What keeps the ABS pump from going into test run?
Thread Starter
Racer
Joined: Oct 2007
Posts: 442
Likes: 15
From: FL
What keeps the ABS pump from going into test run?
The antilock light in the DIC of my 1986 AT coupe comes on right after taking off - but I do not hear the pump do it´s test run.
The system does the initial testing cycle upon turning the key, the lights comes on and goes out as it should.
All fuses are good, the three relays are good, the brake fluid level is good, the wheel sensors resistance values are good, the lateral accelerometer´s horizontal resistance value is good, the pump cycles when I jump pin 30 and 87 of the pump relay with the engine running.
What do I overlook? Can the ABS module be bad?
Pls help, I am out of ideas.
The system does the initial testing cycle upon turning the key, the lights comes on and goes out as it should.
All fuses are good, the three relays are good, the brake fluid level is good, the wheel sensors resistance values are good, the lateral accelerometer´s horizontal resistance value is good, the pump cycles when I jump pin 30 and 87 of the pump relay with the engine running.
What do I overlook? Can the ABS module be bad?
Pls help, I am out of ideas.
Drifting
Joined: Jan 2021
Posts: 1,314
Likes: 384
From: Northern California
I think you are right, you may have a failing ABS Control Module. The wiring diagram and diagnostic chart for my '89 is what I'm looking at, but I don't think the system logic changed much between '86 and '89.
The initial test and DIC light on/off at Key On is correct, but it's triggered via a different input on pin 15 of the module directly from the ignition switch. The fact that the fuses and relays are good is somewhat confirmed by the fact that you can get the pump to cycle jumping pins 30 / 87 - which energizes the pump motor via the relay pins you've jumped. But you're bypassing the control module that way, which normally energizes the pump motor relay by grounding pin 85 on the relay itself. If that's not happening, it means that either pin 85 on the relay itself is not getting a ground from the Control Module, or that the relay is faulty / pin not connecting well enough to provide ground there. Is that relay on the modulator valve replaceable?
Just my two cents from what I see on the diagrams.
The initial test and DIC light on/off at Key On is correct, but it's triggered via a different input on pin 15 of the module directly from the ignition switch. The fact that the fuses and relays are good is somewhat confirmed by the fact that you can get the pump to cycle jumping pins 30 / 87 - which energizes the pump motor via the relay pins you've jumped. But you're bypassing the control module that way, which normally energizes the pump motor relay by grounding pin 85 on the relay itself. If that's not happening, it means that either pin 85 on the relay itself is not getting a ground from the Control Module, or that the relay is faulty / pin not connecting well enough to provide ground there. Is that relay on the modulator valve replaceable?
Just my two cents from what I see on the diagrams.
Thread Starter
Racer
Joined: Oct 2007
Posts: 442
Likes: 15
From: FL
Thank you for the educated reply.
All relays have been swapped out for new or known good ones and cross-swapped back in to find the silver solenoid relay being bad.
I jumpered pin 30 and 87 just to see if the pump itself is functional. I agree that the pump motor relay does not get switched ground, my suspicion is from a defective ABS control module.
Two thoughts on that: First, what inputs could keep the control module from switching on the ground? Lateral accelerator, wheel sensors and what else? Second, what exactly does the solenoid relay trigger and can this assembly have any effect on the pump not doing the test run?
All relays have been swapped out for new or known good ones and cross-swapped back in to find the silver solenoid relay being bad.
I jumpered pin 30 and 87 just to see if the pump itself is functional. I agree that the pump motor relay does not get switched ground, my suspicion is from a defective ABS control module.
Two thoughts on that: First, what inputs could keep the control module from switching on the ground? Lateral accelerator, wheel sensors and what else? Second, what exactly does the solenoid relay trigger and can this assembly have any effect on the pump not doing the test run?
Last edited by colonel328; Apr 26, 2022 at 01:17 PM.
Drifting
Joined: Jan 2021
Posts: 1,314
Likes: 384
From: Northern California
Nothing else - just the lateral accelerometer and the wheel speed sensors. If those are good, then the only possibilities are 1/ failed control module logic internally (bad board or IC, etc.) and 2/ open in the circuit for wire 913 (black / lt green stripe) which is on control module pin 28. Remember, this is for 1989 so check your configuration for 1986 carefully.
I lean more towards control module failure, because in addition to all this stuff we're talking about for activation of the pump motor, the control module also monitors that circuit to see if the motor activated successfully on circuit 985 (black / lt blue stripe) on control module pin 14. If that circuit does not see the motor activating when commanded, the control module logic should detect that and set an error - which it's not doing. Soooo....
In your very first sentence above, is the DIC light coming on when the vehicle starts moving and then going out? Or is it staying on? If it's staying on then that means the module did indeed see the failed command to run the pump. If that's the case, then I'd check that wire 913 closely.
I lean more towards control module failure, because in addition to all this stuff we're talking about for activation of the pump motor, the control module also monitors that circuit to see if the motor activated successfully on circuit 985 (black / lt blue stripe) on control module pin 14. If that circuit does not see the motor activating when commanded, the control module logic should detect that and set an error - which it's not doing. Soooo....
In your very first sentence above, is the DIC light coming on when the vehicle starts moving and then going out? Or is it staying on? If it's staying on then that means the module did indeed see the failed command to run the pump. If that's the case, then I'd check that wire 913 closely.
Instructor
Joined: Oct 2018
Posts: 225
Likes: 81
From: Vancouver, WA
Without this tool, you will be in a "search and destroy" mission and have to test every component and wire in the system,
You may get lucky and find the culprit, but it will take some time.
I bought one off of ebay, but I am 3000 miles away from you. There must be someone near you in FL that can let you borrow one.
Thread Starter
Racer
Joined: Oct 2007
Posts: 442
Likes: 15
From: FL
I lean more towards control module failure, because in addition to all this stuff we're talking about for activation of the pump motor, the control module also monitors that circuit to see if the motor activated successfully on circuit 985 (black / lt blue stripe) on control module pin 14. If that circuit does not see the motor activating when commanded, the control module logic should detect that and set an error - which it's not doing. Soooo....
Do you have access to a 1986-1989 Corvette ABS Tester (Kent-Moore #J35890)? This is the ONLY way to read ABS codes for 1986-1989 Corvettes.
Without this tool, you will be in a "search and destroy" mission and have to test every component and wire in the system,
You may get lucky and find the culprit, but it will take some time.
Without this tool, you will be in a "search and destroy" mission and have to test every component and wire in the system,
You may get lucky and find the culprit, but it will take some time.
Thread Starter
Racer
Joined: Oct 2007
Posts: 442
Likes: 15
From: FL
Circuit 913 has continuity from the control module connector pin 28 to pin 85 of the pump motor relay.
I have ordered a used ABS control module and hope, it will do the trick. Will take some days to arrive.
I have ordered a used ABS control module and hope, it will do the trick. Will take some days to arrive.
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Drifting
Joined: Jan 2021
Posts: 1,314
Likes: 384
From: Northern California
If you have continuity and the connections were good, then I think you're heading the right direction with the control module. Good luck -
Thread Starter
Racer
Joined: Oct 2007
Posts: 442
Likes: 15
From: FL
The replacement ABS module arrived the other day and I just quickly threw it in and took her for a spin: no change!
I did not have the time to look into this any further yet, but I think it may make sense to mention, that my speedo is out and I have an error code related to the VSS. Could a
missing VSS signal keep the ABS pump from coming on?
I did not have the time to look into this any further yet, but I think it may make sense to mention, that my speedo is out and I have an error code related to the VSS. Could a
missing VSS signal keep the ABS pump from coming on?
