Over the last week, my engine coolant gauge has gone to the dark side. At any given time after start up (the car can be dead cold or very warmed up, doesnt matter) the coolant needle shoots up to high, pauses, then shoots back all the way to low, back up to high and on and on. Other times, the needle behaves itself. When the needle does hit high, a fan comes on under the hood and clicks off once the needle is off of high. Now the check engine light is on. it was 28 degrees where I live last night and I just went out and started it and it went straight to high. Impossible. I have an appt. to bring it in Wed., but in the mean time, any ideas??
Thanks guys

 

Get you battery checked first. I have had a few cars that had the battery going, but not bad yet, and the symptom was the gauges fluctuating just like yours. If you have the original battery it is likely time to replace it. I just put a new Interstate in my Aero. The cars now a days are VERY picky about the voltage and current they get and if it isnt up to par they start doing odd things. Good luck.

MikeR

 

Sounds like the engine coolant temperature sensor is failing. When the gauge pegs, you are probably hearing the auxiliary fan for the engine coming on because the control module thinks the engine is getting too warm. And if the ECM does not see a certain coolant temp after and while the engine is warming up, it will bring the check engine light on.
The sensor list (from Mercedes) for about $13 and is real easy to replace.