Symptoms of a bad monovalve? - PeachParts Mercedes-Benz Forum

JamesStein · #1 06-20-2004, 03:10 PM

Does this sound like the monovalve?

Car starts.. AC works great until the car is stopped for a short period of time. 30-60 minutes. Then the AC blows hot air for a variable amount of time. Usually 15 -30 minutes in traffic. It will cool down quicker if you get up to speed.

Am I on the right track here?

vwbuge · #2 06-20-2004, 09:10 PM

I am not entirely sure that a bad monovalve has anything to do with the A/C.

#3 06-20-2004, 10:25 PM

The monovalve has everything to do with the a/c. The refrigeration portion of the a/c system runs all the time. The temperature is regulated by the amount of heat that is added to the cooled air (I know, it sounds very inefficient.)

That said, what you describe does not sound like a problematic monovalve. The monovalve typically fails by leaking and flowing coolant to the heater all the time. The symptoms of that are sweat and then smelly clothes.

What your problem does sound like is an inoperative cabin temp sensor. This is typically not the sensor itself, but simply the tube that supplies air across the sensor.

Remove the glove box and you will see a pipe running horizontally behind the glove box that goes toward the temp sensor that is below the small grill in the center of the dash. See if there is a hose leading from the pipe to the sensor. It was originally a foam hose and disintegrates after a few years.

If the hose is gone, your first inclination will be to replace the hose with a piece of garden hose, heater hose, plastic hose or some other substitute. Don't do that! The foam hose conducted virtually NO temperature through the hose material. Being foam it insulated any heat that may travel through the material of the hose so that the air sample was accurate.

Go to Home Depot and get some foam pipe insulation. This is sold for insulating plumbing pipes in the winter to prevent freeze up. Cut a length of the foam hose and slip in place.

Good luck,

Pete Burton · #4 06-21-2004, 09:03 AM

When my monovalve failed, I had similar symptoms. Of course, in CT it doesn't get used much. What I remember very well was that in the winter I would get no heat until the engine was partly warmed up (about 60C) and I SHUT OFF THE ENGINE AND RESTARTED. Then I would have heat as long as the dash thermostat called for it. I hate to admit it, but I wqas so busy at the time that I lived with it for a whole year. Finally replaced the monovalve, no problems since.....Current to the valve CLOSES it when it is operating properly, but if it is failing, the valve may not close. You could try this....Disconnect the wire to the circulator (if you have one, so there is no chance of it deadheading) and clamp the hose between the monovalve and heater core with a flat clamp (I used a vise-grip type) to shut off the flow. See if that changes anything. Just make sure your hose is in good shape (nice and flexible, not hard or cracked) before you clamp it.

JamesStein · #5 06-21-2004, 09:59 AM

I'm pretty sure the foam hose is intact. I'm at the office right now and can't verify. Both mine and my wifes cars are having AC issues and I'm not sure if I actually checked hers or mine.

I suppose I should stop trying to troubleshoot 2 identical systems with different problems at the same time.

JamesStein · #6 06-21-2004, 02:16 PM

Just got a call from my wife. She says it appears to be related to acceleration as well.

The AC started cooling and she came up behind someone slowing down on a hill so she braked. Then climbing up the hill the AC cut out and never recovered.

Pete Burton · #7 06-21-2004, 03:39 PM

that may just be incidental. Check other things in the system before you go too far. At a fast idle (1500RPM) windows open MAX A/C on (temp switch MINIMIZED), blower on highest setting, see that A/C clutch goes on and stays on. Auxiliary fan should come on and off with temperature switch. Also, there is a pressure switch which will cut out the clutch if the pressure is marginal. You can check that with a jumper wire and a gauge if necessary to see if a problem is charge or the switch, etc, etc. At least the system is cycling on and off, so the troubleshooting shouldn,t be too bad. Good luck

JamesStein · #8 06-23-2004, 11:59 PM

Well 2 things I've found..

#1 AC belt is stretched.. to the point that I cannot tighten it enough to even begin to make it tight. I am actually shocked that the compressor is even running its so loose. I've order a new belt to resolve that issue.

#2 in-car temp sensor is was missing some parts and not even connected properly. The hose was just haing free in the dash and the PO had replaced the foam section with some heater hose. I'll remedy that as soon as I can get to Lowes. Also the part that the hose actually connects to on the sensor is gone.. I'm hoping its just hanging at a low point on the wiring harness, but I really didn't feel like dropping the driver side kick panel. I just stole the one off my car for now since the AC in it is majorly wacked.

I imagine the biggest part of the problem is that belt.