Strange ABS problem
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Joined: Sep 2004
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Strange ABS problem
I'm having a problem with the ABS system in a 1987 corvette convertible.
The ABS system cycles occasionally while I'm driving without the brake pedal even being pressed, until the ABS light finally comes on. It's like it's going through the self test more then once.
I checked the power relay and the ground to see if power is intermittent,
but both are good. I'm thinking it maybe the control module itself.
Can I install the module from a 90 corvette to at least gain the trouble code diagnostics?
Any other ideas?
Thanks
The ABS system cycles occasionally while I'm driving without the brake pedal even being pressed, until the ABS light finally comes on. It's like it's going through the self test more then once.
I checked the power relay and the ground to see if power is intermittent,
but both are good. I'm thinking it maybe the control module itself.
Can I install the module from a 90 corvette to at least gain the trouble code diagnostics?
Any other ideas?
Thanks
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
There are a number of causes of your problem. One is an intermittent wheel sensor or the ABS computer. Most GM dealers won't work on your ABS system because no parts are available and they probably got rid of the ABS tester if they even had one. The best way to repair your ABS would be to find a place that has the ABS tester for an 87.
Thread Starter
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Joined: Sep 2004
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Will a wheel sensor cause the ABS to cycle even though the brake pedal hasn't been pressed?
Does anyone have the abs schematic for a 90 corvette?
I would like to compare it to my 87 schematic to see if it the 90 abs module can be used.
Thanks
Does anyone have the abs schematic for a 90 corvette?
I would like to compare it to my 87 schematic to see if it the 90 abs module can be used.
Thanks
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Yes the ABS will pulse the brake pressure if it finds a speed disparity between any of the wheels, no application of the brakes is necessary. A wheel sensor that suddenly has no output will cause the ABS to run its pump pressure modulator. I have never seen a schematic for the ABS. I emailed Bosch who made the ABS and got no reply.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Shop Manual should have the schematic - mine does.
'90 interfaces with the Body Module for diagnostics and added ASR; doubt it's going to work without hacking up the harness or adding to it.
I'd still guess - as you did - an intermittent at the Relay or the Module so it's Self-testing as it goes off/on, though it might be on the Module Board. Aim a hair dryer at it and see what it does. Shake/vibrate it, etc.
JFB's idea to look for a Shop with the Tester is a good one, though with an intermittent, it isn't going to be easy to find even with the Tester.
I thought I saw that NAPA offered the Module, @ $700 - yikes!
'90 interfaces with the Body Module for diagnostics and added ASR; doubt it's going to work without hacking up the harness or adding to it.
I'd still guess - as you did - an intermittent at the Relay or the Module so it's Self-testing as it goes off/on, though it might be on the Module Board. Aim a hair dryer at it and see what it does. Shake/vibrate it, etc.
JFB's idea to look for a Shop with the Tester is a good one, though with an intermittent, it isn't going to be easy to find even with the Tester.
I thought I saw that NAPA offered the Module, @ $700 - yikes!
Burning Brakes
Joined: May 2007
Posts: 965
Likes: 13
From: turn 12 at Road America or TX
I have an 86 and a 91 FSM, and it looks like the 91 (and probably 90) is an entirely different system from the earlier version. You'll need both 87 and 90 FSMs to be able to do the comparison and adaptation because the wiring harnesses are different too. Looks like a challenge to me. Fixing the 87 might be easiest.
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Joined: Sep 2004
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I guess I'm stuck with the 87 system.
I'm making a list of things to try this weekend.
I'll try re-seating the connector at the module.
I already checked the wheel sensors with a meter, but nothing looked wrong. I'm going to hook a small scope to the wheel sensors and see if anything shows up while I'm driving.
I'll also hook it up to the +12V and see if I'm loosing power at the relay.
Thanks for the help so far. Any other ideas for me to try?
I'm making a list of things to try this weekend.
I'll try re-seating the connector at the module.
I already checked the wheel sensors with a meter, but nothing looked wrong. I'm going to hook a small scope to the wheel sensors and see if anything shows up while I'm driving.
I'll also hook it up to the +12V and see if I'm loosing power at the relay.
Thanks for the help so far. Any other ideas for me to try?
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Advanced
Joined: May 2010
Posts: 89
Likes: 0
From: Morgans Point Resort Texas
87 vert
My 87 vert does something similar. It cycles each time i touch the brake pedal up until the ABS light comes on then of course it goes into the non ABS mode. My brakes still function except i do not have anti lock brakes.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Might help / might not-
Quick check:
ABS fuse(s)
Ensure the ABS wire on the brake master cylinder is connect.
Take some time at the wheel sensor:
Remove each wheel sensor and clean the crap off the tips of the sensors. The tips of these sensors pick up little tiny bits of metal that collect on the face as a result of magnetic forces. Reinstall sensor.
Unplug the sensor conector at each wheel postion and put an meter's leads on each conductor low/high ohm readins should be ~ 800 - 1300 depending on sensors positon/relation to the hubs tach wheel's teeth.
More involved:
Clean/check the tach wheels teeth on the hub positions. Dirt can build up the slots and interfere with proper signal generation.
Tough:
Find someone or a shop with the proper ABS Kent-moore tester and have them check it.
Quick check:
ABS fuse(s)
Ensure the ABS wire on the brake master cylinder is connect.
Take some time at the wheel sensor:
Remove each wheel sensor and clean the crap off the tips of the sensors. The tips of these sensors pick up little tiny bits of metal that collect on the face as a result of magnetic forces. Reinstall sensor.
Unplug the sensor conector at each wheel postion and put an meter's leads on each conductor low/high ohm readins should be ~ 800 - 1300 depending on sensors positon/relation to the hubs tach wheel's teeth.
More involved:
Clean/check the tach wheels teeth on the hub positions. Dirt can build up the slots and interfere with proper signal generation.
Tough:
Find someone or a shop with the proper ABS Kent-moore tester and have them check it.
