Brake Warning Light Mystery - Help Needed

[diesellibrarian]

Hi all -

I am trying to figure out why my brake warning light on the dash of my '89 FJ62 remains lit at all times. Searching this forum, I learned that the brake warning light comes on when the fluid in the master cylinder is low, when the parking brake is engaged, and also when there is an issue with charging (see yellow line in pic below).

I have tested the switches in the master cylinder cap and at the parking brake, and they are both working as designed, i.e. they have continuity under the right conditions.

I then moved on to testing my alternator. It was only putting out 13.5v at idle, and the other diagnostic tests I did showed that it was not making very much juice. So, I swapped in a spare alternator, and I'm now getting a solid 14.4v at idle. Even with the headlights on and fan at full blast, I'm still getting almost 14v at idle. So I believe the alternator is good. But the brake warning light is still on.

In all of the threads I've found on this topic, replacing the alternator solved the problem, but for me it didn't.

Looking at the wiring diagram (below), it looks like there is another wire that is tied into that circuit, and it comes from the "transmission control computer" (see green line in image below). Does it make sense that there might be a fault in the transmission control computer that is causing the dash light to stay on? Can anyone tell me where the T/M control computer is located?

I'm wondering if there is anything else that might be causing the light to remain on. Perhaps a ground fault somewhere? I'm at a bit of a loss at this point, and welcome any input. Cheers!

 

[ActionBastard]

Uh turns out if the brake fluid is a bit low, the brake light comes on….. I’m a genius….

I believe you can test this, or if you have a faulty cap sensor by simply unplugging it and seeing if the light goes off

 

[crutch]

Did a bit more testing last night, and discovered that something is drawing current even when the key is out and all accessories are off. I was getting 12v on the multimeter between the negative post on the battery and the main body ground. It's not a high current draw - less than 2 mA. But there's definitely something bleeding current to ground. I pulled the fuses, but that didn't correct the parasitic draw, so I figure that the short must be between the battery and the fuse box.

Decided to isolate the circuits by removing the various connectors on the hot side of the battery one by one. Sure enough, when I removed the white wire, the parasitic draw stopped. I reconnected it, and the parasitic draw reappeared. I then disconnected the 3-lead connector at the alternator, and once again the parasitic drain stopped.

To me, this says that either the IC regulator or a diode(?) in the alternator has failed, or there's a short to ground in the wire between the battery and the alternator.

Either way, I think that is might be what's causing the brake warning light to remain lit.

From what I understand, this is how the circuit works: at startup, before the alt starts charging, the charge light on the dash is lit because the power from the IG main relay is grounding through the alternator, closing the charge light relay circuit and turning on the charge light on the dash. Once the alternator starts charging, it sends 12v through the Y-W wire, effectively eliminating that path to ground. The relay opens, and the charge light turns off, as the path to ground no longer runs through the charge light relay.

View attachment 3335948

So if the alternator is not putting out a consistent 14.4v or whatever, the Y-W could become a path to ground for that circuit, causing the charge light to turn on. But my charge light behaves normally, so I don't think that's what's happening here.

I'm trying to figure out under which conditions the brake warning light would remain on, but the charge light turn off as normal after a couple of seconds (which is what it does).

I'm also trying to figure out if there's a link between the parasitic draw in the charging circuit and the brake warning light staying on.

So, the mystery persists, but I feel like I'm narrowing in on a possible solution.

Did you by chance ever find the root cause of this? I have similar problem where I've disconnected the brake fluid level sensor and parking brake sensor but I still get a dimmed brake warning light that won't turn off. What is interesting is the light gets brighter when increasing RPM's. The charge light is NOT on.

Sounds like replacing alternator has helped others but I haven't had any starting problems so I haven't replaced the Alternator yet. Figured I'd see if you made any progress. Thanks!

 

[diesellibrarian]

Did you by chance ever find the root cause of this? I have similar problem where I've disconnected the brake fluid level sensor and parking brake sensor but I still get a dimmed brake warning light that won't turn off. What is interesting is the light gets brighter when increasing RPM's. The charge light is NOT on.

Sounds like replacing alternator has helped others but I haven't had any starting problems so I haven't replaced the Alternator yet. Figured I'd see if you made any progress. Thanks!

Nope. My solution was to remove the bulb, haha. I do think it has something to do with the charging circuit - perhaps a failed diode - but i don’t know if or when I’ll find the time to start digging into it again. The car is charging normally so .

 

[crutch]

Nope. My solution was to remove the bulb, haha. I do think it has something to do with the charging circuit - perhaps a failed diode - but i don’t know if or when I’ll find the time to start digging into it again. The car is charging normally so .

I get it, so many things to do! Thanks for the response. I’ll circle back if I ever track it down.

 

[crutch]

Nope. My solution was to remove the bulb, haha. I do think it has something to do with the charging circuit - perhaps a failed diode - but i don’t know if or when I’ll find the time to start digging into it again. The car is charging normally so .

Closing the loop here as my issue has been fixed. I just had my alternator rebuilt which included replacing bad diodes and I added a 6awg cable from alternator to battery. No more dim brake light. Can’t remember if you had already rebuilt your alternator but wanted to at least pass along. Good luck!

 

[diesellibrarian]

Closing the loop here as my issue has been fixed. I just had my alternator rebuilt which included replacing bad diodes and I added a 6awg cable from alternator to battery. No more dim brake light. Can’t remember if you had already rebuilt your alternator but wanted to at least pass along. Good luck!

Much appreciated! I basically concluded that it must be a bad or failing diode in the alternator, but since it seems to be charging fine, I'm choosing to ignore the issue for now. I did swap in a second used alternator which did not solve the problem, but since both alternators have unknown history, it's entirely possible that they're both failing in the same manner. Anyway, glad to hear you got it sorted, and thanks for documenting your solution for posterity.