maxum68

I have an 87 560SL with a climate control issue, I'm sure this has been covered elsewhere, but I haven't been able to locate the exact scenario so here it is.

1. With temperature control wheel ALL the way on cool until it clicks, the car blows cold air.

2. With the temperature wheel at any other setting, the car blows HOT air.

3. The low & high speed fans work fine. With the Fan set on Auto, it blows a full speed with either scenario 1 or 2 above.

As the car blows cool with it on the coldest temperature setting, it would seem that the Monovalve is not the issue?

Is this most likely the climate control panel?

Thanks for your help!

BJonesFL

I'm not sure why you're ruling out the monovalve; did you change it recently? With the temp wheel set to the min position, the click you heard/felt, any power going to the monovalve from the control unit is cut off, hence the cold air. For whatever reason, these monovalves go bad too often, so if you haven't changed it in a year or so, you may want to.

Bob

maxum68

Thanks for your help! I didn't realize the monovalve was shut off when the temp wheel was clicked to cool. I'll try the monovalve.

Thanks!

-Peter

alkraut

The monovalve A/C-on default position is closed, with power to the solenoid. Remove the connector from the top and check the connector sockets with a voltmeter (you may need to switch leads to get polarity correct) to see if there is voltage there when the A/C is on. If not, there is a control problem. If the seal is bad in the monovalve (common problem) the valve stays open regardless of voltage and hot water goes to the climate system. You can also manually "close" the valve by pinching the outlet hose with a vise grip. It's the upper hose that goes toward the driver side. An easy way to know if the A/C is on is to feel the steel pipe by the driver inner fender. It should get real cold when the pump is running.

maxum68

Ok, the monovalve has been replaced, but still having the same problem.

I'll check for voltage next and report back... thanks for the help.

maxum68

Ok, with the temperature wheel set at the lowest setting, but not "clicked" into full AC, there is no voltage to the monovalve.

So..... does the control panel need to be replaced or do I need further diagnosis?

Thanks!

Mich1ael

I have all new parts and I still find that when on 22 the heat will blow and then the cold air comes on. It also happens when it's warm and the air comes on and then it gets a little warm. I almost think the 22 it says to leave the system on in auto mode does not work. It works in my other car. Any suggestions?

brentkhack

Have you replaced the foam that covers the cabin temp sensor. I have heard that these things get very nasty due to age.

david.marra

I have a different issue which I am trying to diagnose. I believe the control panel is the culprit, but I don't know... (1989 560SL)

I am able to control the temperature of the air blowing out of the vents with the temp control wheel. However, I can not get the AC to come on. Specifically I can set the temp wheel to full cold and the air that blows out is just outside air temp, when I set it to full hot, I got full heat. So I assume the monovalve is regulating hot water in and out of the system correctly.

I am however unable to change which vents I get air from. I'm stuck with the two outboard vents and the dash vent for defrost - no matter which vent buttons I select on the control unit, I get the same behavior.

Other than looking for blown fuses I haven't done any troubleshooting.

Is any wire I can jump behind the control unit to manually engage the AC compressor? Is there a switch or relay to the compressor that may have gone bad. The AC hasn't functioned in over a year... But when it went, I remember the temp control wheel acting up a little bit, then the I just had no AC.

Jambrose

I have the same air distribution issue with my '86 560SL but haven't solved it yet. A possible solution to the A/C problem is to add R12 to the system. There is a low-pressure cutoff when the systme is low of Freon. i added some to mine and the A/C kicked on immediately.