I was helping a friend with his A/C. It's on a 2008 Mercury Mariner (Ford Escape). It's a hybrid, and has a belt-driven compressor. It has not been cooling as well as it should since last summer, but it does cool a little (air at vent today was 60-70 degrees when it was 95 outside).

Compressor comes on and stays on - there isn't any cycling off and on.

With ambient ~95F and idling at ~1000RPM, low side = 70-75PSI, high side = ~170
Increased engine RPM to 2700 (highest it would go while in driveway, I assume due to some hybrid feature), and low side went to 55PSI, high side to 210PSI. Let off the gas pedal and it went back to 75/170PSI.

According to all charts, at 95F it should be 50-55PSI on low side and 275-300 on the high side. I haven't found much info for when the low side is too high and the high side is too low, other than possibly something restricting flow or a weak compressor.

Thanks for your help,
-Chris

 

Every time I've ever seen insufficient pressure differential (except in cases of overcharging), it's been due to a "weak" compressor.

 

My friend has owned the car since new and has not had any A/C work done recently as far as I know. Weak compressor diagnosis sucks, as that thing looks like a challenge to get to!

Thanks for your input,
-Chris

 

Still learning but getting a handle on all this a/c stuff! Couldn't this be a clogged or stuck Expansion valve?

 

That would cause the opposite pressure observations.

 

After replacing the compressor and drier at a local shop, all is well with his A/C system now.

Thanks,
-Chris