Brake Warning Light Mystery - Help Needed (1 Viewer)

[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!

 

[ToyotaMatt]

your floor mounted pin switch e brake

un-plug the 2 pin connector when handle down

it may / should go off

if so bad switch

 

[diesellibrarian]

your floor mounted pin switch e brake

un-plug the 2 pin connector when handle down

it may / should go off

if so bad switch

Thanks Matt. I did disconnect the switch to test it, and it seems to be working properly, i.e. continuity when handle up, no continuity when down.

I notice that when I move the handle up and down, it does have a slight effect on the brightness of the warning lamp.

 

[ToyotaMatt]

Thanks Matt. I did disconnect the switch to test it, and it seems to be working properly, i.e. continuity when handle up, no continuity when down.

I notice that when I move the handle up and down, it does have a slight effect on the brightness of the warning lamp.

your pin switch is STILL the likley problem
here


moving part

45+ years old


McDonalds and super big gulps spill on it 45+ years then BUD CANS = problematic sticky and corrosive issues there , once inside the little square box these various foreign compounds like cola are corrosive and will hang up the little springs and check balls inside doing the job of transmitting 12V DC and things like copper contacts will turn Green like the Statue of Liberty on Ellis Island is now from the affects of Global warming based ACID RAIN ..

If you think all this above is phunny for some reason ?

Take a new shinny Abe Lincoln Penny and drop it in a small glass of Coke or Pepsi Soda ( and not the wimpy Diet or FREE type , the Original formula or whatever they call it now , let it soak for a while ….

 

[diesellibrarian]

Thank you sir. I'll take another look at it tonight.

 

[2mbb]

I think @ToyotaMatt is suggesting to disconnect the switch from the harness then check to see if the light is on or not. You should be able to do this with either the parking brake or fluid level switches.

 

[ToyotaMatt]

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?

View attachment 3326102

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!

Having a good visual of the little square box E-Brake pin switch would be a good idea too here …

Take the E-brake handle and pull it up lool
At the tiny black contact dowel pin area in general .
Is it debris-free ?

Loose dirt , grime , fossilized parts of McDonalds Chicken McNuggets , dog hair or human hairs for that matter , bits of what ever combined with goo from any type of spilled liquid like Latte make a binder , this sticky nasty SH#T now is a hanger upper of a spring loaded part both inside it and out , not to mention the corrosion factor

You mention as you move the handle the light flickers on and off during the full sweep range of motion …

This tells me any of the above is possible

Lastly , in it’s fully seated position make sure the pin switch is fully depressed , like fully

There are a few factors at play that can make you think the handle is down and such and hence depressing the pin switch fully hence turning the dash gauge red indicator light OFF

So double check this well here

I have found loose lock down adjustment hex nuts on pin switches from the factory during my dealership days , and also ones that vibrate loose somehow , and also ones installed and adjusted incorrectly by humans at one point recently and such …


Please make sure you report back here with all the tech and also several photographs of the culprit no matter what it is or where it is …

This is so very important because it will help others here on MUD next week or years from now with there brake light staying on issues too .!

A large percentage of the time folks forget to do a simple but all important follow up here in there TECH thread for whatever reason , so this is why I mention this so you will not forget to help others who have helped you ….

This is one of the foundation principles of MUD ….

THIS IS THE WAY​

 

[diesellibrarian]

Oh I will definitely post the solution...when (if) I find it.

I've spent many hours chasing this mystery, and the light remains on.

It's clear from the wiring diagram that the light is ground-switched, so to me it stands to reason that regardless of possible damage/corrosion of the parking brake switch, the light should go off when I disconnect both the parking brake and brake fluid level sensor switches. With those connectors disconnected, there should be no path to ground = no light on the dash. Correct?

But the light stays on. So that tells me that there is a fault to ground somewhere in the harness, and that's what I've been chasing.

The only other variable is the fact that the "charge light relay" is also connected to the brake warning light circuit (see yellow in above diagram). I gather this is to warn of a charging issue. In chasing this problem I have swapped out the charge light relay with a spare that tests good according to the FSM. I've also swapped in an alternator (actually two alternators lol) that test good according to the FSM. So I don't believe I have a charging issue. But it's not clear to me under what conditions the charging light relay would trigger the warning light. Here's the wiring diagram:

So that's where I'm at with this. I've dug into the wiring harness quite a bit, looking for damaged wiring. In particular I was looking for the spot where the lead from the brake fluid level switch (which should be RW according to the wiring diagram but is RL on my truck) ties in to the RW wire that goes to the combination gauge. But there are so many wires under the dash that it's almost impossible to trace a single wire.

I've also checked and cleaned all the grounds under the dash, and have confirmed continuity between the connector at the combination gauge and both the parking brake and brake fluid connectors.

I should add that I've swapped in a spare gauge cluster with the same results, so it's not likely a faulty gauge cluster.

Anyway, that's where I'm at with this. I'll be looking at it again today. I'm going to pull out the front passenger seat and carpet to look for a ground fault in the harness that feeds the parking brake switch. I'll be sure to update if I find anything new.

 

[diesellibrarian]

Did a bunch more work on the brake warning light last night, and unfortunately I'm not much farther ahead.

With the parking brake switch, fluid level switch, and charge light relay disconnected, there should be *NO* path to ground for the circuit, and yet the light remains lit. So there must be a ground fault somewhere after the "GAUGE" fuse. When I pull the fuse, the light turns off.

I pulled the fuse box and dropped the relay block and various bits of trim and heater ducting to get at as much of the harness as possible, but it's very difficult to trace an individual wire. I tried wiggling various sections of the harness hoping to get the light to flicker a little, but didn't see even a hint of that.

I'm wondering if this mysterious "junction connector" is where the problem lies:

Seems like a good location for some kind of short. I looked for quite a while for a connector that has 6 yellow wires running to it, but I didn't see anything matching that description. Any FJ62 wiring gurus have any ideas?

At this point, I'm at a loss with this. It's a very simple circuit, but with all the hundreds of wires all bundled up under the dash, finding a short feels impossible.

 

[hj 60]

My Hj has kind of the same problem when I press the brake pedal firmly about 10 times, the light stays on for a few seconds (alternator vacuum pump issue?)
I am scared it will do this when at the road check and so it is a concern, but never found why (new alternator) and always afraid the road check guy will mention it and then have no clue what to do about it.

But that seems connected to brake pressure (all works fine, no need to ever add oil, no fading, nothing wrong and road test brake check is pretty strict in europe)

Maybe remove the ground from the light and use one wire straight from the battery to test, or from a nearby ground to see if it is off then.
Very nice you checked the wires diagram and maybe something else in that circuit has a bad ground like you see in trailer lights problems sometimes.

 

[kirvesmies]

@diesellibrarian , any headway with the mystery light?

 

[Hojack]

My BRAKE warning light was due to a bad alternator.

 

[diesellibrarian]

@diesellibrarian , any headway with the mystery light?

NOPE.

But I did get my license plate lamps working:

My BRAKE warning light was due to a bad alternator.

I think my alternator was bad, but I've tried two others with no change. The one that's in there now tests good according to the FSM diagnostic procedures. It came with the truck and looks like a freshly reman ND alternator.

On the bright side, I can now swap a 3FE alternator in about 15 minutes.

 

[kirvesmies]

well, 2 outta 3 is not horrrible. good luck with that my friend

 

[diesellibrarian]

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.

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.