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  #1  
Old 07-19-2007, 12:25 AM
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Location of Mono Valve 1991 300SL?

Can some kind soul please post a real picture of the mono valve location for a 1991 300SL. I have a line drawing diagram on CD, but I can't pick out the valve in all the clutter. I suspect my valve is stuck open, as I get heated air in all modes except auto with the temp set low enough to activate the AC. However, with the AC on it gets plenty cold, so maybe the valve is OK. Anyhow, I'd like to have a picture so I know what I'm looking for. Thanks
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Old 07-19-2007, 09:39 PM
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I have a photo that shows its location near the OVP, but I can't upload it to the forum because the file size is too large. If you like, you can send me an email and I will reply back with it.
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Old 07-20-2007, 01:33 PM
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Thanks Bobterry: I found it. I looked on fastlane for a replacement part picture and after that it was easy. But now I have some more questions. I have 12V on the solenoid at all times when the ignition key is "on" and the engine is cold. When I turn off the ignition, after about 1 second, I hear a relay click in the Climate controller and the 12v at the solenoid goes off. Does the 12V at the solenoid cause the valve to open ( pass water )? Or conversely, does the control signal stop the water flow? Are any of the actions dependent on the actual temperature of the water in the system? And are there any water temperature sensors involved. I would think if the climate controller is not "asking" for heat, the mono valve should not pass water. What I really need is a good primer in R129 HVAC operation. Any recommndations? Thanks Tod
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Old 07-21-2007, 03:15 AM
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Tod, voltage applied to the valve closes it.

There is a heater temperature sensor involved in the control of the monovalve. It is located in the heater box. You can monitor its signal through diagnostics as explained here: Climate control diagnostics on '90 - '95 R129 SLs -- you are interested in item "08". If you have the climate control in "EC" and the temperature setpoint at minimum, then the evaporator, heater, and outdoor temperatures should all be about the same.

The Mercedes WIS has a fairly detailed description of the operation of the climate control system and its individual components.
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Old 07-21-2007, 11:54 AM
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Thanks for the info on the mono valve Bobterry. I've been aware of the procedure which is supposed to put the control unit into a mode to diagnose the system, but mine just doesn't respond. i.e REST + Fan 4 has no effect. It just goes into REST mode. The controller will however display error codes and erase them. If I just put it in AUTOMATIC, it heats fine in winter and cools great in summer and maintains the set temperature. All the vents respond properly ( no bad vacuum motors )( defrost OK ) But in EC mode ( nice day, top down, no heat or AC required ) the side vents deliever air which is noticeably warmer than ambient outside air. Everyone who owns one of these cars knows what a confusing mess the HVAC controller can be with all those iconic vent selection buttons, dials, RED BLUE center vent buttons, and various modes. So, I just wanted to monitor the signal to the MONO valve. And, if the MONO is faulty or leaking some coolent past the valve it could explain the warm air. And of course, it's an old car and perhaps some of the vacuum flap seals are leaking past bad rubber. But I'm not ready to pull the dash just to have a look. Some things you just have to live with. Thanks Again. Tod
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