For a couple of years I have suffered from an intermittent problem where the heating and cooling system seems to get stuck on "heat mode" and does a thorough job of roasting you.

I had looked at all of the usual fixes:-

- is the CCU applying the correct voltage to the monovalve?
- is the power connector on the monovalve clean and tight?
- is the monovalve insert in good shape and able to move when power is applied?

etc. etc.

In my case the symptoms are that occasionally it would get stuck in heat mode and just would not switch back to cooling......

In my case it turned out to be the monovalve solenoid itself. Upon close examination the insulation on the coil was corroded/rubbed off and I guess it was shorting in the coolant from time to time. I replaced the solenoid with a used replacement and voila, it works!

This is the second time in my four years of w126 ownership that the cabin heat system has operated as designed. I wonder how long it will work this time?

Tanks for the troubleshotting info.

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84 300D, 82 Volvo 244Gl Diesel