How cold is too cold for AC if there is such a thing
 

I just finished replacing the TX (expansion) valve and receiver/dryer in my 87 190d. Evacuated it , held vacuum,then charged it up with good old R12. My Pushbutton control unit does not supply the proper ground signal to the Kilma relay so I installed a temporary switch in the ashtray which does the same thing. Result is the compressor runs as long as the engine is running and the switch is on, crude but effective. All engine compartment compressor safeties are functional. In my testing I found cruising down the freeway I was getting center outlet temps below 28-30 deg on the third fan speed. I wasn't sure if this was desireable or safe so I switched the fan to high and got 36-37 deg at the centre outlet. Is there a lower limit on safe operating temps for this type of system and if so what is it and why. Cheers Dan

In general, the untimate temperature a system like ours can attain is limited by the temperature of the air outside. The typical refrigeration system can remove enough heat to lower the temperature inside the vehicle by 50 Fahrenheit degrees or more. You do not say what the outside temps were like during your experiment; certainly BC is not AZ. Switching on the "recirculation" should also make it possible to attain a colder ultimate temperature. It is possible that "too cold" could block airflow through the climate control system by freezing atmospheric humidity. Obviously that would depend on the humidity in your area. I do not think that any harm would come to the system.

If the heat load is less than required for the compressor to run all the time, you'll drop the temperature of the evaporator below 32°F. and it will freeze. Once that happens, you've got a huge block of ice with no airflow.

That's the entire purpose of cycling the compressor...........prevents freezing the evaporator.

Not to steal the thread, but where can you wire a switch to make the compressor stay on all the time when desired?

^^it does until the evap gets to a certain temp, if it gets any colder the evap will turn into a black of ice.

Evaporator Temperature Sensor....
 

My 1987 300D Turbo has a Evaporator Temperature Sensor. With the climate control unit working properly, this device keeps the evaporator from freezing.

I know because the one in my car was defective...the symptoms were when the engine was cold, the air conditioner worked fine. One the engine warmed, the temperature of the air also warmed.

It is a small sensor directly above the accelerator pedal on the left front side of the heater...once replaced, the problem was solved...about $15.

The way I found out about it was the ACC Mono Valve. It wouldn't close. That's why the AC worked well when the engine was cold, but once the engine warmed, the air from the vents got warmer. The sensor opens the valve to let coolant into the heater core to keep the evaporator from freezing...the defective sensor was keeping the valve open all of the time...leave it the Germans to over-engineer...

The Tenor Man

Ac stuff
 

We generally don't get temps much higher than 85 or so which is what it was when testing, also humidity is not too high either. I was more concerned with the refrigeration portion of the system getting damaged. If the evap froze up it would have to get moisture from somewhere. Lowrider the driver side connector on the top of push button unit, the one marked with a red band has 12 pins on it. In my vehicle a 190D pin 6 needs to have a ground signal on it to tell the Kilma to turn the compressor on , the wire is blue with a green stripe. I am not sure if all the vehicles have the same color code or pin position so you may have to do some investigating. I just used a jumper lead connected to ground to verify the pin started the compressor. In my vehicle the Pushbutton unit is electronicly protected from shorting unlike earlier units. Yours may be different. Cheers Dan

W124 cars got the same unit pioneered on your 201 in about 1984. The earlier 123 cars lacked the short-circuit protection and a lot of controllers were fried due to aux coolant pump failures, among others.