PeachParts Mercedes-Benz Forum - View Single Post

Jeremy5848 · #10 05-23-2008, 07:08 PM

Quote:

Originally Posted by probear

. . . The vents seem to be a logical place to look but the confusing part is that at max cool, their is still heat, but the monovalve is suppose to be closed, thus not allowing hot coolant into the heater core, yet, I have heat.

As an experiment, run the car and operate the climate control system in a/c mode but do whatever you do to get hot air coming out of one of the vents, as you have described. Now open the hood and carefully pinch the coolant hose that the monovalve is in. Use a clamp or a pair of pliers or whatever it takes. With the hose clamped, does the hot air gradually become cold or at least no longer hot? If so, the monovalve is somehow being opened even though it is supposed to be closed.

Remember that the monovalve is fail-safe: with the power off, it opens and allows hot coolant through. This is so you will always be able to clear a fogged windshield. Anything that interrupts the 12 volts to the monovalve can result in unwanted heat. Check for dirty or misaligned connectors, broken wires, etc.

Another experiment: remove the plug and connect 12 volts and ground directly from the battery to the two pins of the monovalve. I don't think that polarity matters but check and see. With a constant 12 volts on the monovalve, it should be closed and the heat should go away. Does it? Now remove the 12 volts. The heat should come back. Does it?

Be careful not to leave the 12 volts directly on the monovalve for a long time (more than 15 minutes). I'm not sure it was designed to dissipate that much power (even though it has hot coolant going through it). The CCU produces a variable duty-cycle pulsating DC, which dissipates less energy. This is probably overkill but I don't want to suggest something that would damage your car.

The only other thing I can think of is to ask whether your SDL can have different temps set for driver and passenger. My wife's '96 E300D can do that and it has a "duovalve" instead of a "monovalve." But it's a much later car.

Jeremy