Temp gauge not working properly... help?

[Blevinsax]

Okay - this is starting to get on my nerves...

I took the car to Sig's ******** (the only Mercedes shop in Las Vegas that I've seen mentioned on this forum) for some work because I want to get all the bugs worked out before my daughter is born in June. He did a good job on most everything, but when I picked up the car the temp gauge was not working properly. I thought the car was running cool because the temp gauge only read 60-70C, but the car is actually running at proper temperature - it is the gauge that is not reading right. By the next day the temp gauge was completely dead - pegged at the bottom. He just replaced the temp switch (on the manifold between 2nd and 3rd cylinders) but the gauge is still reading low while engine temps are normal.

Any idea what else might be causing this problem? Thanks for the help in advance! I hate having to do research for my mechanic - I just wish I could find someone local who is really knowledgeable.