I am troubleshooting a problem with my AC system and have come across a device in the engine compartment, passenger side, under the big plastic cover by the firewall that I could use someone's advice or help on. This device has 2 water hoses from the firewall coming to it, a couple of electrical actuators on it - one for each hose, and runs a single hose to another device below the air filter which eventually feeds back to the engine. I removed this device, took it apart to inspect the inside, and cleaned up what might have been a sticking plunger. Prior to reinstalling I checked resistance and had 10 ohms from center to left post (viewed from front of car) and 1 ohm from center to right post. Could anyone tell me if this device has anything to do with controlling the temperature in the heater core or might affect the constant heat coming from the side vents. Also can anyone verify the readings I got as good or bad. I can only assume that this part is bad since both coils should probably read the same.
Thanks in advance for any help...

 

The device you are describing is what is caled a "monovalve" As you surmise, it controlls how much heated engine coolant goes to the heater core(s) under the dash. The silicone diphram on older units is natorious for ripping, and requiring replacement. The little diaphram in the 80's use to be a $2-$3 replacement item in the US, but MB decided to only sell it with the electro actuator for $30 or so. Cars with dual heating, have two monovalves, one for each side of the car.

The rip in the diaphram can be small and hard to find. Pulling the actuator out and streatching the diaphram can make rips easyer to find. Older W123's had an electomagnet that fit around the actuator and the electromagnet would cause the plunger to move in and out. I havn't pulled apart the monovalve in my W202 to look and see if it is similar.

The plunger as you mnetioned can be sticky, mostly though when there is a leak and the coolant has gotten back into the plunger. If you follow the heater hose from the engine block, you will find an inline water pump that sometimes can stop working. You will get some heat, but not much, especially in realy cold weather.

 

Thanks for the reply. I was guessing this was the duovalve I saw referenced in other posts. It looks to be a much better design than that monovalve was. Inside the actuator is a spring loaded piston that drives a stem that it slides over. The opposite end of the stem is a beveled hard rubber that is pushed into another hard rubber ring when power is applied shutting down water flow. There is a rubber diaphragm between the sections that looks to be designed to eliminate water from the piston area, but the design probably doesn't totally eliminate it. I have read about the newer vehicles that have individual driver/passenger heat settings where this duovalve would make sense to be applied but my car has only a single setting for the whole compartment.

My problem is not heat, I get more than enough. My problem is cooling. It is 45F here most of the day. I set Auto mode and temp to 65-70. As the interior temp sensor passes about 75F I can hear the switching happening in the dash but I never get cool air again. The fan speeds up to compensate but the cabin temp continues increasing to about 86F-88F where it hovers. At that time the heater core temp is about 140F and the coolant temp is about 175F. If I switch manually to low fan speed to keep the temp reasonable the heater core temp rises because the airflow is not cooling it as much.

I have checked the proper operation of all the air flow flappers and they all seem to switch. I hear no vacuum leaks except when I turn the system off. Then I hear a leak at the vacuum header under the glove box. As soon as I turn the system on the vacuum leak goes away. I pulled all the error codes from the PBU a few days ago, there were 12. I cleared them and the only one that comes back is 52, which I have no idea how to interpret.

I think that duovalve (monovalve) costs about 300 to replace so I kind of want to know somehow that it is indeed bad. I assume the CCU sends it the open/close signal and the 2 times I have checked both actuators had 12V applied. I don't know enough about the system to know if, with my outside temp, the system is supposed to use only outside air to cool or if the system is supposed to close the valve so the heater core temp goes down. The other possibility I have for being a problem is the CCU itself, about 600 to replace. The only reason I can suspect the CCU right now is that it doesn't remember the last setting I had used when I turned the car off. I am sure it used to remember.

With the above in mind I would surely appreciate some other info or troubleshooting tips from someone before proceeding with costly parts swapping. And I am hoping someone can check their resistance on the same psrt for me. It's real easy to get to the posts to check...

Thanks again for any help anyone can offer...

 

I do have cool air sometimes within the first 10-15 minutes, but after that never again. I also can hear and feel the AC compressor kick in at times after those first 10-15 minutes. Hope this helps in someone's reply..

 

Thanks again sph17. I went to the junk yard with my meter and checked 2 other monovalves. Both had 0.00 readings across both coils. The folk there applied 12V to each side and I checked by blowing as hard as I could on each pipe. Cost was 85.00 from junk yard. Dropped the replacement in, took about 10 minutes. Voila - cool air from all vents (including side vents) and warm air when needed. Heater box temp hovered around 125F when heating and dropped to about 90F when cooling.

Only problem I have left is CCU not remembering it's last setting when starting vehicle. No big deal. Junk yard quoted 125.00 for replacement CCU. Maybe some day.

 

I was having problems with a Monovalve on one of my W-123, and I hesitated spending $125 to replace the whole unit as the electo-magnet was bad. What I did during the summer, so as not to have hot air, was to clamp the hot water hose coming out of the monovalve, before it went into the firewall. Vola, no hot air.

I finaly broke down, and bought the new mono-valve as I wanted to sell the car and I didn't want to have to give the new owner my favorite pair of vice-grips with the car attached to the heater hose. I saved the new diaphram for my other W-123 as I know sooner or later it will go bad, again, as it has about six times allready in 20 years.