ABS Problem

[calypso325is]

My ABS is not working - the rear wheels lock up under hard braking. I had a BMW shop read the fault codes and there are no faults. They think the problem is an incompatible hydro unit. The part number of the installed unit is 34-51-1-090-428, while the BMW parts database indicates the correct part is 34-51-1-090-427 for E36s manufactured up to 08/94.

 

Before I go searching for a replacement ABS hydro unit, I was wondering what others have installed in their E36s. That is, does the part number end in 427 or 428?

 

Also, any suggestions as to what the problem might be would be greatly appreciated.

[matt325]

Did you recently replace the hydro unit? Was the ABS working in the past? Did this problem just start?

 

Does the ABS light come on at any time when the rear wheels are locking? Can you feel the ABS working on the front wheels?

 

This sounds like an odd problem for an ABS. Have you checked the brake lines, calipers for sticking pistons, etc.?

 

You could have a problem with the wiring in the hydro unit where the solder joints have cracked and its enough to keep the ground preventing a fault code/light, but when the solenoids go to activate, the connection doesn't work. I have had that happen after I have rewired an ABS hydro unit.

 

I have a 1/94 build 325is, but am not able to get to the car for a few days to check. IIRC, either part number will work, but I am not positive.

[laze1]

I was having some brake-lockup, but that went away after rebuilding the calipers...

[calypso325is]

In response to matt325's questions:

 

- I bought the car about a year ago (4/94 build). The previous owner had paperwork showing that both the ABS hydro unit and control module were replaced in 1999 by the dealer. The ABS has never worked since I've owned the car. In fact, I found that the ABS bulb in the dash as well as both ABS relays were missing after I figured out I was having problems. I installed a new bulb and bought new relays to replace those that were missing

 

- The ABS light does not come on during braking (the light does come on and then go off when starting the car like its supposed to). I can't feel the ABS working on the front wheels.

 

- The flexible brake lines have been replaced. Front calipers have been rebuilt and rear calipers have been replaced.

 

- I checked the resistances through the solenoids in the hydro unit from both the 55-pin connector at the controller as well as the 12-pin connector on the hydro unit. All were within the specifications shown in the Teves Mk IV manual. I also applied 12 volts to solenoids from both of these connectors. I could hear each solenoid open/close when voltage was applied/removed. So the wiring appears to be okay.

 

I should also mention that when the rear wheels do lock up, it causes the car to rotate very quickly to the right. Both rear wheels lock up simultaneously.

 

Any insight would be appreciated.

[matt325]

Seems odd that the pump and the ECU were replaced, and then someone went through all the trouble to deactivate the ABS by pulling the relays and the bulb.

 

Sounds like you have all the right troubleshooting of the mechanical stuff. That just leaves the ABS ECU. Have you tried swapping it out with a known good one?

 

Oh, and have you checked and cleaned all the sensors? You can put a multimeter across some pins on the connector and spin the wheels and make sure voltage varies.

 

I feel for you man, ABS problems suck. Good luck.

[dbgeek]

Mine was missing the ABS bulb (and locking up rear tires). When replaced, it was on constantly - found a dead wheel sensor front left corner. Replaced, and now have wonderful brakes.

[calypso325is]

I replaced the control module with a unit that I pulled out of a '93 E36 in a junk yard. I have no assurance that this unit works, but it seems unlikely that both the original and the replacement would be faulty.

 

I have checked the resistance in each wheel sensor from the 55-pin connector, and all were within the allowable range. I'll try spinning the wheels to see what voltage signal results. Also, I'll pull the sensors and clean them. Is there any trick to getting them out? I can't get them to budge even after spraying with WD-40.

 

Thanks, all, for the suggestions.