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Old 05-11-2017, 04:17 AM   #1
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Default What can make only the brake warning light come on?

I have this wonky issue developing where the brake light sometimes won't turn off, sometimes won't turn on with the parking brake. It also will very occasionally come on while driving, but generally not. Just eventually turn off.

Historically, the light would turn on and off with the park brake reliably. I have fiddled with the switch, and I can't actually do that any more. What's more, according to the wiring diagram, the only thing that should turn that on, and that alone, is the park brake. I did fiddle with the brake fluid sensor, but that turned on the ABS light as well.

I cannot figure out how to remove the switch, but the wiring diagram shows a diode somewhere in there. I guess I would like to know if anyone else has hunted this gremlin, and also, how to get that darn switch out!

Thanks!
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Old 05-12-2017, 02:12 PM   #2
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Yours may be different being newer, but on my 86 the e-brake light came on when my brake caliper started leaking fluid. My symptoms were exactly the same as what you're experiencing though, so it sounds like it could be the same problem.
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Old 05-12-2017, 04:37 PM   #3
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Does your truck have a trailer wiring harness?
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Old 05-12-2017, 06:27 PM   #4
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My Brake warning light came on at start up as it should and goes off.
It also comes on when the e-brake is engaged.
I'm trying to track down the cause of my Front brakes dragging.
When I pulled the Master Cylinder to bench test it, I noticed that the Brake fluid pressure sensor located at the base of the MC was disconnected.
After I reinstalled the MC I reconnected the sensor and the Brake warning light would not turn off. I pulled a sensor from another truck and soldered it in. Works fine now.
I can only assume that the previous owner had the same problem with the sensor and just yanked it out. This may be the problem.
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Old 05-14-2017, 03:59 AM   #5
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I think I can eliminate the brake fluid level/pressure/whatever it is in the master cylinder. I have had it disconnected with no change, and when I jumper the terminals, my ABS light comes on. It looks like, in the wiring diagram, that the master cylinder switch is wired in such a way that it will turn on the BRAKE light directly, while also indicating to the ABS unit that it is grounded. In any case, this switch doesn't seem to be the problem.

At this point, the light is on almost all the time. It's awesome I have thought about disconnecting the ABS unit to see what that would do although it seems to be functioning properly and puts out no codes.

Quote:
Originally Posted by jp2code
Does your truck have a trailer wiring harness?
It does! Related maybe? I have a newer converter for the brake light signal, about a year old.

Has anyone removed their parking brake switch? I cannot figure out how it is retained on that subframe.
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Old 05-14-2017, 09:25 PM   #6
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Quote:
Originally Posted by highest_vision View Post
It does! Related maybe? I have a newer converter for the brake light signal, about a year old.
The old trailer brake light splices are probably coming lose back there.

It isn't hard to fix, but it will take a little time.

Take the tape off of the connectors and resplice them. You will be surprised how nasty it is under there.
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Old 05-19-2017, 08:02 PM   #7
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Well I now have everything disconnected except for the park brake switch. Still got that light on! I have also found out that the park brake switch is NLA... I will need to see about accessing that and unplug it to see if i have a wiring issue, but it seems that I have a switch that is wonked out.

I have learned in this process that unplugging the ABS unit will not illuminate the BRAKE light, but it will illuminate the ABS light, and that the master cylinder fluid level switch will turn on both the BRAKE and ABS lights. Not sure if this is useful info for anyone.
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