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Old 12-07-2000, 11:11 PM
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I did a search and read about the monovalve. Although reading about this widget, I am uncertain as to where it is exactly and what it looks like on a 89 420SEL? I have no heat and it's suppose to snow tonight. Could anyone provide some clarity for my non-mechanical brain? Freezing in DC. Thanks!
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Old 12-08-2000, 07:17 AM
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I'm not familiar with your particular car, but I can offer some general information that maybe will help.

First of all the infamous monovalve problem is with the 123 series cars from about '81 to ''85. I expect that yours has a different monovalve that is much more reliable. My '88 124 car has the newer style that seems to be much more robust.

The monovalve can most likely be found near the battery. It will have a heater hose going in and another going out. There will most likely be a two wire connector attached. If it is like the 124 car, you can disconnect this connector and it will open the heater flow wide open. If you disconnect this connector and the system starts making hot air again, you probably have a control problem, not a monovalve problem.

I don't know how knowledgable you are, so don't be insulted if I tell you some basics that you may already know. If I were about to freeze I would probably be in a panic to get information.

Make sure that the coolant is not low in the radiator. If so, is the temperature guage showing that the engine is coming up to operating temperature? If not, it could be that you need to replace the thermostat. I have had to replace a failed MB thermostat a couple of times over the years for this reason. Is the heater fan blowing in manual mode? In the cars of the late eighties, the climate control problems were more often in the climate control pushbutton unit than anywhere else. As I said I'm not completely familiar with your particular model.

Give us as much more information as you can, and we will all do our best to help.

Best of luck, and bundle up well,
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Old 12-08-2000, 02:16 PM
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Join Date: Aug 2000
Location: Houston, Texas
Posts: 322

I have an '82 240D and I have seen a valve body by the battery as you described. In my case, if I remember correctly, this valve body has three openings. There are two valves (and corresponding two openings for each valve)that are mounted on the ends of the valve body and are connected to control cables that lead to the inside of the car. I have been able to trace these two individual valves to the two big rotary knobs inside the car. I believe they are to control the amount of heated water traveling through the heater core....correct me if I'm wrong.

The third opening on this valve body leads away from the valve body to the front of the engine near the water pump.

Anyway, is this the famous monovalve because what you describe seems to be something different from what I have. Do these valves, like mine, give alot of trouble? I have been operating the heater with no trouble whatsoever. I live in Houston, Texas where it has been getting chilly (around 35-40 at night) and the heater performs exceptionally well.
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Old 12-08-2000, 03:06 PM
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No, what you describe is the heater valve as used on the manual heater as seen on most all 240D's. The 300D of about '81 or later has an automatic climate control system. In this system the monovalve is electrically operated by the automatic climate control system.

The heater valve on the manual system such as yours is, in my experience, bulletproof.

Good luck,
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Old 12-08-2000, 09:49 PM
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I'm making progress

Thanks for the guidance... still need some help
1. Temp gauge is OK.
2. Manual Fan is fine both high speed and low.
3. I found the monovalve as you described.
4. Disconnected the 2 prong connector and still no hot air.
5. All push buttons seem to be working.
Should I buy the monovalve kit?
Sincerely and thanks,
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Old 12-09-2000, 07:01 PM
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I don't think your car has the monovalve. I believe that it has a simple heater valve like found in a 124 car. If it does indeed work like a 124 car, and you did not get coolant flow when disconnecting the valve the problem is elsewhere, most likely the pushbutton control unit.

If it does indeed have the monovalve type sytem, there will be four screws on a square plate at the top of the valve. If it is like the 124, it will be made in such a way that it cannot be disassembled.

I was hopeful that someone who was familiar with your big body car would chime in and let us know if your system is like the 124 car, or like the monovalved 123 car.


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Old 12-10-2000, 07:39 PM
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Still Puzzeled

Thanks for your comments...maybe someone who has a W126, 420SEL can provide me with some guidance. I thought I found something that look like a monovalve with 2 hoses going in and out near the battery. Maybe someone will see this post and help out...cold in DC. Anyone got heating/AC experience with the W126??? Thanks again Larry!
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Old 12-10-2000, 09:43 PM
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Join Date: Dec 1999
Location: Spring Hill,KS USA
Posts: 17

I have an 87 560 SEL (126 body like yours) and had the same exact problem last winter as you are having now. You have a monovalve located near the battery. It is 1 inch square with 4 small screws and a 2 wire connector. Get a new valve diaphram ($38.00 at the dealer) and simply remove the connector, the 4 screws, and the diaphram. Upon close inspection you will find a small tear. It is a simple repair. Good luck.

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