Testing the ABS system - PeachParts Mercedes-Benz Forum

84 500SL Euro · #1 10-17-2008, 10:08 PM

I have the Service Manual Library MB CD and section 42-720 concerns testing the ABS system and utilizes a 'special' ABS tester with adapter.

I don't have access to such and hope that someone not only knows how to do the 15-20 tests cited but without the tester described and has posted this information thus making it accessible to all, or at least the 'essential' tests...... that a well armed DIY person can do.

Specific references [hopefully for 107s] ?

Gratitude to all,

Courtney

pwogaman · #2 10-17-2008, 11:28 PM

Are you having problems with the system? If so, and you can trouble shoot based on the specific symptoms then there would not be that much need to go through all the tests.

Robert Squires · #3 10-18-2008, 12:54 PM

* In my experience, the ABS isn't a very troublesome system. But when it starts acting up, there are a few basic things you can do to take care of many of the symptoms. Although these comments relate to many cars, they specifically relate to the questioner's R107 car.
* First, clean the speed sensors. One at each front wheel and a third at the differential. They attract metal particles and grime and then don't sense wheel rotation correctly. Also, make sure that the wire connections to the sensors are clean and secure.
* Flush the brake hydraulics once a year. I use a liter of ATE SL, a DOT 4 fluid, per car, per flush. Essentially you're clearing out the brake hydraulics using fresh brake fluid. Read some service literature on the correct procedure. Use a catch bottle at the bleed screws (rather than just blowing fluid all over the floor) to keep air from backing into system and to see what you're taking out. Note for cars newer than R107: if you have SBC brakes (MB > 2003?), you will need to disarm the SBC electronics. Suggest reading up on servicing SBC brakes, if this system is in your car. It can be dangerous if you don't do it right.
* Another ABS issue could be intermittant power supply. Put in a new over-voltage relay. You can "test" the original one, but it's not conclusive (in my view) and better to just replace the relay. And, make sure that the connections at the ABS unit are clean and secure.
* And here's a general suggestion for an older car. Replace all the old fuses with new ones. You'd be surprised how many "idiosyncracies" you can get rid of just by putting in fresh fuses. That's also a good time to make sure the fuses are the correct amperage rating and to clean the fuse holder contacts if needed (an ink erasure on a pen works pretty well). Good luck.

stevebfl · #4 10-18-2008, 01:16 PM

You might be benifited by locating a shop with the bosch LED tester. It was required tools for Bosch Service centers of that era, As such we have one and it sure simplifies testing.

If the light is on all the time then look to powers and grounds on the wiring diagram. Check the relay inside the hydraulic unit by removing the units black plastic cover. The relay looks like a standard "icecube" relay.

The most common issues of ABS light on all the time are: bad OVP, bad pump relay, and bad control unit. The only test for the control unit is another one. They are all the same for years so swap one with a buddy (put yours in his car to keep from scrambling his if there is a weird circuit issue).

The sensors won't cause a continuous ABS light unless they are open circuited. Use an ohm meter to test.

If the light comes on after hitting 10kph then the sensor function (one or more of them) is implausible. It can be checked with a voltmeter but a scope does a better job.

84 500SL Euro · #5 10-20-2008, 09:01 PM

Thanks to all who responded to my dilemma.

After reading Mr. Brotherton's post, I tested the old sensor, that had been damaged by battery acid, for continuity and found to my surprise that it was not open.

So, I cleaned up all contacts on the plug end after 'chiseling' off the original plug which was crumbling from the acid, leaving merely the metal plug itself. I then cleaned the sensor end and lubed it before reinstalling. I used tie-wraps and electrical tape to secure the plug end, after finding out a new sensor is about $200. Outrageous. I also did the same to the entire length of the cable and used more tie-wraps to minimize underbody/hood chafe.

All now seems well except that it takes so much more pedal pressure to stop this car ['84 500SL Euro] than the other ['82 380 SL] which has no ABS system.

84 500SL Euro · #6 12-22-2008, 10:07 PM

It worked for a while but I'm back where I started.

My most recent effort was to unplug the sensors, jack up each wheel in turn, put an ohm meter on the plug while spinning the wheel and I got output in each case. They may be defective but ARE working to some extent, so..........

Since I recently lucked up and got a new ABS unit for less than a sensor, my next move is to replace the 25 year old unit with the new one and see if that 'fixes' it, on which I'll report back.

Thank you to all who've responded,

Courtney