Climate Control Acting UP..... - PeachParts Mercedes-Benz Forum

Cummmins · #1 07-22-2013, 11:01 PM

I just got my a/c working (see no compressor engagement thread). I was very excited on my way to work that the a/c was working great. It was 101* when I left work this afternoon and the a/c was still working great (had climate control on auto and max cold). I stopped by the bank, and when I got back in and started the car it was just blowing hot air. I popped the hood to be sure that the a/c compressor was running - it was. I tried several different settings and the fan speed would change, but it would just blow heat. I noticed that when I moved the temp wheel from max heat to max cold that I could not hear the relays like I normally could. I took apart the ccu to see if it had any cracked solder joints, etc, and everything looked ok. I have only had the car for a couple of months, and have noticed that the climate control would sometimes do weird things. It never seemed to blow that hot, even on a hot day. Today it was very hot. Do these sound like symptoms of a bad climate control unit. Or, is there something else in the system that could be causing similar issues? Thanks for any help.

rocky raccoon · #2 07-23-2013, 06:33 AM

Even though your monovalve has no function except when in heating mode, it can affect the ac if it has failed.

The normal (unpowered) state of the monovalve is OPEN. A control voltage closes and opens the solenoid valve to regulate heat. A torn rubber diaphragm (common) will allow hot water to pass through the monovalve at all times and will overwhelm the ac. Similarly, a loss of control voltage at the monovalve connector will have the same effect.

You can measure the presence/absence of the control voltage at the connector and inspect the monovalve diaphragm very easily by disconnecting the connector, removing four screws and withdrawing the solenoid piston assembly. If needed, a new piston assembly is in the $30 range. Should take no longer than an easy 15 minute DIY.

Cummmins · #3 07-23-2013, 11:30 AM

Thank you for the reply. Is the monovalve what some may refer to as a heater control valve on other vehicles? Just to be clear - the valve needs power to close, to stop the circulation of coolant into the heater core? I can try using pinch-off pliers on the heater hose to see if that helps. I am still concerned that I no longer hear the climate control unit relays clicking when turning the temp wheel. Maybe there are a couple of issues...

rocky raccoon · #4 07-23-2013, 01:48 PM

12vdc present CLOSES the valve thus preventing hot water circulation. You can pinch off your heater hose of course but simply inspecting the monovalve is almost as fast and has no potential for damaging the hose.

Up against the firewall on the passenger side. The monovalve has a square top held down by four screws. Easy to see and access. Pull the plug off, unscrew the 4 screws and lift the top off to withdraw the monovalve solenoid. You will lose no coolant if you are parked level. Look for torn/rotted rubber on the diaphragm. Obvious when you pull it out. The complete solenoid assembly is available at a low cost. It may be worth having one on hand before you inspect the existing one. If it hasn't failed yet, it soon will.

Cummmins · #5 07-23-2013, 02:18 PM

Where is the best place to get one?

JamesDean · #6 07-23-2013, 03:17 PM

1985 Mercedes-Benz 300D Sedan - Heater System - Page 1

Cummmins · #7 07-23-2013, 06:22 PM

Thanks.

Cummmins · #8 07-30-2013, 09:56 PM

I replaced climate control unit and the ac started working again. I replaced the mono valve as well because one of the seals was torn. I went a full day with ac! BUT, now the fuse has blown for the hvac/horn, etc. The circuit is only drawing a couple of amps right now, it must spike at some point. Will have to spend some more time on it.

Cummmins · #9 07-31-2013, 10:36 PM

Update! I found that when I put the car in reverse it was drawing 39amps! After searching, I found that the back up wire at the trans had been rubbing on the pan and had grounded out causing the high amp draw and blowing the fuse. I repaired the wiring and all is good.

rocky raccoon · #10 08-01-2013, 02:31 PM

Fault isolation is most often a matter of logic isn't it?

Cummmins · #11 08-01-2013, 11:06 PM

Yes, it is. A little patience, although sometimes hard to come by, can help too!