Peter,
I agree there is probably one in the controller. However, the component locator I am referencing is from Star Tek Info.
http://www.startekinfo.com/StarTek/ (Mercedes sponsored) clearly shows two engine coolant sensors, one for the gauge and one for the GP's. The one for the gp's apears to be a three wire affair. I keep getting a check engine light and a code of P0308 which indicates a problem with the glow plug system. Since the car has no problem starting I am suspecting teh coolant sensor may be giving a bad signal intermittantly. I want fo find the sensor referenced in the literature I have and take voltage readings and compare them with what the factory asys they are supposed to be.