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  #1  
Old February 3rd, 2018, 07:30 PM
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anyone have to replace an ABS control unit before?

I'm about 95% sure the controller is bad in my 91 LSC. The ABS light is on and it refuses to give codes. Some poking and prodding determined that the ABS power relay is not being triggered.

The relay operates like any other, ground, fat constant hot from the battery, fat switched hot to the ABS controller, small switched hot from the ABS unit to pull the coil. I get no power from the ABS unit to the relay. Wiring is fine, ohmed out and even jumped to switched hot at the controller connector in the trunk to prove it. Best I can determine is that whatever inside the ABS controller turns that relay on is dead.

Anyone ever have that fault, or is there something else I am overlooking? Powering the relay manually does make the ABS light go out. I haven't test driven the car to see if the ABS actually works like that though. It was all working when I parked it a few years ago but of course sitting does nothing any favors.
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Old February 4th, 2018, 12:19 PM
W.V. Kelly W.V. Kelly is offline
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WARNING: Quick reaction answer without a lot of study.

Just to be sure, we're talking about the "Anti-Lock Power Relay" in the lower right corner of the two page diagram. The coil is powered through circuit 513 (BR/PK)* and the power goes through circuit 532 (O/Y). Both of these come from the control module which gets its power (in run) from circuit 687 (GY/Y). If you get power at 687 and not at 532/513 the problem is internal to the control module or wiring. If no power at 687 check fusible link "N". The hydraulic system (pressure switch and pump motor relay) takes its power in parallel with the control module through circuit 687 so it's possible to have one without the other.
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Old February 4th, 2018, 06:45 PM
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I have power at circuit 687 with key-on

no power at 513 with key on, key off, jumping up and down or doing anything else.

I ohm checked the wiring for circuit 513 and get less than 1 ohm from the relay socket to the ABS control unit connector in the trunk. Just as a sanity check, I connected a lamp from the br/pk wire to one of the grounded grey wires in the relay socket and jumped from circuit 687 to 513 at the control unit connector. With the key on, the light came on so the wiring seems fine from the trunk to the engine compartment.

I did try powering circuit 513 manually by jumping from another key-on hot. That did give me power through 532 and turned the ABS light off, but it didn't make the ABS actually work.

Best I can figure the control unit died for unknown reasons. The constant yellow light is because the ABS power relay is not engaged. The warning light circuit grounds through that if the relay never pulls. It will also stay on with no relay and the control unit plugged in. I haven't tried pulling codes with the relay out and the control unit in. Possibly it will blink since the light won't be on through the relay contacts.
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Old February 5th, 2018, 09:47 AM
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It's beginning to look like your initial diagnosis was correct. Do you have a spare to swap out?
W. V. Kelly
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Old February 5th, 2018, 05:31 PM
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Unfortunately no. Mostly I was just after a sanity check to make sure there wasn't some obvious "hey dummy" thing that I was overlooking before begging, borrowing, or buying one. It worked when parked but that was a while ago. I half suspect the controller perished from jump starting the car. I lost a tripminder that way once.
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Old March 10th, 2018, 04:36 PM
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so to circle this back around, I found a 91 Mark VII in the junkyard today and liberated it's ABS unit. I now have working ABS brakes again, so it was indeed the controller.
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Old March 11th, 2018, 11:39 AM
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Quote:
Originally Posted by gadget73 View Post
so to circle this back around, I found a 91 Mark VII in the junkyard today and liberated it's ABS unit. I now have working ABS brakes again, so it was indeed the controller.
Good deal. Out of curiosity, how many miles on your unit that you're replacing? Also, what kind of job is it to pull one of these? I guess I shud try to find one.
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Old March 11th, 2018, 05:57 PM
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Car has just shy of 250K. It sat a while, the battery went bad and it was jumped a few times. I suspect thats what ultimately killed it. The first time I ever jumped the car I lost the tripminder.

Removing one is dead simple. If you have the carpet in the trunk that sits under the third brake light, drop that and the control unit is mounted right to the back seat frame. 4 small screws around the outside hold it to the bracket, one bolt in the middle of the connector gets it undone. The only real trick is that its deep in the trunk, you end up laying halfway in there to access the blasted thing.
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