2000 GMC Jimmy pedal fade when engine on

Z06SCHMAK

MEMBER
2000 GMC JIMMY
6 CYL
4WD
MANUAL
108,000 MILES

Brake problem: Soft pedal that will travel to the floor when engine is on. I have bled the system, and changed the master cyl. The pedal feels great when the truck is off. ABS and Parking Brake lights come on after a few minutes of driving. Proportioning valve? The vacuum check valve seems ok on the booster. The vac line seems fine too. What do I need to do? Thanks a ton.

5 Replies

SWILLIAMS

MECHANIC
597 POSTS

Are you losing any fluid? If not I would suspect a bad ABS module that is leaking by internally.

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Saturday, January 3rd, 2009 AT 4:24 PM

Z06SCHMAK

MEMBER
3 POSTS

Thanks a ton. No fluid leaks that I know of. No drips atleast. Had the wheels off the other day, changing the master cyl, but didnt notice anything. Is there any way to 'check' that module? Dont want to buy a new one just to see if that fixes it. Know what I mean? Autozone, etc. Cant read the abs codes, and the dealer wanted 80$ just to read the codes. I would assume that's the real way to check that module?

Thanks again

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Sunday, January 4th, 2009 AT 8:01 AM

SWILLIAMS

MECHANIC
597 POSTS

Yes it is possible to test it, BUT you need a scan tool and the test procedures to cycle it through the tests.

However you could just disable the ABS and see if the brakes are OK. That will give you a better idea if they are the problem.
If you look at the EBCM you will find there are two connections. One is the wheel sensor inputs and the other is the power and ABS light controls. Disconnect the power feed and see if the brakes now work OK. If they don't leak down and they stop you fine the ABS unit is the problem. Just remember that you don't have ABS.

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Sunday, January 4th, 2009 AT 9:07 AM

Z06SCHMAK

MEMBER
3 POSTS

Hmm.
Is the ebcm the one under the dash right in middle of the car? There were 3 plugs, 1 small and 2 identical larger. The one small one didnt seem to do anything, except the radio didnt turn off when shut car off and open door. And the other two wouldnt let the car run. The brakes were the same with the small plug undone.

Maybe I have the wrong module in mind.

Sorry i'm being such a hassel.
Thanks a lot

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Saturday, January 24th, 2009 AT 1:01 PM

SWILLIAMS

MECHANIC
597 POSTS

The EBCM is located under the hood. On the drivers side fender. It has the lines from the master cylinder coming in on two ports and the lines out to the brakes on others. You will find 2 wiring harness connectors that go to it. One has wires that are all the same size, that is the input line from the sensors. The other will have two larger power leads in it. Disconnect that one and test your brakes again. With that disconnected the ABS unit will not effect the brakes.

The module you found is the body control module. Different item.