Brake Warning Light On

Robert Petersen

Robert Petersen

Member
Earlier this week my brake warning light started coming on intermittently. The light progressed to being on full time. My brakes feel completely fine and there is no noise, pulling, sticking, and the brake fluid reservoir is full.
I bled the brakes 2x and never had any air come out.
While the light was still in it's intermittent stage the parking brake switch was working fine but I took it out anyway and cleaned it, light still on.
Checked all the brake pads and looked for leaks, pads are good and no signs of leaking.
When I disconnect the parking brake switch the light goes out. I'd like to say "problem solved" but that reminds me of our supreme leader's solution to Covid, "If we stopped testing people the infection numbers would go down", not a good solution.
I'm at a loss of what to check next.
 
3 things can turn on the brake warning light:
-Handbrake switch
-Brake fluid level sensor in reservoir
-Vacuum switch on brake booster (diesel engine L300s).

Unplugging those things should stop the light coming on. Plugging them back in one at a time should tell you which one is faulty.

You've already found the plug for the handbrake switch (tucked under the carpet or handbrake surround trim just in front of the handbrake lever).
The brake fluid level sensor and vacuum switch can be unplugged from below the dashboard, but access is easier if you also remove the speaker. The level sensor has a blue plug with two wires, and it's up high in the dashboard. The vacuum sensor is much lower, just above the brake pedal, looks like an oil pressure sensor (lower right on the photo). The plug comes off sideways: that'll make sense when you see it.

Screen Shot 2020-07-28 at 09.34.18.png
 
Growlerbearnz said:
Unplugging those things should stop the light coming on. Plugging them back in one at a time should tell you which one is faulty.
Click to expand...
I just want to make sure I understand what you're saying.
  1. Unplug all 3 of the switches/sensors.
  2. Plug in any one of the three lets say the first one is the brake fluid level sensor in reservoir.
  3. If the light comes on it is the faulty sensor.
  4. If the light doesn't turn on, unplug the brake fluid level sensor in reservoir and proceed to the next sensor.
Am I understanding correctly?
 
Yup, that's how I'd do it. Though you missed step 1.5:

1.5. The light should now be off. If the light stays on then something really weird is going on.
 
Try adding some brake fluid till the light goes out. Probably the pads getting worn and the fluid is taking up the wear. Just make sure it doesn’t overflow when you put new Brake pads on.
 
Okay, I looked at the wiring diagram and simplified it for just the brake warning light for the L300 (assuming a 1991 diesel is the same as your car)
3.jpg

So, it's either a bad parking brake switch, a bad fluid level switch, fluid level too low, or the vacuum switch.
 
Growlerbearnz said:
3 things can turn on the brake warning light:
-Handbrake switch
-Brake fluid level sensor in reservoir
-Vacuum switch on brake booster (diesel engine L300s).

Unplugging those things should stop the light coming on. Plugging them back in one at a time should tell you which one is faulty.

You've already found the plug for the handbrake switch (tucked under the carpet or handbrake surround trim just in front of the handbrake lever).
The brake fluid level sensor and vacuum switch can be unplugged from below the dashboard, but access is easier if you also remove the speaker. The level sensor has a blue plug with two wires, and it's up high in the dashboard. The vacuum sensor is much lower, just above the brake pedal, looks like an oil pressure sensor (lower right on the photo). The plug comes off sideways: that'll make sense when you see it.
Click to expand...
After following Growler's instructions it turns out to be a bad vacuum switch. Looks easy enough to swap out, hopefully it'll cross reference to something I can get stateside.
 
Is it possible that the vacuum switch is working and there may be an issue with supplied vacuum to brake booster?
 
DustAngel said:
Is it possible that the vacuum switch is working and there may be an issue with supplied vacuum to brake booster?
Click to expand...
Possible, but if the vacuum on the booster is low enough to trigger the switch the brake pedal feels super hard and requires a lot of effort to brake. You'd definitely notice it (or maybe *I'd* definitely notice it.) The warning light really is a warning that something's about to go wrong.

(I got curious/bored and tested it one day. Used an electric vacuum pump until the light *just* went out, pressed brake pedal. I really need a hobby.)
 
Assuming all the sensors are working correctly... ;-)
-Low vacuum: the brake pedal is going to be *much* harder than you expect, if you're not prepared for it you're probably not going to push hard enough.
-Low fluid level: the brake master cylinder is about to take a big gulp of air, and your pedal will be *way* too soft
-Handbrake on: the rear brake shoes are going to overheat, possibly boiling the brake fluid, possibly catching on fire.

Of course if one of the sensors is telling lies then the light is just a light. Generally, red warning lights mean "stop and figure this out or you'll crash/be stranded/wreck something". Yellow lights mean "You might want to know about this but no big deal"
 
Growlerbearnz said:
Assuming all the sensors are working correctly... ;-)
-Low vacuum: the brake pedal is going to be *much* harder than you expect, if you're not prepared for it you're probably not going to push hard enough.
-Low fluid level: the brake master cylinder is about to take a big gulp of air, and your pedal will be *way* too soft
-Handbrake on: the rear brake shoes are going to overheat, possibly boiling the brake fluid, possibly catching on fire.

Of course if one of the sensors is telling lies then the light is just a light. Generally, red warning lights mean "stop and figure this out or you'll crash/be stranded/wreck something". Yellow lights mean "You might want to know about this but no big deal"
Click to expand...
So, red light means I will die, yellow light means I have a 50/50 chance if I ignore it. Got it, lol.

I think I'm going to with a bad sensor as everything looks and feels great. Thanks again for all the help!
 
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