I need the help of this forum troubleshooting this problem. Any idea is welcome.
On my 1991 300E, I noticed warm air was coming out of the air vents when the fan is on even when the temp dial is all the way to the coldest. I did a search on the forum and everything is pointed to the mono valve as the culprit. I bought the whole unit(this model has a sealed unit and does not have the repair kit). I installed the new mono valve and was very confident it will take care of the problem.
Road tested the car and it seems its okay until the following day when I noticed hot air was flowing through the vents again. I observed that hot air comes out of the vents only when I start the car after the engine was already hot from a previous trip. Hot air blows through the vents for about 2-5 minutes then the temperature drops to the outside temp. This happens every time I start the car with the engine at operating temp. What is the possible cause of this?? Anybody out there encountered this problem in the past? Is it posssible that the new mono valve is defective or there are other areas I need to check. How does the mono valve work and how much current is supplied to it.

This what I think is happening. When the car is parked with the engine "hot", some of the coolant is escaping to the heater core through a valve somewhere. This valve however gets shut completely after you start the car. The hot coolant in the heater core causes it to blow hot air for a short period until the outside air temp cools the heater core and by then everything is normal. Are there other valves in the system other the mono valve. Please advise. Thanks!!!!

Anybody out there who had encountered this same problem in the past, please provide some advise. Still waiting for the MB experts in this forum to share some ideas. Thanks in advance.

I'll throw this one out.

The default state of the monovalve is wide open. With the engine off, coolant is allowed in the heater core. If you start a warm engine, the heater core will continue to be warm for a while.

MB's solution in the W126 is to have an air path between the evaporator and the center vents that bypasses the heater core. There is an alternative path to the center vents downstream of the heatercore.

If the W124 system is comparable, then you might have a bad center vent flap actuator (the center vent flap is downstream of the heater core) allowing heated air to go to the center vents. Eventually the heater core cools down because the monovalve closes when the engine's running and the AC compressor is engaged.

The monovalve is closed in cooling modes, open in defrost mode and othewise cycles to modulate temperature. I suppose you can measure the duty cycle of the monovalve to test that it's working but I don't know what the correct frequencies should be for a given cabin-desired temp difference.

Sixto
91 300SE
87 300SDL
83 300SD

Thanks, Sixto. What you said really makes sense. Hoping the 126 and 124 are similar in the way the vent system is designed, and the center flapper is bad explains why some of the heat seem to be coming from under the dash. How hard is it to get to the center vent flap actuator? Would it require the removal of the dash? A DIY job or had to take it to the shop.

I'm not familiar with the W124 so I can't help you there.

Your best bet is to start a new thread with a subject like 'W124 AC vacuum actuators' or something like that.

If the W124 is anything like the W126, you remove the lower dash panels and the console side panels (you have to look for the screws buried in the carpet). That will expose the inner workings of the AC. I don't know which of the (possibly) 4 actuators you'll see is the center flap actuator. I think it's the lower one in the center. Another thing you should see is a box with about 7 vacuum hoses leading to it. This is the bank of vacuum switches that control the actuators. Some quick testing with a MightyVac will tell you if a swtich or actuator is bad.

Sixto
91 300SE
87 300SDL
83 300SD