Now that it's plenty cold here in CO I've discovered that my SDL has some heat issues. For starters, it only has heat when I click the defrost button (left most button on the control) and with the dial control on MAX heat.

The heat starts flowing and soon it's boiling hot. If I then click any other button (leaving the dial alone) the heat will quickly subside and turn cold pretty quickly (with fan either high or low).

So to keep the temp comfortable inside the cabin I have to continuously keep cycling between Defrost button (most left) and Off button (most right)...

I've searched and found references to the Monovalve - I plan on inspecting that first. Beyond that, it seems that there were some references to the vacuum system - specifically the pods behind the dash (sure hope that's not the case, but it seems probable).

Can someone suggest some other checks beyond the monovalve?!

I should also mention that the dash has an aftermarket plastic cover on it - could that be affecting the cabin temperature sensor and confusing the Climate Control Unit?!

Thanks,
James

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K