Well,

After having ALMOST everything replaced on the A/C of my car, they seem to have missed something. I figured I would go to the real pro's so when I chew my mechanic's ass until it's sore, I will seem like I know what I am talking about.

The A/C works for about 10 to 15 minutes blowing out of the left and right vents. Then for some reason, the center vents come on and the air becomes HOT. Not ambient....HOT!!! As if the heater was put on full blast.

If anybody has any ideas of what they missed, I would be glad to hear them.

If it's blowing hot with the temp set to cold, it's probably the monovalve (heater control valve) opening when it shouldn't. The two most likely causes are the valve itself failing (this is common, and the valve can be rebuilt for about $20), of the climate control failing (these also fail, but not usually like you're describing). Either way, it may not be related to the AC work.

Thanks Craig,


I have been reading up on the monovalve going bad and at first, it didn't seem like it might be the cause. I have seen monovalve rebuild kits online - is the monovalve pretty easy to get to or is it quite involved? I am mechanically inclined -I do "most" of the work on my car. Oh, where would the valve be located?

Thanks again!!

It's easy, the monovalve is the electric valve in the heater line on the firewall, right next to the battery. It gets 12V to close and 0V to open. If it fails to close, it will give you heat.

If you remove the 4 screws on the top, the internals can be removed. It looks like this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41U40YBX4S&year=1983&make=MB&model=300-DT-001&category=All&part=ACC+Mono+Valve

This is the repair kit for $56:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41U40YBX4S&year=1983&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

I thought they were available cheaper, so check around.

Thanks again.

I guess the first thing I should do before ordering a repair kit would be to check the signal to make sure the valve is getting the 12 volts required to close. Any idea on where that signal originates?

I think I saw the repair kit for about the same price, but I will do a bit of shopping.

The signal comes from the push button control unit in the dash. When the temp is set to cold, you should get 12V at the valve. Some people have tried blocking the heater hose to verify the problem (carefully using vice grips, etc.). You can also check to see if the AC compressor is running and the AC lines under the hood are getting cold. If the AC is OK, it's probably just being overpowered by the heat.

The fact that the vent configuration is changing at the same time the heat comes on suggests to me that what we have here is not the typical monovalve failure. If the vent line-up changes, the problem sounds like the CCU or temp control regulator.

The center vents should be on when the heater is off, and vice versa.

Good point, there may be more going on here. But, I would still check the monovalve to rule it out. I don't know why he's only getting AC from the side vents, this doesn't sound like a typical CCU failure either.

have you checked the foam tube beside the glovebox? Mine was acting very funny, with temps swinging way out of whack with that thing crumbling.

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I haven't checked anything actually. I paid to have the work done. I'm just a bit peeved having spent so much to have a heating system in the summer time.

Either way, it sounds as if I'm not done spending money.

Just to reiterate, the A/C blows cold for about 15 minutes out of the very left and right vents (when the fan is on high). Then the center vents kick on and the heat follows.

I called the guys today and even stopped by, but they weren't there.

Update:


I'm at work, so there is only so much I can do at the moment. I took some electrosolve and cleaned the terminals to the monovalve. The monovalve is made of plastic; does the rebuild kit work on plastic monovalves or just the cast iron ones?

When I get home tonight, I will check to see if there is still 12 volts going to the valve.

Yes.

Check to see if the compressor switches off when the heat comes on. If it does, you can probably forget about the monovalve repair kit. A poor electrical connection at the monovalve can cause the heat to come on when it is not selected, but that won't cause the center vent configuration to change. It could be working backwards due to a simple vacuum plumbing error, but it's not going to change unless the CCU "thinks" it has changed modes.

The monovalve is powered through fuze 14 and grounded directly through the CCU in the OFF mode. In the remaining modes except DEFROST, it is grounded through the temperature control unit.

True, but he can test it by putting a volt meter across the connector. He is just trying to verify if it is getting a signal to close (12V) when the CCU is set to cold. If the voltage is correct and the monovalve isn't closing, that's at least part of the problem. If the correct voltage is not getting to the valve, he needs to look elsewhere in the system. Also, if you plug and unplug it with the temp set to cold you can hear it click open and closed if it's working. Just a starting point.

Thanks everyone for their input!!

I'm leaving work right now and will check on this either when I get home or in the morning. Hopefully this will be an easy fix once I get to checking things out. I have already verified that the compressor does come on. It better as it's BRAND NEW!! Stay tuned!!