Here is the latest puzzle in the life of a 92 400e. Air cond works great til car is hot. ..then the ac comp cycles frequently and is not sufficient to cool the car. High side gets near 300lbs and the low side around 50 or so. When the car is cold the preassures are about 225 high side 38-45 low at idle. ambient temps around 98 so this is still normal. compressor has 186 k on it but makes no noise at all and makes cold air on cold start up...Ideas?
Thanks for any help with our puzzle.


P.S. The comp stays running at fast idle. when i let it return to normal it cycles at high side 225 low side at 60. New expansion valve, receiver dryer.

Brian

Is the aux fan/fans coming on ? If they dont come on, the pressure at the high side will remain high and that would cause compressor cut-off/cut-in cycles. Did you recently charge the system ? It could be slightly overcharged. If the aux fans do come on at the right times, I would try to remove some refrigerant from the system, get the high side down to between 185-205. Another possibility is that you have air in the system, is there a leak in the system ? Have you charged the system without evacuating ?

__________________
Saumil S. Patel

Hi Saumil.Thanks for replying. I've had more time to check out the car. The auxillary fans work great. The expansion valve is done, the receiver drier, and now the evaporator temp sensor has been replaced. The compressor stays engaged when the rpm's are maintained at acceleration but when I let it idle after the car is hot the comp disengages and will not reengage until I raise the rpm's. This is only true after the car is warmed up. When cool or cold the comp stays engaged full time. I"m scratching my head. Any input will be greatly appreciated.

Brian.

What is the high side pressure when the car is hot and compressor just cuts off ? Belt is nice and tight and the compressor pulley is oil/grease free ? Battery/Alternator voltage is o.k. at idle when warm ? Is there a difference in the battery voltage when cold vs. warm ? The cold vs. warm and low vs. high rpm tests you did were with the car stationary right ?

__________________
Saumil S. Patel

225 on the high side and cooling perfectly...no slippage on the belt...the compre3sser can be made to rengage the compressor without a restart so i know it's not the crank match mismatch circuit. I don't know about the voltage and yes the car was stationary....does this help?
Thanks...Brian

PS..When I replaced the components. (changed out the expansion valve because of high sides over 300 low sides in the upper 20....clogged or failed expansion valve I think) Drew a vacume to -30lbs for 4 hours. It held off the pump with zero leaks over night. Charged exactly 2.2 lbs R134 as the book calls for. I'm in florida so at 180-205 high side at idle with the outside air temp at 98 would I believe be undercharged. Could a bad relay cause this?

I would suggest making sure the fins on the condensor are clean and clear of bugs and debris. The condition you are describing is consistent with over-pressure due to the condensor not cooling sufficiently at idle. If the system is really not overcharged then that is where I would look. The reason it kicks back in when you rev it is because the fan speeds up and cools the condensor more and pressures drop below the cutoff (300 PSI). Clean out the condensor and between it and the radiator and see if that helps. Sometimes you can find quite a mat of crud between the condensor and radiator which blocks airflow. Cleaning it out will help both the A/C and the car's cooling system so it will not be time wasted even if it does not cure the immediate problem.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz