By: Gary Harrison
I recently repaired my 94 318is ABS unit. After searching for some time for diagnostic help, I made a couple of break-throughs. The keys were finding the diagnostic manual for the Teves Mark IV (http://www.autodiagnos.com/multitester/manuals/teves4eng.pdf) and the ease of releasing and accessing the 55-pin connector to the ABS controller. It has a very nice hinged latch which releases the connector and plenty of "slack" harness to allow you to access the connector down and use the connector as a break out box. The Bentley wiring diagram for my ABS system was also helpful.
I ran the diagnostics in the Teves Mk IV manual using wire jumpers to cross-connnect pins of the 55-pin connector and a sensitive Volt-Ohm Meter (VOM) to measure mostly resistances and a few voltage checks. Since you are measuring resistances of less than 10 ohms, it is necessary to have a good VOM which can be accurately calibrated for low resistance measurements. Everything checked OK except the Rear Axle Inlet Valve (solenoid). It showed 500 ohms when should have been 6-8 ohms. A post by Matt325 in the Bimmerforums (see below) in the following thread suggested the likley fault was the thin film wiring to the solenoids.
I removed the ABS pump module w/o too much difficulty. I found it necessary to remove the air intake box/duct and lower the evaporative emissions cannister from it's mounting to gain access. Surprisingly, the tubing connections to the ABS were not super-tight as I've frequently found on other vehicles. I was able to easily break w/ regular open-end 11mm wrench. The supply lines from the brake fluid reservoir are a PIA due to the permanent hose clamps which must be released w/ a dremel tool to grind away the retaining tab. Hoses are still very hard to release.
On my work bench, I opened the ABS pump/valve assembly by removing the base cover. This exposed the 6 solenoid valves and thin film wiring harness. It was not apparent which was the Rear Axle Inlet valve. I had to check resistances across each until I found the failed valve. A close visual w/ a powerful magnifying glass revealed no obvious flaws in the solder joints or thin film circuit. I got intermittent results by touching the thin film circuit. I tried re-soldering with no improvement. After reviewing Matt325's photos, I was unsure of the wiring arrangement and did not want to take the time to re-wire the entire solenoid set. It's very tedious and difficult to splice in wire as he's done. Wire must be restrained to the contact pin to allow soldering and in tight quarters it takes time, a surgeon's hands, and patience to accomplish. I tip my hat to Matt's patience. I decided on a hybrid scheme. I clipped the damaged thin film lead short and covered ends w/ tape and bridged two wires over the flawed thin film circuit to the single solenoid. Still took me an hour to accomplish to my satisfaction. I re-checked all the resistances to the outlet plug on the ABS pump/valve assembly (Bentley is handy here to identify the connector pins by the numbers on the connector. )
Re-installation was uneventful, except for mighty exertions and loud oaths to coax the supply lines from the brake fluid reservoir back into place.
After installation, I rechecked all resistances to 55-pin connector again. All OK. BTW, All wheel sensors checked OK.
You can check that all the solenoids and pump are working by supplying ground through the 55-pin connector (w/ ABS controller disconnected) and jumpering the ABS relay to energize the pump and solenoids. Solenoids are individually grounded at the 55 pin connector and when you jump the ABS relay (pins 2-8), you will hear them close. I also sequentially opened the inlet/outlet valve pairs (3) and cycled the pump a few seconds to eliminate any air in the ABS unit.
I was bitterly disappointed when I started the car and dreaded ABS light remained ON. Then I remembered an incident during breadboarding on the 55-pin connector. I accidentally grounded the power lead (pin 2) briefly. When I checked, I had burn't out the ABS 10A fuse. New fuse and finally NO ABS LIGHT.
It's been a long pull, but I've learned a lot about ABS. Fundamentally, ABS is quite simple. The BMW Pump/Valve assembly is 6 solenoid valves and a pump. Once the 55-pin connector is available, it's easy to check the health of the unit and wheel sensors. The Teves Mark IV manual I found on the www provides the specs, but even w/o you could puzzle your way w/ the Bentley wiring diagram.
Gary Harrison
June, 2004
Katy, TX