'91 GMC K2500 Brake Indicator Problem

[CSR]

The brake indicator in the dash of my '91 GMC K2500 either stays on or blinks. When it is on, applying the parking brake makes the light brighter. When it blinks, it is 6 short blinks followed by a longer one (7 total).

I jumped the A and H terminals and got the same 7 blinks. I have read on this site that the problem could be either a shorted isolation valve or bad ECU.

First off, what is the isolation valve and where is it?

Second, how can I tell if it is the isloation valve or ecu?

Thanks, Chris.

[ChevLoRay]

Well, now......for a new guy you ask a lot of questions!!!


Seriously, welcome to the board. You have come to the best place to get your questions answered. Hopefully, it won't take too long for a knowledgeable person to chime in....we have several who are experienced mechanics with the brainpower to 'figger' out your problem and offer solutions as needed.

[ChevyTech]

You have done well to get the code and know what it pertains to.

Code 7 = Shorted isolation valve or defective ABS module.

Code 7 involves a shorted isolation-valve circuit or faulty EBCM. A shorted isolation circuit could be caused by the coil shorting on itself, or the wiring to the coil shorting.

The isolation and dump valve are in the same part. It’s located out by the master cylinder. The isolation/dump valve has 4 wires going to it, has two solenoids on it, and brake lines connecting to it. The combination valve has one wire going to it and is more long and skinny.

This site has general information about RWAL.
http://www.babcox.com/editorial/tr/tr89850.htm


If you have a good digital volt ohm meter here are some tests you can do.

Key off, isolation dump valve unplugged, probing the wire connector that is on the Isolation dump valve.
Terminal A to terminal D - should be 3 to 6 ohms.
Terminal A to the valve housing – should be greater than 50,000 ohms.
If the resistance is to low on either of these tests, the valve is shorted. – replace it.

Isolation dump valve disconnected, Key on, ALDL Terminal H connected to terminal A – test voltage between terminal A and D of the harness that goes to the isolation dump valve. You should have one volt or more. If it fails this test it points to the RWAL module.


Welcome to the message board.

[CSR]

Thanks Chevy Tech. You sure know your braking systems.

Can I assume terminal A is the top one and D is the bottom one on the plug?

[ChevyTech]

Originally Posted by CSR

Thanks Chevy Tech.

You’re welcome!

Originally Posted by CSR

Can I assume terminal A is the top one and D is the bottom one on the plug?

No, it is the other way around when looking at the wires on the RWAL module. It is the bottom connector on the RWAL module, and the top wire is “D” and the bottom wire is “A”.

A = Green wire
B = White wire
C = Blue wire
D = Black wire

Or better yet- the position they are in on the RWAL module:

D = Black wire
C = Blue wire
B = White wire
A = Green wire


The person that designed these brake systems must have been dyslexic because many items are backward of what is commonly expected. And on newer trucks the technical data, DRAC ratio inputs, and DRAC jumpers, are backward and contradict logical order and some of the documentation.

[CSR]

Thanks again ChevyTech.

Heres's what I have so far.

We pulled the isolation valve harness out of the ecm and got 4.6 ohms across the A and D terminals (key off). So I think the valve is ok.

I am a little confused on the ecm test procedure. We unplugged the valve harness from the ecm, jumped the A and H terminal under the dash and turned the key on. I thought your instructions were to test the A and D terminals on the valve harness but I wouldn't expect to see any voltage with the harness unplugged. We went ahead and tested the A and D terminals on the ecm and got 0 volts. Did we do this correctly?

Thank you very much for your help, Chris.

[ChevyTech]

Yes you did it correctly.

My wording was poor, and you are correct that there would be no voltage on a harness that is unplugged. The test you did for voltage at the module terminals A to D and finding not voltage does indicate a problem with the RWAL module.

DO THE OTHER TEST TOO.
Make sure the terminal A it not shorted to the valve housing or it may destroy a new module as soon as you connect it and power it up.

If you have a U-Pull-It close by, many of the RWAL modules seem to be interchangeable, and I believe the same part number fits many vehicles except the 1 ton trucks.

ZPRWAL on 30 series HD models have a different module and the plastic where the wire harnesses plug into it is a different color.


Beginning in August 1990, a software change was made in the ABS controller on all General Motors RWAL equipped vehicles. The change transforms codes 6, 9 and 10 into "soft" (non-latching) codes that only illuminate the warming light as long as the fault is present. If the fault is intermittent and no longer exists, the light will go out with the next ignition cycle. The conditions under which the codes are set remains the same, as do the basic diagnostic procedures.
A scan tool must be used on the newer GM applications to read any soft (non-latching) codes that might be present because grounding the "H" terminal on the ALDL connector erases soft codes.
Source:
http://www.babcox.com/editorial/tr/tr89850.htm

The new Modules are different but I think are totally interchangeable with the older design. I would prefer an older one for working on the system so I could read all code without a scan tool, but then I would prefer a newer one for driving the truck so false codes do not get set.

[CSR]

ChevyTech, once again thanks for your help.

How can I tell if the A terminal is shorted to the valve housing? Will it be visible like a burned spot on the valve or is it an electrical check?

[ChevyTech]

Originally Posted by CSR

ChevyTech, once again thanks for your help.

How can I tell if the A terminal is shorted to the valve housing? Will it be visible like a burned spot on the valve or is it an electrical check?

Terminal A to the valve housing – should be greater than 50,000 ohms.
If the resistance is to low on either of these tests, the valve is shorted. – replace it.

An electrical test.
With the harnes unplugged from the module, this would be testing between WIRE "A" to the valve and the outside aluminum housing of the Valve. They should not be electrically connected.

[CSR]

If that is the same test as the one you mentioned earlier (see below) then it must be the ecm. That test didn't show a ground.

"Isolation dump valve disconnected, Key on, ALDL Terminal H connected to terminal A – test voltage between terminal A and D of the harness that goes to the isolation dump valve. You should have one volt or more. If it fails this test it points to the RWAL module."

[ChevyTech]

There are multiple tests.

One test is between wires “A” and wire “D” of the valve testing the resistance. This tests the valves coiled winding end to end.

The test between terminal “A” and Terminal “D” on the module tests the module. Which it tested to be bad.

The test you need to do is testing the resistance between wire “A” of the valve, and the aluminum outside housing of the valve, with the harness unplugged from the RWAL module. This will test to see if the insulation has failed in the valve so that that coiled winding is “shorting out” (in electrical contact) with the metal outside of the valve, which could be why the module failed. The test should show that there is no connection when doing this test, if the part is good.

[CSR]

ChevyTech, we have done all of the tests you recommended and all indications are a bad control module. I ordered one tonight and it will be in tomorrow.

Is there anything special about installing the module besides disconnecting the battery cable while you do it?

I really appreciate your help. Sorry for bombarding you with so many questions. I don't know much about ABS and know even less about automotive wiring. I have learned more from your posts the last couple of days than I knew before.

Thanks again and keep your fingers crossed that the light will stop blinking, Chris.

[ChevyTech]

You are welcome!

Is there anything special about installing the module besides disconnecting the battery cable while you do it?

That is all you need to do.

All the questions are ok with me. I save what I write for the next person that asks the same question, so you are helping me refine my file about RWAL systems.

It is much more enjoyable for me to help when the replies come back fairly quickly, as this thread did.

Let me know how it goes.

[ChevLoRay]

Thanks for all the help you gave, Chevy Tech. Looks like another satisfied customer.

[CSR]

ChevyTech, you are the man!! I finally got around to replacing the ABS module today and that took care of the problem. No more blinking brake indicator and the rejection sticker is off the windshield.

Thank you very much for your help.

I don't know if you know as much about AC systems as you do brake systems but that will be the next problem I tackle.

Thanks again, Chris.

[ChevyTech]

You’re welcome. A thank you is always appreciated.

Glad to hear things worked out, and thanks for letting us know what the problem was.

I will watch for your AC post. I am busy to the point I may not be able to get to the message board during the week but will try to reply to at least a few threads on the weekends.