ABS problem

arnebret · 03-23-2002, 06:39 AM

As you might have read, my 951 is back! But there are still some things i can't understand. One thing is my ABS:
When i start the engine, i can hear all systems doing their thing and then shuting down as they should. When i start driving all is fine but i can hear some motor doing hard work. I'm almost sure it's the abs motor. It just keeps on going and when i hit the brakes the abs comes in. After driving for about 30 secs the motor stops and the abs light turns on. From that moment on i can brake normally but the abs won't function. When i turn the engine off again and start it right away the abs light turns on quicker (just a few secs) and i can't seem to hear the motor now.

I don't know how the abs functions in detail, so i was wondering if anybody could explain me how it works and what could be the problem.

Thanks!

Scott R · Scott R's Garage

Sounds like one of the rotational sensors is out of whack. You have one sensor per wheel. Pretty easy to check, just jack the car up and place a multi meter across the sensor plug, spin the wheel and look for impedance change. A failed sensor will either be open loop or not be showing an impedance change under rotation. The other that could be happening is that you have a leak in the ABS distribution manifold which is inside the passenger side fender. That would be bad and would require the replacement of the central ABS unit. Hope this helps. And good luck!

arnebret · 03-24-2002, 02:44 AM

What do you mean by leak??

Scott R · Scott R's Garage

Well the distribution block has an attachment for each brake line in the system, so four calipers, four connections, another connection to the master cylinder and a return. Inside the distribution is a cyclic valve that proportions the fluid off during ABS activation. You can have internal leaks in the distribution block, or a leak in the pump to distribution system. So the pump would attempt to pressure the distribution block and then would fail. There is also a pressure sensor in that block that can go bad as well.

On a second note I was thinking about this again last night, and I have a question for you: Have you had the brake system apart recently? If so and the system was not correctly bleed then you could have air in the distribution block.

arnebret · 03-24-2002, 10:35 AM

Hmm, the system is properly bled by a mechanic a few days ago. But the abs is doing this for over a year now. In that year i never ever touched braking related stuff and the fluid has always been at max-level. I'm not really thinking a leak here, but when i have the money and time i might dig up the abs stuff and see if i can do anything about it myself, else i will sent it to the garage again.

arnebret · 04-02-2002, 12:23 PM

I'm going to do some sensor checks now. I did a relay-check and all was fine, it switched nicely at 12v, no problem. I will now check sensor by sensor and see what ohm-readings they give me while driving. Now, what i need to know is where can i find the ABS-control unit? In the footwell? As soon as i find the unit i can unplug the large connector on it and plug my ohmmeter on each sensor very easy without having to make a big deal of trying to connect the ohmmeter on the outside etc.

Thanks for any info

Scott R · Scott R's Garage

Search the archives on resetting the light. I found that plug and followed it back up to the brain. Mine was under the dash, not sure where yours is since mine is an 87, so I dont want to say for sure.