PeachParts Mercedes-Benz Forum - View Single Post

tinypanzer · #5 10-28-2009, 06:35 PM

It could be a torn monovalve diaphragm, but the fact that you get strong heat when you pull the plug might indicate otherwise. It's really easy to pull the diaphragm out and check for a tear, so I'd do that just to be sure, before moving on to what I am going to suggest next.

The CCU pulses the monovalve solenoid to allow in however much hot coolant the CCU thinks is needed to accomplish the temperature you have it set to. I'd start CCU diagnosis by testing the in-car temp sensor (in the dome light assy.) with an ohmmeter to make sure that its resistance corresponds to the temperature correctly. I don't have a chart for that sensor off hand, but I'm sure one exists. Basically, if the temp sensor thinks the interior is hotter than it is, it will not open the monovalve as much.

Turn the temp wheel all the way to hot until it clicks. That should make the monovalve open completely and stay open, just like when you pulled the plug. If that produces heat like you would expect, then you have either a CCU or interior temp sensor problem.

Still, I would start by checking the diaphragm and making sure it isn't torn. That is a typical fault with those. A flat blade screwdriver is all you need, it takes 5 minutes.

Also, the fact that your radiator isn't getting hot might indicate that you have a stuck open thermostat, in which case your engine will never get up to proper operating temps. If you're not getting to 80-85c on that 7 mile drive, then I would strongly suspect the thermostat (providing your dash temp display is accurate).