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1987 M6 ABS issues

Discussion in 'E24 (1977-1989)' started by 025570, May 2, 2018.

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    025570

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    I had my 87 M6 restored by Korman Autoworks and they did a stunning job with the entire car. One issue is still plaguing us though. The brakes occasionally won't release completely and drag, heating up the rotors and causing a significant shimmy. They thought they had it fixed when they found a loose pin in the ABS control unit but the issue came back. Everything at the wheels are new and we have replaced the hydro unit. The issue got less frequent but still is there. Has anyone encountered this issue? The car is beautiful but this issue concerns me and makes the drive not anywhere near as pleasant.
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    charlson89

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    Does this seem to occur more when the brakes are hot? Meaning after driving for awhile, not when left to sit or vehicle stored.
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    MGarrison

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    I'm not totally sure, but I think ABS is disabled if you disconnect even one of the sensor leads. If it's an issue with the ABS brain, I'd think that should be pretty easy to test by disconnecting one or all the sensor leads and seeing if it happens. ABS warning light should be on if ABS is disabled/not functioning. I don't know offhand if BMW had any additional traction control systems such as ASC on premium models like your M6 or the 7's in '87; in the US, '86 was the first year they put ABS on the 3's, I think it was on at least 7's at least a year or more before they added it to the E30's. Even so, if it does have something like ASC, I believe any braking component of that system is tied to the ABS system - if no leads are connected, there's nothing to generate the signals that normally actuate the hydro unit. You could also disconnect the plug from the hydro unit to test, if it's like the E30's, there's at least one small threaded bolt to unscrew before you can prise the plug free, maybe more. The old plastic on the abs sensor lead connections and the harness plug for the hydro unit has the potential to be fragile, if they're original or old; exercise care, lest connectors break and you find yourself having to get new ABS sensors, or worse, plugs for the wiring-harness-side connectors, and you really don't want to damage the hydro-unit harness plug/connector, although fwiw, I had no problems with unplugging the hydro-unit plug on my '87 E30 a few years ago; take your time. Assuming they're like the E30's, the ABS sensors are easier to disconnect, but the rears might be buried underneath the rear seat or something like that, not sure if they're easily accessible or not. The fronts are probably like the E30, easily accessed in the engine bay. I suppose if you're getting a shimmy in the steering wheel, it could be one side's clamping more than another, or, variable abs application between either front wheel... unless the shimmy is from a rotor getting warped due to the malfunction. As for any test driving with the ABS off, that's probably better done on dry roads as opposed to a rainy day.

    If you disconnect the sensors & the symptoms stop, I'd be thinking ABS brain unit, wouldn't imagine sourcing another used one off ebay or parts houses would be too hard to come up with without too much expense, and could swap in a diff. brain, see what that does.

    If it still does it after disconnecting the leads, then maybe try pulling the hydro-unit plug, just to eliminate the ABS brainbox generating actuation signals for some oddball reason of bad circuits, grounds, or whatever; phantom self-sourced signals or elsewhere, but not generated by the ABS sensors. Now, if the harness-side leads were doing it somehow from somehow picking up a signal or something weird like that... well, okay, but that would seem unlikely... voltage has to come from somewhere. And, ugh, having to run replacement harness leads through the car to the brainbox?? Ick. That's the kind of thing that would make you think a variable short from a harness lead's insulation abrading through somewhere, or something like that - not expected since the cabling is secured and clamped in place.

    With a new hydro-unit, you wouldn't expect that to be the cause, and if that's unplugged and you still get it, then you're down to something with the brakes themselves... master cylinder, sticking pistons (wouldn't seem likely with all new calipers, etc.), but, something with the mechanical side of the brakes apart from the ABS system.

    Charlson can add insight here if I'm off-base or missed something obvious in thinking about that part of it; presumably his response query is relating to considering potential issues for the brakes between operating warm/hot, or cold. :)
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    025570

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    Thanks for the quick responses guys.

    Does not seem to be related to brake temperature per se. If I'm very careful with braking, and let it roll before final stop, it seems to happen less. Maybe pointing to the control unit and sorting out signals.

    I will try some of the ideas MGarrison has. Allen at Korman's seemed to think swapping control units might uncover the issue, however I'm reluctant to buy one for the sake of the experiment.
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    charlson89

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    I have seen a master cylinder cause a similar issue but seem to happen more with hot brake fluid. The two stage master first cylinder uses the foot pedal pressure to hydraulically actuate the second piston. I have seen the second piston stick and the brakes thus sticking. Mgarrison has given great advise on ruling out the ABS/traction control devices.

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