Brakes: 850: '96 850 with mechanical ABS problem-need help please

[Pier1]

Hello everyone,

Two week ago I went to a local garage to replace the front lower Control Arms with the ball joints. I remember the mechanic hammering the older arms to get them off and about the same to get the new ones on.

When I left the garage I noticed that the front left ABS was kicking in many times when I was about to stop, let's say when I was at 3 to 5 miles per hour or so and until full stop. It would happen only about 30% of the time and no warning ABS light would ever come on.

Last Sunday I pulled the front wheels off, removed the sensors and cleaned them. They both had a good layer of black grease on them and I was hoping the cleaning would have solve the problem. Not so, now the problem has worsened. The ABS kicks in on the front left wheel all the time now when I'm near to stop the car. It kicks in when I'm down to about 8-10 MPH I think.

I now wonder if the mechanic could have bent something when he was hammering to remove the Control Arm. By the way, the joints of the control arms where both broken and the wheels had some play before, but I never had the ABS problem prior to install the new Control Arms.

Because it is only the front left wheel with the problem, tomorrow I'm thinking to remove the speed sensor again and raise it by placing a thick washer before re-installing it so that the sensor will have a greater distance to the ABS rotating disk. Does anyone thinks this may work?

I welcome any suggestion from people with previous experience on simial problems. I confess I don't know much about the ABS. I would also be willing to disconnect the plug from the sensor if this would solve the problem.

Thank you very much for any comment.

Pier

[ShadeTek]

I would check the wiring to your front sensors. 95-96 had known issues with the cabling. You could try doing a continuity check between the ECU and the connectors. I had a similar problem and whenever I did any work at the front wheel area, I would get a prolonged ABS light on. Replacing the cabling sorted out every ABS issue I had. There even an entry in VADIS on how to re-do the cabling.

The chances are that one or both of the reluctor rings has broken. These are the castelated rings on the CV joints which rotate under the sensors; when they get old the high frequency vibration which comes from hammering nearby causes them to split and that split opens up! Once the split has opened (by 40 to 50 thou of an inch (1mm or so)) the ring becomes oval causing the voltage induced in the sensor to be greate at one part or the wheel rotation that at the rest - the computer can cope with this until one front wheel rotates slightly faster than the other then the computer thinks it has lost a signal and the ABS modulator kicks in. This also happens at slow spped - 5 to 10 mph. Quick test is to remove the sensor and drop a 1/2 inch wooden dowel into the hole, rotate the wheel and feel if the dowel moves up and down (it will only be by 10 or 20 thou of an inch (1/4 mm or so)) - if it does the ring has gone!

DO NOT put a washer or shim under the sensor! it will move the sensor further away from the reluctor ring and lower the induced voltage. This wil exacerbate the problem

[Mike Smith]

just a thought - can you swap the sensors over and see if the fault follows the sensor?
Might help

[Pier1]

Thank you all for your help. I think I took care of the problem this morning.

After removing the sensor of the problematic wheel I checked inside the hole where the sensor is mounted with a flash light while turing the wheel.
I noticed that one groove ( or tooth, whatever you call them ) of the ABS mechanism was squashed ( most likely by the hammer of the mechanic who installed the Control Arm ) and part of its metal ( about 5 millimiters ) was laying accross the next groove at about 45 degree from its original position.

I was able to free the metal from the groove and apparently that did the fix. I assume the sensor was detecting something unusual and was triggering the ABS mechanism on that particular wheel. After that I drove the car and stops and go numerous time and no more problems.

Stevelong, by the way, at first I did try putting a washer between the sensor and the mount, and you were right, it did made it worst.

What lesson did I learn out of this? Dont take your Volvo to a Muffler shop unless what you want is just to change the muffler.

Thank very much to you all for your help. I'm happy that I could save myself few hundred $$$ today. This forum is great! Happy Holidays to all of you!

Pier

[Mike Smith]

Cheers Pier
Glad to here all's OK