My A/C doesn't get fully cold until Max on the temp wheel is selected -even then it is intermittent (mostly cold though). Any other position from Max I get hot air mixed with the cold air...sometimes alot of hot air. I have a new mono valve (the valve itself, not the exterior housing) and am now curious if the electro magnet in the mono valve assembly is not performing, or whether I am not getting the correct voltage to the electro magnet.

I understand that the electro magnet regulates the mono valve to supply hot water to the heater core when heat is called for. While I had the mono valve apart replacing the valve itself, I had my son go to max on the temp wheel and the electro magnet would only sometimes push the valve closed, other times if I were to pull the valve, the magnet would hold it. As soon as he came off of Max, the valve would go to the default hot position.

Since it defaults to hot, that the voltage required when hot air is called for, I am assuming the voltage would be 0. I am also assuming that full cold would be 12 volts to the electro magnet within the mono valve assembly, and other temperatures would be somewhere between 0 volts and 12 volts, depending on the amount of heat or cold called for.

I have several questions in order to trouble shoot this properly.
First: Am I assuming the voltage values correctly?
Secondly: What supplies voltage to the mono valve assembly? Does it come directly from the climate control, or is there something between the climate control and the mono valve itself (perhaps an amplifier or such)?

The A/C itself is working properly and is charged with freon. The issue here is one of the mono valve letting hot water through the heater core and ultimately in to the cabin.

Any trouble shooting advice would be helpful!

1983 300D Turbo 190,000 miles
Thanks in advance!

When at the max cold detent, 12 volts is applied to the monovalve all the time. At the max heat detent, the voltage is removed. (BTW, switching is in the ground side, so don't let that confuse you). In normal operation, the monovalve is switched on and off. (In other words, it does not run at partial voltage, it is either on or off).

Two things, first, clean the connector terminals. If the contacts are crusted over with oxidation, the monovalve does not get voltage, therefore is in the defaullt full heat condition.

Secondly, I have had a monovalve with a new repair kit operate intermittantly, similar to what you describe. The problem was due to the monovalve plunger sometimes physically sticking in the bore. Apparently there was a dimension problem in the repair kit parts. I was able to troubleshoot the problem visually by removing the monovalve and hooking it to a battery (CAREFULLY). I could see that it would sometimes not fully close, therefore intermittantly alowing some coolant leakage past the valve. BTW, if you try to check it by blowing air through it as you are turning the valve on and off, be advised that the monovalve plastic has an absolutely horrible taste to it.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

Thanks Palangi.

I will clean the connections and check the voltage. I should have done that first, but I didnt have my voltmeter handy. I did, however, check the new mono valve by pulling on it to determine if it extended properly without getting hung up. As I mentioned, it sometimes work with voltage applied as I was watching it, but mostly not. I will check the voltage and clean up the connections and let you know what happened.
Thanks again.

Also remember that the default 'position' (no voltage applied) of the monovalve is "open" to let hot water through. The monovalve "works" by turning "ON" and NOT allowing hot water through to the heater core, thus allowing the a/c (which I believe runs full blast whenever it's engaged) to cool the air passing through the evaporator. I've been told that this is deliberate, for safety reasons, so that if there's a system failure you will always be able to get hot air to clear your fogged or frozen windshield.

One test that you can perform is to pinch off the water hose with a clamp or vicegrip (padded jaws, please). If you get nice cold air in a/c modes, then you know the problem is in the monovalve circuit. In your case, it sounds as if the ACC is at least trying to modulate the valve. Palangi's analysis is correct. Fortunately I didn't have the problem he describes when I kitted my monovalve.

I don't remember the switching frequency that the ACC uses on the monovalve. It would be interesting to put an oscilloscope on the thing and see what the pattern looks like. Maybe I'll try that one of these days.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Well, I checked to voltage to the mono valve and am only getting 10 volts, which is obviously not enough to close the valve to keep hot water out. I am thinking now that the climate control is the issue as I am not getting sufficient voltage.

Check your auxilliary water pump - the monovalve is just like the water valve in your dishwasher or washing machine, it requires water pressure to operate it. It has a primary valve, operated by the solenoid, that controls a bleed to allow the water pressure to operate the main diaphram, and if there isn't enough water pressure the monovalve will allow water to flow, even when it has the proper voltage. I replaced the whole thing in my '82 380SL with a vacuum controlled heater valve ($17.00 from NAPA, 1974 ford), and a switchover valve, ($38.00 from Performance Products, 1990 Mercedes 124). The switchover valve is operated by the monovalve control voltage, and in turn operates the vacuum controlled heater valve, regardless of water pressure, so the auxillary water pump can be eliminated. You will notice that ALL Mercedes vehicles that have a monovalve also have an auxiliary water pump. My setup works really well, keeps the heater turned off when the AC is on full blast, and modulates through the temperature wheel range very well.

__________________
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

I've got Monovalve problems as well. Could you post a rough drawing of this setup? Thanks!

Here are some pix of my monovalve replacement. The solenoid valve is the same valve used on the newer climate control system to control the flap valves, also used as a switchover valve on 124's. As I said before, this will eliminate the monovalve and the aux water pump, and costs less than $60. Works very well.

Attached Thumbnails

       

__________________
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

Nice setup Richard.

Couple of questions:

1) Usually those vacuum solenoids will connect the bottom port with the horizontal port when deenergized. When energized, they connect the top port with the horizontal port.

So, if it is connected as follows:

Bottom port: supply vacuum
Horizontal port: to valve
Top port: open to atmosphere

The valve would get vacuum when the solenoid is energized and would bleed off vacuum to atmosphere when deenergized.

Would this work better than your existing setup?

2) I'm not understanding the "bleed" with the wire attached??

3) Have you encountered any very cold temperatures (say 10F. or so.)? If yes, how did it behave without the aux. pump?

That's possible, but before you go too far try replacing your #14 fuse (just for fun). I recently discovered that a failed #14 fuse on my 82 300D will result in full heat (open monovalve). ACC failures are also common, but your case sounds a little different.

Hi Brian,
1. You are correct in that the valve works as you described, ie I could have left off the plug and the bleed. I felt it gave a little smoother action with the bleed than without. No, I haven't tried it in extremely low temperatures, in fact we never get those kind of temps here in SW Washington. I've never seen it go below 18 degrees.
2. The stranded wire lets a small amount of air pass through between the strands, a very easy way to control the bleed air, as evidenced by Mercedes's use of this kind of bleed in all of the ACII systems. You can just cut the wire to whatever length it takes to get the bleed you want.

On a different subject, I checked in my factory manual, it has a good description of the operation for each pushbutton, and in all positions except economy and off the AC compressor is engaged continuously, and the heat is controlled by adding heat to the AC. I suppose this gives the system a dehumidifying effect. Also, I expect that the heat would be mostly shut off in warm weather.

The problem I was having with my monovalve was that when I slowed down to 25mph or less, the heat would come on even with the AC turned to max cool. This setup fixes that!

__________________
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

Thanks Richard.

I'm inclined to try it on the SD. I'd love to eliminate the problematic monovalve. The aux. pump is already removed. However, it does get a bit cold in the cabin when the temp is down around 15 degrees or less.

Did you pick the vacuum operated flow control valve for any particulat charachteristics or because you know Fords? The reason I asK is I have one for a Chrysler New Yorker that operates the same way.

I picked that one out of the parts catalog at Napa because it looked like it would fit the location. No other special reason.

__________________
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

Cool, thanks! My monovalve is shot. Coolant got into the electrics and shorted it out and these are no longer being made so a good design that works for a reasonable price is worth trying! Nice setup Richard!