Bleeding the ABS/ABD system

[edc153]

As an addendum to my previous posting about repairing the brake booster, I thought it would be helpful to some to show how the ABS/ABD assembly functions.








As you can see, there are 10 solenoids used to control the fluid. There are 12 pins on the connector. In this case this means 2 of the 12 are 12v+, the rest go to ground within the ABS/ABD control unit. The chart shows which pin is which. I am 95% sure using this chart enabled me to completely bleed the system. All that was necessary was to provide battery + power to pin 15, and ground the solenoids for each circuit while allowing the fluid to be pumped through that circuit by the ABS pump. The pump has its own ground so I connected battery + to pin 3. Doing this moved fluid through each circuit speedily.

I was hoping my ABS and ABD warning lights would go out after doing this; they remain lit.

If anyone can tell us why doing this cannot actually completely bleed the WHOLE brake system, I, for one, want to know why. And of course, I would really like to get rid of those last two glowing dash lights.

Advice anyone?

[David Br]

Thanks for the informative post. Do you have an update on your problem? I too have air in my brake system (very mushy pedal) which occurred after a brake caliper replacement and I have tried all the obvious remedies so far without success. I've bled the system repeatedly, new m/c, different ABS pump, and most recently, the process you described above. I've read in other forums and posts that the ABS controller could cause a soft pedal. Have you tried that yet?

David

[HelmetHead]

Originally Posted by David Br

Thanks for the informative post. Do you have an update on your problem? I too have air in my brake system (very mushy pedal) which occurred after a brake caliper replacement and I have tried all the obvious remedies so far without success. I've bled the system repeatedly, new m/c, different ABS pump, and most recently, the process you described above. I've read in other forums and posts that the ABS controller could cause a soft pedal. Have you tried that yet?

David

Experienced this same issue after caliper replacement. The pump needs to cycle to get the air out...I ended up taking the car to a mechanic with Durametrics equipment.

[edc153]

I resolved my abs/abd dash light problem with the help of TRE Motorsports. They plugged into the car's obd port with their genuine Porsche multi year code reader computer, which told us that the problem with the dash lights was an apparent broken circuit from the right rear abs sensor. In reality, it was two things causing the light to come on. The first was a corroded terminal lead on the abs/abd control unit; the second was a wire fault just behind the passenger seat.

David, you might try deliberately actuating the abs unit on a clean patch of dirt somewhere, or perhaps an icy parking lot. This will get the fluids moving in the system. There are times when it is seemingly impossible to get all the air out. In my case, I got it first try. Several people drove the car and tested the brakes besides me and all seemed well. And this after having reassembled the whole brake system from it's individual parts.

It is possible that the abs/abd unit has some really old, dried out brake fluid gumming up the circuits.
I can't imagine how the electronic control unit could cause a soft pedal. It's only function is to open the internal fluid solenoids during a wheel slippage event. The rest of the time, the system is inert, and the brakes act like a non abs system. A simple test of your theory may be to unplug the abs/abd control unit, or perhaps simply remove the correct fuse.

Lights out forever!
I hope

[David Br]

Good to hear that your problem is solved. I've tried to get the ABS to turn on in order to cycle the solenoids but it feels as though I'm getting wheel lockup on the fronts and that the rears are not receiving enough braking input (ie, a bias to the front). I have a used ABS Control unit coming today or tomorrow and I can do the bleed, drive and skid, bleed routine. I'll report back as soon as I have some info.
David