blinking brake warning light on dash

gptom · 04-21-2013, 10:07 AM

I have a 72 911. I completely rebuilt the brake system including a new master cylinder. I have been unable to get the brake warning light to stop flashing. I have unhooked the battery. I have pulled the parking brake housing & grounded the wire to its switch.

The new m/c came with a switch with 1 connection. My old m/c had a switch with 2 connections so I installed the old switch into the new m/c and hooked up both original wires.

How do I get the light to turn off?

Thanks for your help

Tom

HarryD · 04-21-2013, 10:59 AM

Most likely it is a failed switch on your MC.

There is no "reset switch" on an early car MC. The battery thing does not work either.

If you unplug the wires from the switch, does the light stop?

Does the light stay stopped when you reconnect the e-brake microswitch (with handle down of course)?

The MC switch is a USA only thing. You can leave it off or replace at your convience.

86 911 Targa · 04-21-2013, 11:58 AM

Quote:

Originally Posted by gptom

I have a 72 911. I completely rebuilt the brake system including a new master cylinder. I have been unable to get the brake warning light to stop flashing. I have unhooked the battery. I have pulled the parking brake housing & grounded the wire to its switch.

The new m/c came with a switch with 1 connection. My old m/c had a switch with 2 connections so I installed the old switch into the new m/c and hooked up both original wires.

How do I get the light to turn off?

Thanks for your help

Tom

Tom,

Try shorting the two wires on the harness side of the switch connector,
then turn on the ignition switch.

Good luck,

Gerry

pm me as needed.

jstobo · 04-21-2013, 05:05 PM

Your two wire switch has one brown wire(ground) and a brown/white wire(switched ground). The br/wh goes to the ground side of the brake warning lamp. When the switch closes (to indicate a bad MC), the switch connects the Br/WH wire to the Br ground wire and the circuit is completed and the warning light goes on. You have a bad two wire switch if the MC is working fine.

Connect the BR/Wh wire to the new one pole switch. The one pole switches uses the body of the car as the ground. So when the switch closes the Br/wh goes to ground to light the warning light. Just leave your Br wire disconnected. You do not need it with the one pole switch.

Also, after you put the new switch in, you'll have to re-bleed the brakes. Because the pressure will be low as you bleed, the warning switch will activate. After bleeding, push in the reset button on the warning switch.

I used my old two wire switch because it tested good on the bench. But when I put it in the MC it failed and I cussed myself for not using the new switch which I had on hand. Now I had to rebleed the system again. I waited until I upgraded my brakes and in the meantime just pulled the wires off the MC switch. No more light unless the handbrake was engaged.

HarryD · 04-21-2013, 05:45 PM

I am not sure you need to rebleed the brakes. When I replaced my bad switch, the chamber where the switch toggle was held was dry.

gptom · 04-24-2013, 03:22 PM

Thank you for all of your replies. I have jumper-ed the 2 wires & no joy. I than ran another wire to ground and still no joy.

Is there a relay or ??? that the ground goes to and that controls the flashing light?

Tom

HarryD · 04-24-2013, 05:20 PM

Did you just disconnect the wires from the MC and handbrake? Do not ground them, just suspend them in air. Is the light out?

E Sully · E Sully's Garage

The warning light uses the commom flasher relay. If either Br/W or Br/Y reads continuity to ground the light will flash. I am not sure which of the following diagrams relates to the '72. First diagram is from '71, second is '73. I haven't looked into the effect of the Y/R for the seat belt light in later diagram.
'71

'73

gptom · 04-25-2013, 11:16 AM

HarryD You are 100% correct, I was going at this backwards. I disconnected all of the wires and the light stopped. grounded parking brake & it started as it should. Connected brown/white to switch & it started blinking

Thanks to all of you. Now to find the old m/c with a new 1 wire switch in it

Tom

HarryD · 04-25-2013, 08:38 PM

Quote:

Originally Posted by gptom

HarryD You are 100% correct, I was going at this backwards. I disconnected all of the wires and the light stopped. grounded parking brake & it started as it should. Connected brown/white to switch & it started blinking

Thanks to all of you. Now to find the old m/c with a new 1 wire switch in it

Tom

Tom,

Glad to help. I had the same issue a few years ago and it took me forever to figure it out because I did not want to remove my tire and look at the master cylinder.

Have fun.