Working on Brake Warning Light; How it works

[DaveMcKenz]

Hi guys and Merry Christmas,
When I picked up my car a couple of years ago, I drove it home with the Brake Warning Light shining in my face. I temporarily disconnected the light from behind, but knew someday I would address it. When I was doing my pads and rotors the other day I noticed that all the brake sensors were previously removed. This gave me the impetus to fix my warning system.
Ignoring the seat belt warning, on my 84 there are two brake warning lights. One for parking brake reminder and low brake fluid, the other for brake pad wear. The circuit board for this system has inputs from the brake fluid reservoir cap float switch, parking brake position switch, oil pressure switch, and the brake sensor loop. The two brake warning lights should turn on when ignition switch is turned on, and if all is well they should turn off when the engine starts and develops oil pressure. I almost took my parking brake lever apart to adjust the switch, until I read that the lights need engine running to extinguish.
There is some conflicting information on the brake pad sensor warning circuit. It is said to be a single wire system. That is true, but that single wire enters and exits each wear sensor and extends to the tip of the sensor where it is positioned in the brake pad. When any pad wears down sufficiently that the sensor tip get ground down enough to cut the wire, the sensor circuit shows a break in continuity. This condition triggers the brake pad warning light.
Apparently there is a reset procedure, once sensors have been fixed, but I haven't gotten that far yet. My new sensors come this week.
I will update.
Thanks,
Dave

[HorstP]

there is no reset feature or reset required

[DaveMcKenz]

Originally Posted by HorstP

there is no reset feature or reset required

Cool!
Thanks,
Dave

[McLaren-TAG]

Same issue with my 86. As far as problems, this is pretty low on the totem pole so I've just lived with it. I removed all of the sensors since I do regular maintenance and my pads get changed as part of regular upkeep since I AX the car heavily.

I know the parking brake is adjusted and the brake fluid is good and topped since I flush is yearly. When I removed the sensors I twisted together the loop from the old sensors and put a butt connector on it and brought it inside the car in the front, and wire tied it around the trailing arm.

Replaced both brake switches early this year so my options of what it could be a slim. I think it's possible that one of the sensors I customized may have been damaged or whatever and I was planning on taking a look at them over the next month or two when I work on the car.

I'll keep an eye on your progress.

[DaveMcKenz]

Hi McLaren,
Remember the two brake warning lights on 84 maybe to 89 react to different things. The light on the left only reacts to fluid level and parking brake. The light on the right only reacts to a break in the brake pad sensor loop. The brake switches are now longer monitored.
The float switch in the brake reservoir can stick, so it can be eliminated by disconnecting it at the cap. Again, only affects the left hand light.
Good luck,
Dave

[schoward]

There was a reset on my SC lamp assembly that I used and it worked. E brake light was stuck on.

The light modules are clearly different but this is free and easy if want to try. Apparently those modules are fairly complex little things. With everything off, reach in through the clock hole and disconnect the plug on the back of the seat belt / brake light module and leave it for a minute or so (presumably to let any remaining charge in the capacitors drain) . Then plug back in and start engine as normal. With mine I think I popped the whole assembly out of the dash by nudging from behind then disconnected/reconnected the plug where I could see it.

This corrected or 'reset' my brake light which was stuck on to the point that the PO just popped the cover and pulled the bulb. Took 10 minutes and e brake light has worked flawlessly for 10 years since doing this.

Seems you have kind of done this already with your initial disconnect but may be worth a try again.

[DaveMcKenz]

Thanks, Scott.
I have found it's pretty hard to get the module out of the dash. I pushed pretty hard from behind, but those four little plastic tabs just would not budge. I had to take the lens off and work from the inside. Not sure that's the correct way though.
Dave

[steely]

This probably doesn't apply to you but on early 911's (or was it 914's?) the reset button is located on the single wire type brake switch on the master cylinder.

[timmy2]

Reset my '78 when it was stuck on (relay latched) due to a loose bulb, by disconnecting the battery.
Way easier than pulling the module.