Heater/AC

[benz909]

I have a 1983 300D. My AC works if I rotate the wheel all the way to the detent for AC. My heater seems to be on full heat in any other position on the temp wheel. So I seem to have heat on full at any position on the temp wheel except the max ac. At the max AC detent position the AC seems to work fine. Why cant I adjust heat.

thanks for any help

[tangofox007]

Except in either (min or max) temp selector detent position, the control of the monovalve is in the hands of the electronic unit for temperature control. Evidently, you have a problem in that circuit. The "min" detent provides a constant ground path to the monovalve (except in defrost) which keeps the valve closed.

[benz909]

Ok Im pretty much a newby to Mercedes as this is my first and I have only owned for a couple of months.
I think I know what the monovalve is. Mounted on firewall under hood behind engine.
However can you help me out with the electronic unit for temperature control. Is this the wheel itself?

[tangofox007]

The electronic unit for temperature control is located near the glove box, under the dashboard. The problem, however, is not necessarily with the electronic unit itself. High on the list of possible culprits would be failed solder joints in the push button unit.

[cmbdiesel]

Resoldering every solder joint on the circuit board of my SD made all the difference. Only takes about an hour to remove/repair/replace.

Now that works, I'm actually tempted to try and get the AC functional.


Maybe start with this thread -
DIY page. Introduction to Resoldering Climate Control Unit