Parking Brake Switch/Light Troubleshooting (1 Viewer)

[MatthewMcD]

As I wrap up my Parking Brake rebuild on my 75 FJ40, I am trying to figure out the Parking Brake Switch circuit. The switch works fine. I have it operating correctly (based on my multimeter) when the brake is set and released. The problem is with the ignition on and the brake set, the brake warning light does not illuminate. Since the wires enter the wiring harness and disappear I don't know how to troubleshoot this further. I did run a bypass jumper to ensure it is not the switch and I know the bulb works.

Anyone know what to check next?

 

[charliemeyer007]

Interesting. Do you have a good enlarged print out of the wiring diagram? I had the one in Chilton's for my 72 blown up to like 18x24 and then laminated - very handy.

There are 2 sensor/switches on the brake master and the one for the e-brake switch that "worked with that brake light on the dash. My dash lights no longer works either. I just make sure I set the e-brake enough it stalls the motor during normal take off - better warning than an idiot light IMHO.

I think you try one of those circuit tattletales devices that you plug in, and then follow the wire with the probe/detector.

 

[Green Bean]

@Coolerman has a very thorough writeup about the parking brake wiring: Cruiser Wiring - http://www.globalsoftware-inc.com/coolerman/fj40/wiringmainframe.htm
I just worked on ours and I think that I determined that the + wire to the bulb on the dash gets 12v switched. The other side of the bulb of the bulb gets its ground from the harness tied to the switch on the parking brake. When the parking brake is engaged the switch completes the ground to the bulb. This may not be the factory correct way, but it works on our 1970.

 

[mattressking]

Just chase the wiring and make sure it is all working.

http://www.globalsoftware-inc.com/coolerman/fj40/schematics/FJ40/1974FJ40/HaynesFJ401974.jpg

 

[MatthewMcD]

Interesting. Do you have a good enlarged print out of the wiring diagram? I had the one in Chilton's for my 72 blown up to like 18x24 and then laminated - very handy.

There are 2 sensor/switches on the brake master and the one for the e-brake switch that "worked with that brake light on the dash. My dash lights no longer works either. I just make sure I set the e-brake enough it stalls the motor during normal take off - better warning than an idiot light IMHO.

I think you try one of those circuit tattletales devices that you plug in, and then follow the wire with the probe/detector.

Thanks, I have a wire tester. I was trying to avoid that process. But that's probably what it'll take.

 

[MatthewMcD]

Just chase the wiring and make sure it is all working.

http://www.globalsoftware-inc.com/coolerman/fj40/schematics/FJ40/1974FJ40/HaynesFJ401974.jpg

Thanks, that's the plan. First I need to test the brake on a hill. But it would be nice if the lights worked the way they are described.

 

[MatthewMcD]

@Coolerman has a very thorough writeup about the parking brake wiring: Cruiser Wiring - http://www.globalsoftware-inc.com/coolerman/fj40/wiringmainframe.htm
I just worked on ours and I think that I determined that the + wire to the bulb on the dash gets 12v switched. The other side of the bulb of the bulb gets its ground from the harness tied to the switch on the parking brake. When the parking brake is engaged the switch completes the ground to the bulb. This may not be the factory correct way, but it works on our 1970.

I don't see the write up you mentioned. I'll try the wiring diagram and see if the wires match up. Also, need to dig around in the console and make sure I didn't bump something else loose.

 

[GA Architect]

Be sure your parking brake circuit is completing the ground at the brake master cylinder, item #34 on the wiring diagram in post #4 above. For help see post #19 < click here > in this thread.

note: Post #11 in that same thread, Matt posted the Toyota Manual's diagram as well.

 

[MatthewMcD]

Be sure your parking brake circuit is completing the ground at the brake master cylinder, item #34 on the wiring diagram in post #4 above. For help see post #19 < click here > in this thread.

note: Post #11 in that same thread, Matt posted the Toyota Manual's diagram as well.

The brake master cylinder switches are connected. I'll clean up those contacts this morning and try and rule them out.

 

[45Dougal]

Check the fuse is OK (heater fuse I think).
Then I'd start by grounding the red- white wire at the handbrake switch and see if that makes it light up.

If that works then try grounding one of the master cylinder connections while the handbrake is on. If nothing, then hit the foot brake.

You'll then know which wire is broken.

 

[MatthewMcD]

Check the fuse is OK (heater fuse I think).
Then I'd start by grounding the red- white wire at the handbrake switch and see if that makes it light up.

If that works then try grounding one of the master cylinder connections while the handbrake is on. If nothing, then hit the foot brake.

You'll then know which wire is broken.

View attachment 3513538

Thanks! I'll be back at it after Christmas!

 

[Aloha Jen]

I just went through this exact same scenario on my ‘75 Fj40. I confirmed all brake light switches and bulbs were good but still had no dash brake light signal come on with the key turned on when the brake handle was pulled. After some frustrating troubleshooting and some thorough Mud searches, the problem turned out to be the short wire/plug section that connects from the main wire harness to the 2 sensors on the brake master cylinder. The wire under the sheath was badly corroded and not allowing enough electricity to pass through the master cylinder sensors to complete the brake system circuit (just an educated guess). Not sure if I’m using all the right technical terms, but hopefully you get what I’m saying. After replacing that section of wire from the main harness to the brake master sensors the dash brake light all illuminated like they are supposed to. Hopefully it is an easy fix like this to get your brake monitoring light to work properly.

 

[Green Bean]

Another easy test: pull out the bulb socket. Use a grounded12v tester. With the key on there should be 12v constant. This will at least tell you if your issue is with the positive or the ground.

 

[45Dougal]

Thanks! I'll be back at it after Christmas!

Merry Christmas. See you on the other side

 

[MatthewMcD]

I just went through this exact same scenario on my ‘75 Fj40. I confirmed all brake light switches and bulbs were good but still had no dash brake light signal come on with the key turned on when the brake handle was pulled. After some frustrating troubleshooting and some thorough Mud searches, the problem turned out to be the short wire/plug section that connects from the main wire harness to the 2 sensors on the brake master cylinder. The wire under the sheath was badly corroded and not allowing enough electricity to pass through the master cylinder sensors to complete the brake system circuit (just an educated guess). Not sure if I’m using all the right technical terms, but hopefully you get what I’m saying. After replacing that section of wire from the main harness to the brake master sensors the dash brake light all illuminated like they are supposed to. Hopefully it is an easy fix like this to get your brake monitoring light to work properly.
View attachment 3513858
View attachment 3513859

Thanks! I'll check this when I get back to it.

 

[MatthewMcD]

Another easy test: pull out the bulb socket. Use a grounded12v tester. With the key on there should be 12v constant. This will at least tell you if your issue is with the positive or the ground.

Sweet, thanks!

 

[MatthewMcD]

Check the fuse is OK (heater fuse I think).
Then I'd start by grounding the red- white wire at the handbrake switch and see if that makes it light up.

If that works then try grounding one of the master cylinder connections while the handbrake is on. If nothing, then hit the foot brake.

You'll then know which wire is broken.

Grounding the Red/White makes it light.
Grounding the master cylinder switch makes it light.
Bypassing the e-brake switch with a jumper has no effect on the light.
Attaching the switch to the Red/White I tested above and grounding the other side of the switch lights the light unless I push the switch and then it goes out. So the switch works.

I will trace the "return" ground wire and see if there is a break. Otherwise I could just run it to a new ground.

 

[45Dougal]

So if I understand correctly; if you ground either of the two wires at the master cylinder while the handbrake is applied, then the light comes on - that's good because it shows that everything is working except the last stage. .

That sounds like you have either a bad connection to both master cylinder switches, or from the MC body to ground, or they're stuck open, or you have residual pressure in your brake lines holding those pressure switches open.

Test the continuity of each of those pressure switches with the foot brake on and off a few times. They should be open circuit while the foot brake is applied, and closed circuit when the foot brake is released.