67 911S - brake lights always on

[jimhuiz]

So I'm waking my car from a slumber and in checking the lights, everything works except the brake lights are always on. Car is a 66 built 67S. First I've noticed the problem but it may have been there all along.

Thoughts on where to check first? Seems like the wires must be shorting out somewhere between the fuse box and the pressure sensor at the master cylinder.

Haven't gotten under the car yet but thought I'd ask.

Thanks!

Jim H.

[mac73s]

So I'm waking my car from a slumber and in checking the lights, everything works except the brake lights are always on. Car is a 66 built 67S. First I've noticed the problem but it may have been there all along.

Thoughts on where to check first? Seems like the wires must be shorting out somewhere between the fuse box and the pressure sensor at the master cylinder.

Haven't gotten under the car yet but thought I'd ask.

Thanks!

Jim H.

If there is a contact switch at the brake pedal - as with my 70 and 73 - I would start there by disconnecting one of the wires and see if the lamps go out. Again, if there is a pedal box mounted switch, make sure the pedal has not moved toward the floor during its slumber and therefore engaged the switch. If such an early car has no pedal box mounted switch I defer to the 'early' experts.

[jimhuiz]

Thanks. I'll be getting under the car today. I don't think my car has the pedal box switch (from my research) but will look to confirm.

[haul]

2nd option:
the contact to the brake light signal sender -see #18 / part # 901 613 401 00- at the master brake cylinder under the steering protection plate in front is corroded or damaged and only half of the voltage is going through.
the rest is diverted directly to the brake lights and they are always glimming on.
replacing the part is doing the show...
also replace the rotten rubber cap #19

[jimhuiz]

Thanks for the input (and the diagram) haul, that was the direction I was going in.

Does anybody know if the pedal cluster has the rocker switch discussed above for a 66-67 911?

[haul]

your mj66 pedalcluster should not have the box with the brake light sensor. this came 68 onwards...
easy way is to measure the voltage arriving at the brake light. it should be around 12v +- if it is clearly below, f.i. down to maybe 5 something, then it is the brakelight pressure switch, and at your mj66, front axle, master brake cylinder. I know it means flushing and filling the system with brake fluid, but hmm new fluid....

[Jim Breazeale]

your mj66 pedalcluster should not have the box with the brake light sensor. this came 68 onwards...
easy way is to measure the voltage arriving at the brake light. it should be around 12v +- if it is clearly below, f.i. down to maybe 5 something, then it is the brakelight pressure switch, and at your mj66, front axle, master brake cylinder. I know it means flushing and filling the system with brake fluid, but hmm new fluid....

Has your car been converted to a "dual master cylinder" brake system?

[jimhuiz]

Has your car been converted to a "dual master cylinder" brake system?

Nope, still has a single master cylinder.

Just got from under the car. The brake switch terminals look great on the outside. There is some corrosion/rust at the junction of the switch with the master cylinder. The wires themselves are in good/great condition. Pulling the wires results in the brake lights going out (no surprise) and reconnecting them to the switch immediately turns them back on.

Can it be anything other than the switch at this point?

[NeunElf]

Wiring diagram L59 (which is the correct wiring diagram for a '67 911S) shows that power comes from the # 1 fuse to the brake light switch and then to the brake lights.

It's not a true short circuit (which properly would send current to ground with no resistance and either blow a fuse or start a fire). A wiring problem is unlikely: you would need an insulation failure on two wires in the same place so that power could flow from an always-energized wire into the brake light wire. And, you would need something holding the wires together to maintain the circuit.

Could the brake lights be mis-connected to the parking light wires? If so the brake lights would only illuminate with the light switch pulled out one notch.


I'm guessing the only likely failure is the brake light switch. There's an easy way to check without a voltmeter: get under the car and pull one wire off the brake light switch. If the brake lights go out then the current path is through the brake light switch. That means there's a problem with the switch unless there's something in the brake cylinder somehow pushing on the switch (which would probably make the brakes work badly). A voltage drop at the brake light switch might give a clue to the specific failure mode but you still need a new brake light switch.


By the way, it's possible to replace the brake light switch without having to bleed the brakes. Work quickly: the brake fluid will tend to flow out and not let air in. If a small bubble should get in it may work up to the fluid reservoir when the pedal isn't depressed. I did do it not too long ago and I had a high, hard pedal without bleeding the brakes afterwards.

But, be prepared to bleed the brakes. If the car has slumbered for a year or more it's time for new brake fluid anyway.

[jimhuiz]

Thanks Jim. I should have just pulled one wire. I'm pretty sure that must be the problem.