Need help please. SL55 temp guage dropped to 40 then car became undriveable setting P0117 code. Limped home. Installed new temp sensor (even tho old one benched out OK) and checked wiring to ECU--all good.
Car would start (cold) with cleared code but soon after idles rough, sets check engine light and P0117 code again and radiator fan on high. Temp guage needle did not move off 40 (bottom mark) All happens within a minute.
A couple of days ago, car started fine and temp guage moved up normally to 85-90. Drove the car a couple of miles and guage bounched around between 50 and 90 and various points in between.
Guage bottomed out at 40 in my driveway and engine faltered (rough) with check engine light, rad fan and P0117 code. Car would not start at normal operating temp.
With key on, engine off, (stationary) guage bounced up and down with flickering action.
Coolant level good, new thermostat also. A/C in off mode all the time.
Star system just points to coolant temp system still.
Bought the car after an Indy Shop did $8500 worth of repair to the ABC ride control system. Has had SBS brake system done also recently.
Have read that the car may respond to reflashing the controllers with the dealer system since bugs may have been left by previous Indy service techs.
Have consumer battery (new Bosch) drain issues--I think car errantly locks itself too.
Any thoughts please?? ECU?
Thank you.

My thought would be a thermostat - maybe your new one installed wrong?

Could be the computer behind the passenger seat . This wrecks havoc in all electrical items .

Old thermostat also benched out OK but installed new one since I had already bought it. Weird thing is the temp needle bouncing around if it rises above the bottom 40 mark. ECU does not recognize a good signal almost immediately from the temp sensor--at least the guage does not respond--and sets check engine light and P0117 code. Neither temp sensor or thermostat could cause the needle to bounce around the way it does on the occasion it rises from the bottom peg mark of 40.
Appreciate the discussion.
Best!

Thanks. I have a gut feeling it is the engine control module or another computer/module with an issue.
Best looking and driving car I've owned--can only afford it by being a DIY owner but these electrical issues are becoming daunting--until I kick the turbo in again!

Solved!! Made up a new temp sensor harness and tied into ECU/M. All works great again.Old harness appeared to be OK so I can
only assume problem was in the harness plug that connected to the sensor--just made a parallel circuit w connector and whip I found
on the Internet that matched the MBZ temp sensor.

Glad you got it solved. Good smarts to figure it out...

Thanks for posting the resolution - it will help others down the road.

Great solution and fix!