ABS problem finally resolved

[the red barron]

I had been having issues with the ABS on my C350 for the last 2.5 years, initially the ABS light would come on along with the other warnings but would then go off after a couple of days. The car went into a indie and was diagnosed as the front l/h wheel speed sensor faulty, new sensor fitted and that was that or so i thought. Couple of weeks later all the warning lights returned and a few days later went off again, very annoying. The car went back in to the Indie where yet again front l/h wheel speed sensor was the culprit or could be the magnetic reluctor ring all of which was replaced.The same scenario played out again, confidence in the indie dwindled so the car went into Mercedes dealer for diagnostics, result "front l/h wheel speed sensor faulty". I said i didn't think that was correct as it had been replaced twice already and took the car away.
The last 6 months had seen the ABS warning lights on constantly so i decided to do some checking myself, when i checked the harness from left wheel speed sensor up to the abs control unit all was good, i then diconnected the large connector block at the abs control unit all looked fine so plugged it back in. My thoughts were it must be the control unit itself "expensive". I must add that i removed the r/h wheel speed sensor and swapped it with the l/h the fault stayed with the l/h side so eliminating the wheel speed sensor. After doing all of this i switched the car back on and shock no warning lights my thoughts now were that there must be a fault in the connections at the abs control unit but how do you test for this! This is what i did, i bought a used abs control module (£80) and cut the back off. I had already established that pins 45 and 46 were the l/h wheel speed sensor and looking at the circuit board could see there was enough of the pins sticking through the circuit board to solder new wires directly in through the back of the control unit and rejoin the harness at the front thus eliminating the connection in the large connector block.
Before doing any work on the original unit still fitted in the car i carefully measured the exact position of pins 45 and 46 as i didn't want to remove the whole back of the control unit, i then removed the unit from the car which is a very snug fit, measured and marked where to cut the rear of the casing just enough to get access with a soldering iron and soldered the bypass wires into position, replaced the piece of plastic casing after cutting 2 small slots into it to allow for the new wires refitted the unit back onto the car and soldered the new wires onto the wires coming from the l/h wheel speed sensor. I then switched the ignition on and no warning lights came on, took the car for a test drive and was able to use cruise control and speed limiter all of which had been disabled due to the faulty connection.
This was done over a month ago and the car is still perfect but what a job to eventually sort out. Gave myself a pat on the back. The picture (not mine) shows where the problem was.

 

[ajlsl600]

Excellent I lov to read of a, successful owner wriggle, and u learnt somat. And god enuf to pass it on
Prob today is car repairers, are not taught, encouraged to think outside the box "just connect up and replace components"
I try real hard not to replace anything without having a go at fixing it, or finding out what broke and why.
One can get a good idea on if something is fixable, by the effort maker makes to stop you getting inside, impossible, fairly easy to open up. Fixable, one near has to destroy it to get access.
Well done that man..

 

[DSK]

Excellent! Ditto the above comment.

This is my gripe with modern cars, the technology is there but the repairs are just silly as illustrated above. I have nothing against having to pay to get something put right but, having it put right, first time is rare in this day and age.


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[malcolm E53 AMG]

Well done! ‘Diagnose Dan’ on you YouTube would be proud of you

 

[supernoodle]

Well done for fixing. By the sound of it you've bypassed a short section of wire, the connector with its female receptacle and the male pin in the module.
There's not much to go wrong in that chain other than the receptacle in the connector opening out, so not making contact with the male pin. Given that garages had been trying to diagnose, quite possible someone had been pushing probes in, opening the receptacle out.
Once you'd broken into the module did you check for continuity to work out where the problem was?

 

[the red barron]

Well done for fixing. By the sound of it you've bypassed a short section of wire, the connector with its female receptacle and the male pin in the module.
There's not much to go wrong in that chain other than the receptacle in the connector opening out, so not making contact with the male pin. Given that garages had been trying to diagnose, quite possible someone had been pushing probes in, opening the receptacle out.
Once you'd broken into the module did you check for continuity to work out where the problem was?

Hi supernoodle, my original thoughts were that there may have been a dry joint at pin 45 or 46 which after openening up the secondhand controler and looking at how well all of the joints were made i dismissed. I never actually determined which pin was faulty just decided to bypass both.
I dread to think how much the end bill would have been had i left it at the dealer with "don't give it back until it's fixed" which had crossed my mind. Up to the time i sorted it myself i had spent the best part of £500 and achieved nothing for it.

 

[Botus]

this sort of small issue is why the repair services are a good idea - unusual fault - sounds much closer to the old issues they accidentally added on ABS units in the 1990s, today is usually ESP pump brushes

Mercedes CLS ABS (Pump & ECU/Module Combined)

Mercedes CLS ABS (Pump & ECU/Module Combined)

www.ecutesting.com