AC almost tweaked - PeachParts Mercedes-Benz Forum

pselaphid · #1 06-07-2007, 12:39 PM

After much tinkering and head scratching, my 134 converted (by PO) AC works fine around town. I've rebuilt the mono valve (twice) and it seems to be opening and closing as intended in low speed operation, with hot air immediately after a hot restart, then cooldown, but I can live with that. CCU has also been replaced with reman, and dash sensor was replaced. The final piece of the puzzle is highway operation of the AC. After five minutes or so of operation, the AC starts blowing lukewarm (not hot) air and will not, under any settings, deliver cool air. Two to five minutes later, operator delivers a stream of expletives, shuts off AC and opens sunroof and windows. Vent operation, blower speeds, and heat operation all seem okay. Possibly, coolant is being forced through the monovalve due to the increased pressure from the water pump at continuous high rpm, but it seems not to cool down with return to slow speed operation? Perhaps compressor is shutting off at sustained highway speed and staying off? Has anyone experienced this particular variation in CC maladies and successfully tracked it down? Gettin' hot down here in south Louisiana and I am fighting the temptation to take it to the local indy and tell him to "fix my AC" but I will if it seems like a failing compressor.

vstech · #2 06-07-2007, 01:01 PM

pull your cc back out and clean the contacts in the cables with a pipe cleaner. the power not going to the switchover valve can cause the car to dump the cold air and throw you into defrost mode. does your air come out the defrost vents when it cuts out?

Kuhlrover · #3 06-07-2007, 02:22 PM

The climate control could be an issue but it could also be an A/C system issue. For example you could have a problem with moisture trapped on the inside of the system that freezes and cloggs the expansion valve. You could also have problem with possibly the evaporator building up ice on the outside of it and therefore restricting air flow. I suggest you measure the temperature at the evaporater (not the air duct) after 5 minutes running at very high idle and then again when it starts acting up. If the evaporator is at the same temperature then you have an air mixing issue. (i.e. hot air is mixing with the cold air when it is not suppose to.) If the compressor is running, there is good pressure differential from the high side to low side pressure on the system, and the evaporator is warm, then it probably an expansion valve that has iced up.

TimK

pselaphid · #4 06-07-2007, 02:57 PM

Thanks, an evaporator/expansion valve problem seems more likely. Probably will take it to the indy, but will be sure he tells me what he did to fix it and will report back for folks who might be having the same symptoms. I don't think the CCU is the problem. The duct routing never changes (all vacuum pods were rebuilt too) as it would with a switchover fault. I was pretty meticulous about the connections when I replaced the CCU.