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  #1  
Old 05-31-2007, 08:52 PM
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Join Date: Dec 2000
Location: Battle Ground, WA
Posts: 576
Monovalve problem and possible fix

I am commenting on a problem I had with my '82 380SL's heater/AC unit and the fix I made to eliminate the problem. Have any of you people had this problem? When I use the AC, occasionally, when at a stop light, or when the engine is running at idle, the heat would come on and override the AC, resulting in warm to hot air coming out the vents. At first I attributed this to a possible lack of charge in the AC system, but when I checked, it was fine. As I got into the problem, it became apparent that the monovalve would not shut off the heat when the engine was idling. Apparently the monovalve has to have some water flow to operate correctly, and it didn't get enough at idle. (The auxiliary pump is probably bad, and I'm thinking that the main reason the auxiliary pump is there in the first place is so the monovalve will work.) Since I had replaced the guts in the monovalve only about six months ago, and the old diaghram was good when it was removed, I decided to dump the monovalve in favor of a more positive acting valve. I got a vacuum operated valve, (the parts book listed it for a '74 ford), and made some plumbing using 1/2" copper tubing to route from the new valve to the upper hose, and am using a vacuum valve operated by 12vdc to switch the new valve on and off. I must say that it works 100% better than the monovalve did! The AC stays cold regardless of the engine speed, and the heater control is very responsive. Does this scenario remind you of any problems you've had? Let me know! By the way, the new vacuum valve was only $17 at NAPA, and it all fits where the monovalve was, doesn't look bad at all. (I had the vacuum valve, don't know what it would cost, but certainly the two together are a lot cheaper than a monovalve and auxiliary pump! If this rings any bells for anyone, I could probably take a few pix of it all...

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C
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  #2  
Old 06-18-2007, 01:12 PM
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Join Date: Jun 2007
Posts: 4
Richard,
This sounds exactly like the situation I have with my 1982 380SL. I have already ordered a monovalve rebuild kit so I will probably go ahead and install it since that seems to be pretty simple. If it doesn't do the trick, I may be asking for pics of your setup! Very creative!
Regards,
Raymond
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  #3  
Old 06-18-2007, 11:01 PM
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Join Date: Dec 2000
Location: Battle Ground, WA
Posts: 576
I posted some pix on one of the diesel threads, as someone there had the same problem also... I'll see if I can find the thread.

Here it is:
http://www.peachparts.com/shopforum/diesel-discussion/188981-mono-valve-voltages-post1524566.html#post1524566
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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C
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  #4  
Old 06-24-2007, 10:53 AM
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Join Date: Apr 2001
Location: Joliet Illinois
Posts: 309
Monovalve

Richard, I admire your ingenuity.

Last night the aux heater pump on my '88 260e started leaking and I'm looking at options. First off, im really not sure what this pump does. I suspect that it aids in supplying hot water to the heater core but other cars don't have them and is it really necessary? If my monovalve is okay, do you think I could just install a piece of tubing in place of the aux pump and still get adequate heat in the winter?

Thanks,
John

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