AC works then off and HOT - PeachParts Mercedes-Benz Forum

tangofox007 · #1 06-19-2009, 09:55 AM

Quote:

Originally Posted by msgaevans

I replaced the fuse.

Which fuse, exactly?

Quote:

Originally Posted by msgaevans

When I adjust to full cold but automatic temp control (other side of the click), I get warm air at the vents that eventually becomes very hot. As expected, I do not have 12 volts to the monovalve when this is occurring. Moving the temp wheel back to full cold past the click gets cold air after the heat has dissipated and I have 12 volts at the monovalve.

Voltage to the monovalve is not controlled by the CCU. You should have system voltage at the monovalve any time the key switch is in the "run" or "start' position. The monovalve is modulated by switching the ground side of the circuit as controlled by the solid state temperature control module (except in "off," "max heat" and "defrost," where the ground is controlled by the pushbutton control.)

Quote:

Originally Posted by msgaevans

What should I do next?

Plugging or clamping a heater hose should provide immediate relief.

Quote:

Originally Posted by msgaevans

What components are involved in the feedback loop that controls 12 volts to the monovalve? What are common failures?

A bad solder joint in the CCU would deserve a top spot on the list of probable suspects.

msgaevans · #2 06-19-2009, 11:24 AM

I replaced Fuse 14 - the lower one on the right.

As to the monovalve. If I understand correctly, ground is switched in response to the CCU.

I believe I will follow the DIY instructions for CCU re-solder this weekend and see if that helps.

Meanwhile, the AC seems to work OK at full cold past the click.

Is there a simple way provide continuous 12 v to the monovalve?

Thanks,
Mark

ah-kay · #3 06-19-2009, 12:32 PM

Quote:

Originally Posted by msgaevans

I

Is there a simple way provide continuous 12 v to the monovalve?

Thanks,
Mark

Hardwire a 12V to the monovalve. NOT from the battery but from a fuse which is ON when car is running, like the window fuse or others. You only want it to energize when car is running. I went to a junk yard and got the same connector plug, measured the polarity of the plug and wire it to a window fuse. Remove it during winter. I did this for a short while until I realize I had a hairline crack with the fuse.