I have a '98 740i with the newer ABS/DSC setup (precharge pump next to the master cylinder). The brakes are currently semi locked "on". I can move the car, but it takes a bit...no coasting at all, and the front tires don't rotate when lifted off the ground. I replaced all 4 wheel sensors, disconnected the ABS module, but it didn't change anything.
I suspect the ABS module, but am wondering if it might be that DSC precharge pump that has gone bad. When I clear the codes, I can hear the relay clicking, but nothing happens. I am not sure if the ABS module is sending the signal, or if it goes the other way.
Any thoughts on where to go from here? Thanks!
Tim
Additional question: I haven't pulled all the fuses for the ABS/DSC system. If the brakes pop loose, does that confirm the module and or pump? If not, does it mean the Master cylinder is bad? Thanks!
Last edited by goinstrong; 10-11-2018 at 12:23 PM. Reason: more info
Try jumping the relay and see if you can hear the pump engage.
Disconnect the loom from the ABS/DSC unit - that should turn all the valves OFF and release the brakes. But, I've never heard of this happening before - but I have heard of the master cylinder causing this problem, so my money is on the master cylinder!
Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i
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I did disconnect the loom from the ABS/DSC unit, but the brakes didn't release. I thought they would as well. If I touch the brakes, they still activate, but actually grab when BARELY touched. Maybe it is because they are already partially activated. Your point about the master cylinder is a good one, unfortunately! It will be next week before I can get to it, but I will update.
Could a bad pressure sensor cause the brakes to be locked up? It is a lot simpler to check/replace than the master cylinder. Thanks.
Tim
It's very unlikely - with the ABS/DSC disconnected the sensors/motors/valves are de-activated and it reverts to a simple brake system - and the only thing that causes the brakes to lock is the master cylinder.
Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i
My BMW Repair YouTube Channel
My Current 840ci Sport Individual
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My E31 Repair and Information Website
My E38 Repair and Information Website
My E63/E64 Repair and Information Website
Chase - Heroes to a generation
Thank you for your guidance. It is truly appreciated. I will update the post after I have a chance to replace the master cylinder...hopefully that it solved the problem!
Tim
UPDATE: With a suggestion from my sons, I had my wife apply the brakes, and I cracked the brake line from the master to the precharge pump. A burst of air, then fluid, and the pedal went to the floor. After tightening it up, topping off the fluid, the brakes work much better. I will still bleed them.
BUT...the DSC light flashes on any acceleration. I push the DSC light so it stays on, and the car functions perfectly. Is one of the new wheel sensors bad? Any simple way to tell? I will update again after bleeding the brakes. Thanks!
Tim
Check the tyre sizes axle-to-axle, and the tyre pressures. If the rear tyres rotate faster that the fronts you will get this problem.
Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i
My BMW Repair YouTube Channel
My Current 840ci Sport Individual
My Current 650ci Sport Individual
My E31 Repair and Information Website
My E38 Repair and Information Website
My E63/E64 Repair and Information Website
Chase - Heroes to a generation
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