420SEL A/C problem and question
 

I took my 420 to the dealer re erratic operation of the A/C, during which the compressor cycles off and on in an irregular fashion... working just fine in one operating instance and then not coming on at all in the next run .... The dealer reloaded the Freon and checked some things, but could not isolate the reason for this action by the system and they could not say that the compressor is the problem... it appears to be some kind of problem with the CC or a valve or sensor that causes the system to do this ?? Any ideas??

Also, what is the Mono Valve on this car ... don't recognize the name of this unit, but know it was replaced once .... thanks, Ben Carter

A/C & Heat
 

blue -

I can't give you technical help, but I know I saw a thread that will lead you to some good answers and understanding of the a/c system and the various sensors etc. that affect its operation. I think STEVEBFL even did a comprehensive write up about our systems that is linked in a thread. Do a search...

That being said, I wouldn't go back to that dealer with the problem if that is as far as they got the first time (its reasonably simple for somebody with the training and diagnostic tools to track down the source of the problem). Maybe they don't want to work on it?

The monovalve controls coolant flow to the heater core. Its on the firewall, almost dead center in front of the passenger. Look for a little assy. with a square black plate on top with a stainless cylinder (about the size of a dime) popping up through it. Its a five minute job to pop the little valve out and check the rubbers; $20+/- part to replace if the rubbers are shot.

Kevin

Take it to a Air Conditioner shop and ask him to put an extra 1/2 lb of freon in the system, thats what I did and the cycling stopped and now it like a meat locker in the car ! :o

Must have been a Volvo..... or other expansion oriface, pressure cycle system. Adding a extra half pound of refrigerant won't help a cycling MB. It might tear up a compressor though if the right amount is already there. If the compressor cycles irregularly on that MB it is most likely an electrical problem..... unless you have too much freon or too much pressure cutting out the compressor. The low pressure switch is also a high pressure switch and if the fan doesn't come on and you are full of refrigerant (or too full) the high side pressure gets to the threshold of the switch (somewhere around 400psi I think) and cuts the comp out. Then at some lower pressure it comes back on.