked first. And the rear brakes never stopped functioning (I checked them several times) and it was only the warning light that alerted me that there was a problem.

[timmy2]

Feeds to Brake warning light :

Brake Lamp Bulbs or Master Cylinder switch failures.(Tied together)

Hand Brake Switch. Not grounded

Oil Pressure Switch.

Any one of these not in it's normal state will make the lamp light up.

Maybe lift the wire at terminal HB and see if it works? wire touching ground somewhere?
Needs to see oil pressure switch?

[DRACO A5OG]

+1 hand brake, it does come loose sometimes

[john walker's workshop]

disconnected the wire to the oil light.

the hand brake light and switch functions because you can see it getting a little brighter as the lever is moved up and down. this is normal.

can't be the master cylinder causing the light to not reset at all. no pressure required.

[group911@aol.co]

This is a wild shot. On some of the early systems with dual stage master cylinders, if the pedal had gone down far enough at some point to turn the warning light on, they had to be reset.
The tricky part about the reset is you have to push the pedal down to the point where it is past the first stage but not into the second. You can feel where the pedal stops but isn't all the way down.
They were tricky because you have to crack a line or bleeder and it takes quite a bit of pressure to get to that point and once you are there, it's easy to go past and turn it back on.

[wwest]

Originally Posted by john walker's workshop

usually it resets by turning the key on, disconnecting and reconnecting the + battery clamp and then starting the engine.
did that many times, along with the - clamp.

sometimes the brake light switches need replacing. did two sets.

sometimes it's a bad connection at the warning light plug, but very rarely. checked that.

82 does not have pad sensors or reservoir sensor switch.

it's not over-charging.

the brake lights work and are not corroded.

the plugs on the switches are clean and tight.

one wierd thing i noticed, i pressed the pedal down with a rod, key on, battery clamp on/off, started car and the light went out, but when the pedal was released, it came back on. wierd.

so i've had a few over the years that wouldn't reset regardless of any amount of hair pulled from my head. i have an 82 right now that's being difficult.

the circuit board on the light has wires to the oil light switch and the seat belt buzzer, which is gone.

has anyone found some obscure thing that finally turned the light off? maybe the circuit board in the light is wacky. nope, tried another one. about the only thing left is the master cylinder, although the braking feels fine.

The battery is a really good, STELLAR "sink" for momentary voltage excursions, spikes. A few hundred milliohms of resistance from the battery all the way back to the engine compartment will also contribute to the problem.

Check the voltage in the engine compartment to see how elevated it is in comparison to the battery terminal voltage.

If you have access to an O'scope...

A shorted stator phase winding, or intermittently so, can result in the two remaining phases outputing excessive voltage in order to keep the average correct.

[86 911 Targa]

Just a thought.

Have you tried re-flowing the solder joints on the P.C. board?

Also, I removed the seat belt buzzer years ago, which has had no effect on the indicator lights.





Good luck,

Gerry

[timmy2]

Disconnect and bypass the MC switches with a jumper, disconnect battery for a reset and see if it goes away with the jumper in place.

[john walker's workshop]

ok, got back on it today. got the ohmmeter out and compared open circuits and resistances between the 4 new switches i tried. they were all different. there is supposed to be an open circuit between the two outer terminals and upon getting pressure, they close. i got about 3milli-ohms on 3 of the 4 switches. between the center terminal that goes to the light and the side terminals, some had resistance to lets say a right terminal and some had resistance to the left one. damn off-shore crap. so i went through all my saved master/booster units and found two that ohmed out the same. fixed! ya just can't assume anything these days.

[RWebb]

Jeez Louise, John! what a mess - glad you finally got it & hope your customer is properly appreciative