Help - hand break light stays constantly on! Why? How to fix?

[PorscheCollecto]

Hi fellow Pelicans

My and my friend are fighting a problem on our common Porsche 911 SC from 1978 (the car is very original), which we hope you perhaps can help us with.

The problem is that the hand break light is constantly on. It doesn't switch off after the usual 5 seconds when the engine is turned on. It's on no matter if the hand break handle is released or not. We have already tried to adjusting the switch within the hand break, this doesn't seem to be the culprit. Then we have tried to exchange the hand break + seat belt relay in the dash. Again doesn't seems to be the problem.
None of the wheels indicate that the breaks are not working fine or hanging or anything. And the car breaks just fine. Both normal breaks and hand break.

Anyone have any clues or ideas of what could be wrong, what next to pursue?
Anyone had a similar problem and got it fixed?

Thanks in advance for the help.

Best regards
Chris

[Jdub]

This dashboard light typically stays on when there is an issue with the alternator. Have you noticed any surging in idle combined with a dimming and heightening of the brightness of your headlights? Note any rotten egg smell in the car?

If you do not have any records of the alternator being rebuilt, and am sure the grounds from your battery and alternator are in good shape you might investigate this.

[Rich Gas]

I'm assuming Super Carreras have brake pad sensors? Check to make sure they are in working order.

[wwest]

Originally Posted by Jdub

This dashboard light typically stays on when there is an issue with the alternator. Have you noticed any surging in idle combined with a dimming and heightening of the brightness of your headlights? Note any rotten egg smell in the car?

If you do not have any records of the alternator being rebuilt, and am sure the grounds from your battery and alternator are in good shape you might investigate this.

+1 battery being over charged, bouncing tach is next.

Address SOON or sacrifice CDI module.

[porwolf]

I had the same problem with my '79SC. Read on Pelican that the circuit inside the warning light has to be reset by disconnecting the ground cable from the battery with the ignition switch in the on position. That did for me. It it fixed the hand brake light being on even if the handle is fully in the down position.

[timmy2]

I had a loose bulb in the brake light that kept the light on. Reseated bulb and had to disconnect battery to reset latch circuit.

[universeman]

If there is a voltage spike while driving, the brake light will come on and stay on (this is what I understand...who made that decision?) To reset it you have to follow porwolf's post - there is a procedure.

I had to totally replace the switch in my handbrake but while the handbrake is open, you can test to see if it's the culprit by grounding it manually to the car...if light goes off when you do that but not when you have the handle installed, you have a switch adjustment/replacement to make. If light stays on no matter what you do with that switch, your problem is elsewhere.

[MichaelN]

Long story that I'll try to summarize. When I bought my '81 SC a few years ago I noticed that under heavy braking (autocrossing) the fronts would lock up quite easily (aside from this, the pedal felt quite normal in routine driving). I confirmed that the back brakes were working, but apparently not as well as the fronts. I was new to 911s, so I just concluded that SC brakes were biased heavily to the front.

Several months went by and one day the brake light would not go out. I researched the problem and determined that the warning light was indicating a irregular pressure reading at the master cylinder. So, when I changed out the master cylinder the brake light went out, the braking performance improved HUGELY, and the fronts were no longer locking up prematurely.

So it may be possible that your warning light is indicating a partial failure in one of the two circuits in the master cylinder, but it has not become critical enough to be detectible in the feel of the brake pedal. I'm not sure ,though, how you can confirm this without changing out the master cylinder. Maybe someone else will know.

[grant lyon]

+1 on the MC. On my '76 (may be close enough but haven't checked schematics) if there is a pressure differential between the front and rear brake circuit it trips the handbrake warning light. Routine confusion when bleeding brakes to get the light to turn off. Solution in my case (no power assisted MC) is a little reset button on the side of the MC after completing bleeding. IIRC, if using a power bleeder with low pressure the pressure differential Fr to rear isn't enough to trip the switch.