1997 S-500 Transmission Control Unit

[JFawcett]

Well, was driving down the freeway...look down and the Speedometer is showing 0MPH, the yellow triangle is on, the ABS light is on, the BRAKE light is on, the Brake Pad Warning LIght is on, and the ASR Warnimg light is on.....in short, I have a full Christmas Tree on my dash. And then, all is well. The car goes right back to normal.

Next day, same thing happens - only this time, the power steering gets very firm and the car won't up-shift.....glorious.

SO...take to the mechanic - he plugs into the magic Mercedes machine and the car tells him that the transmission control module is out as well as the brake pedal position switch......so we replace and all seems well with the world. That was yesterday.

Today, crusiing down the road....the same thing happens. After several explicatives, the car "fixes" itself again.

SO it would appear that the car lied to the mechanic yesterday and is now not happy.....does anyone have any ideas here?

Obviously not the TCM or the brake pedal sensor. Trust the mechanic implicitly.....

THanks!

[stevebfl]

Have you got the right tire sizes?

The issues all seem to revolve around the speed sensor signals or the cars ability to accept them as plausible. BTW, the MB computer is no better than the one using it. I'd be very leary of anyone who replaced a TCM based on stored fault data.

[JFawcett]

yep.....Michelins.....and problem started well after the new tires were installed.

Are there any other components that could cause all these system failures? You mention speed sensors.......?

[JFawcett]

The car is fixed!

It was the Transmission Control Plate that receives the data from the other systems - the transmission control module is plugged into this board. So what happens is this board gets damaged or confused and shorts out the TCM. The board is not too expensive ($250 or so) but it is buried in the transmission - seems like a design flaw to have electronic componenets immersed in Transmission fluid......either way, that was the problem and I am glad it is fixed.

BTW, mechanic did mention this is a HUGE problem on the W210s. What happens is the seals go and the plug actually wicks transmission fluid into the housing and shorts things out.