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Unread 01-12-2008, 10:04 AM   #1
sharktooth
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Brake Failure Warning Switch - How It Works?

Hey, folks;

Just a quick question on a 1990 4.2l wrangler - does anyone know how the brake failure switch (built into the power brake distribution block in the engine compartment) works? Is it always closed, or always open? I built a new dash board (it looks great), but I'd like to wire in a new idiot light in there "just in case" so I know when front or rear braking fails.

If it is usually "on" then I would have to wire in an inverter. If it is off, it will be easy enough to wire it straight to an LED.

Anyone know?

Joe

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Unread 01-12-2008, 10:27 AM   #2
timatoe
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I've never heard of a failure warning light. The only brake light I've seen on my dash is the one for the e-brake. Not, saying it's not there, just that I've never seen it or heard of it.
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Unread 01-12-2008, 10:43 AM   #3
Peter Nuss
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It's open for wiring, but, it works by the brake rod part being over centered. If either front or the back brake systems fail, the sensing rod over centers and trips the switch. In order to reset it, the switch has to be removed and the rod will center until the brakes are applied again. If the brake system has been repair it fine, but if not it will trip the switch again. PS, you should remove the switch to bleed the brakes also. Cause it can trip doing that process also.
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Unread 01-12-2008, 10:44 AM   #4
Ken Lessard
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I beleive it is always centered in the off position. When either system fails it forces the plunger in one direction or the other makes contact and completes the circuit and turns the light on. Just my thoughts from past experience on vehicles other than Jeeps... Ken
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Unread 01-12-2008, 10:48 AM   #5
ctm
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The brake warning light is connected to the proportioning valve. If the brake pressure between the front and rear brakes becomes too great (i.e. one or the other springs a leak or has no pressure) the prop valve pushes on the metal rod in the switch and makes a ground connection. That ground is connected to the negative side of the dash warning light that has +12V from the ignition.

By the way the e-brake is hooked into the idiot light also.
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Unread 01-12-2008, 11:17 AM   #6
sharktooth
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Quote:
Originally Posted by Peter Nuss
PS, you should remove the switch to bleed the brakes also.
Will I have to bleed the brakes if I remove it? Now it was also mentioned

Quote:
Originally Posted by ctm
the prop valve pushes on the metal rod in the switch and makes a ground connection.
Does that mean it is grounded through the body? (Which would mean that both lines are always connected, if one goes to the e-brake. I suppose that is just a method for allowing both the brake failure switch and the e-brake switch to turn on the single light. Is that correct?)

Joe
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Unread 01-14-2008, 08:02 PM   #7
Big Trev
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That is good info, thanks.

I am waiting for the answer as I am trying to wire in a brake fail light on a race car, but I am having issues.
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Unread 01-14-2008, 08:47 PM   #8
pat8942
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You could be a teacher.
I know how they work but if I didn't I would have after your explanation.
Quote:
Originally Posted by Peter Nuss
It's open for wiring, but, it works by the brake rod part being over centered. If either front or the back brake systems fail, the sensing rod over centers and trips the switch. In order to reset it, the switch has to be removed and the rod will center until the brakes are applied again. If the brake system has been repair it fine, but if not it will trip the switch again. PS, you should remove the switch to bleed the brakes also. Cause it can trip doing that process also.
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Unread 01-14-2008, 08:50 PM   #9
pat8942
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Quote:
Originally Posted by sharktooth
Hey, folks;

Just a quick question on a 1990 4.2l wrangler - does anyone know how the brake failure switch (built into the power brake distribution block in the engine compartment) works? Is it always closed, or always open? I built a new dash board (it looks great), but I'd like to wire in a new idiot light in there "just in case" so I know when front or rear braking fails.

If it is usually "on" then I would have to wire in an inverter. If it is off, it will be easy enough to wire it straight to an LED.

Anyone know?

Joe
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Unread 01-15-2008, 11:26 AM   #10
jbolty
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related topic. The wire connector on my proportioning valve is broken off. In the parts catalog it is not listed as a seperate part so I guess unless I replace the whole valve I will have no warning light. Any thoughts on this from anyone?

thx
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Unread 01-15-2008, 04:09 PM   #11
ctm
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it is not a separate part. It happened to me when I tried to take off the connector it just broke right off. $70 later I had a new connector - and a new prop valve.
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Unread 01-15-2008, 05:47 PM   #12
pat8942
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IMO you can do without it.
If you have a brake problem you won't need a light to tell you.
Even if the light comes it won't fix the problem so light or not you still will know somethings wrong when you try to stop.
I think that was some kind of Government mandated thing.
Quote:
Originally Posted by ctm
it is not a separate part. It happened to me when I tried to take off the connector it just broke right off. $70 later I had a new connector - and a new prop valve.
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Unread 02-10-2011, 07:49 AM   #13
carolinacop58
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brake failure switch

QUOTE=ctm;4679987]it is not a separate part. It happened to me when I tried to take off the connector it just broke right off. $70 later I had a new connector - and a new prop valve.[/QUOTE]

I just had to replace the prop valve on my 81 CJ7 when rebulding the brakes and received one with a single wire connector whereas the original switch had two. How did you wire the new failure switch into the old two wire harness? So far I have just left it disconnected. Thanks.
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Unread 02-10-2011, 08:36 AM   #14
sharktooth
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Quote:
Originally Posted by carolinacop58 View Post
Quote:
Originally Posted by ctm View Post
it is not a separate part. It happened to me when I tried to take off the connector it just broke right off. $70 later I had a new connector - and a new prop valve.
I just had to replace the prop valve on my 81 CJ7 when rebulding the brakes and received one with a single wire connector whereas the original switch had two. How did you wire the new failure switch into the old two wire harness? So far I have just left it disconnected. Thanks.
I didn't use the old wiring harness. My problem was that I had the old switch and a new, "handmade" harness because I became very weary of fires under the hood.

If it had one wire instead of two, usually the "other" side of the switch will be connected to the body. You can connect a volt meter to the pins (one at a time) and also to the body to see if you have a good continuity, and when you find the continuity, you'll attach the OTHER one to the single wire.

Or just leave it off, as others have said. I'd rather figure out where it goes, though. Just my 0.02 .
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