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bleeding ABS brakes

Discussion in 'E30 (1984-1993)' started by mvinco, May 15, 2008.

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    I will be changing out my brake fluid soon, is there any extra steps or tricks that need to be done to get a complete fluid change with a car with ABS. It's a 88 325is
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    You can bleed the fluid in an ABS equipped car just like a car without ABS. Yes, there will be some old residual fluid in the ABS unit that is not flushed but it is a small amount.

    If you really want to flush *all* of the fluid you will either need to activate the ABS pump while the car is stationary and you are bleeding the brakes or bleed the brakes, take the car for a drive, find a loose surface (such as gravel) and brake hard enough to engage the ABS. Drive the car home and perform the bleeding procedure again.

    I do not know how to activate the ABS pump while the car is stationary but I believe it can be done by jumpering the ABS relay. YMMV. Attempt at your own risk. Etc.

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    I think BMW has some special tool for activating the ABS unit. I've bled my brakes countless times, never had a problem from whatever residual's in the ABS unit. However, when it's rainy, (and there's no traffic anywhere near me, or behind), and I have plenty of room ahead, I'll stomp the brakes a time or two here and there to get the ABS going, and make it cycle.

    I would not recommend wetting down the driveway and making a auto-x type launch and run towards the house, or the street, expecting to get the ABS going, (just in case anyone gets any 'bright' ideas).

    Wait for a rainy day, really. ;)

    agranner guest

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    Good advice about activating ABS periodically when it's safe to do so. I'll do this in the future.

    When the ABS is activated, is it only the front circuits that are affected? Or are all four lines pumped? If only the slipping wheels are pumped, you could be only cycling brake fluid in the front circuits. Maybe trying to induce some over-steer (trail braking on a wet skid pad) while exercising the ABS would flush/cycle all four brake circuits. What do you think?

    -- Aubrey Granner
    '93 325i 5sp
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    This is more guess than specific knowledge, but, here goes anyway -

    Can't speak for an E36 - on my E30, there's a single brake fluid line to the rear, which then splits to the calipers.

    When I've had ABS engage, I don't think I've ever had the rears lock -

    So, if the rear wheels don't tend to lock up, then either the ABS unit must affect/modulate the brake line pressure to the rear line to keep the bias such that the rears would tend not to lock, or the bias is set so the rears are unlikely to lockup.

    However - I doubt the ABS unit is all that selective; I'd think that if you engage ABS, it's going to do everything that it does, and it would be unnecessary to do anything further than simply getting the ABS going.

    I think I remember some BMW ad or demo showing a car with 1/2 the car on a slick surface and 1/2 on a dry surface, to demonstrate the control offered w/ ABS under such a driving condition/situation. Can't say I've tried something like that for my E30, but whichever vehicle it was, some modulation would have to be going on at the rear wheels so as to not have a skid induced.

    In any case, if your car doesn't have individual ABS sensors and pickup wheels on the rear wheels, then it's unlikely it's capable of having the rear wheels individually modulated.
    Edit (2019) - a more definitive answer now - the ABS unit acts on each front brake circuit individually, but both rears together; there are 3 fluid lines going out of the ABS unit, one to each front caliper, and one line to the rear, which then splits to each rear caliper. If the ABS brain detects slippage from either rear wheel, the ABS unit modulates the single rear line, and thus both rear calipers. ABS was made standard across the BMW line for '86 models, so '84-'85 E30's, lacking the system entirely, obviously won't have the wheel sensors.

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