Antilock works @ low speed not at high speed

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Antilock works @ low speed not at high speed

Postby robertbweltzien » Mon Oct 17, 2016 5:08 pm

Hello fellow MK VII enthusiasts. Any one ever have this problem? On my '89 LSC, the amber antilock light comes on intermittently and when off, the system functions normally at low speeds, i.e. 20 to 30 mph, and on a sandy road the antilock feature works. A hard brake application on clean pavement at highway speeds causes the amber light to come on and although it goes out again, the antilock feature doesn't function and the brakes lock up. The system seems to work normally otherwise. It has fresh, clean fluid (flushed several times) a fairly new accumulator, with three brake pumps between accumulator pump activation, good pads that are half worn, rotors and calipers (which have all been replaced a few years ago) and the standard self diagnostic "bump" is felt in the brake pedal a few seconds after starting. The amber light also rarely goes on unless the vehicle is moving. The red e brake light never comes on unless the e brake is set. For starters should I check all the sensors for continuity? The factory manual requires a special computer that is out of reach for most of us and the diagnostic procedure is a nightmare and who likes fishing expeditions unless you're actually fishing? It doesn't seem as easy to work on as the air suspension!

Bob
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Re: Antilock works @ low speed not at high speed

Postby CDW6212R » Mon Oct 17, 2016 9:55 pm

I've never heard of that odd symptom, but I agree that the ABS system is the toughest to trouble shoot. I would also suspect the ABS sensors or their wiring circuits. That sounds more like a control module interpretation issue. Either the module is bad, or the signals it is receiving are causing the faults.

Remember that the basic logic of the ABS is to disable the system instantly if it detects any fault in the expected sensor signals. That's all basic logic, and the actual shop manual diagnostic procedure is the best way to handle it if you can. See if you can find that in the manual first, and read though it. I'd start there.
Don
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98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Antilock works @ low speed not at high speed

Postby robertbweltzien » Tue Oct 18, 2016 2:44 pm

Will do, Don. At least there's an elaborate algorithm in the manual that doesn't require special electronics. Interestingly, when there is a wheel lockup on clean pavement it is always the right rear wheel so that gives me a place to start. Thanks again for the reply, Don.

Bob
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Re: Antilock works @ low speed not at high speed

Postby oldschool1 » Tue Oct 18, 2016 6:19 pm

I too have not heard of the issue that you're having BUT, I do have an idea about your right rear lock up issue.

Your left rear may be out of adjustment. Test this out by stopping using the ebrake (in a parking lot or somewhere with no traffic). Pay attention to the steering wheel during braking. It should NOT pull one way or the other. In your case, it sounds like you could have a left rear caliper sticking or clogged.
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