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Why is the pressure high on the low side of my A/C Loop?

We noticed that the air conditioner in our 2000 Hyundai Accent isn’t as effective as it once was and I began to investigate. My hypothesis was that the car is 10 years old and may just have over time become low on refrigerant. After checking that my car does indeed use a R-134a based system, I purchased a canister of refrigerant with a hose and gauge for connecting the canister. Following the directions on the product, I attached the gauge and operated the A/C at full blast. Initially the pressure read 45 psi which would suggest that the system was probably about where it should be given the outside ambient temperature. I continued to observer the gauge and over a few minutes the pressure rose to around 100 PSI before the compressor stopped turning (the clutch released, so the outside of the pulley turned normally, but the center of the pulley stopped.) As expected the cool air ceased and the blower at this point only moved around the warm air still in the cab.

All I have done at this point is observe the pressure on the low side of A/C system and witnessed the compressor stopping when the pressure got high. My suspicion at this point is there is a much bigger problem for me find? I know I can always take the car to a specialist, but I was really hoping for a simple and economical do it yourself solution.

Does anyone have any advice or confirmation that its a big job for a specialist?


Does the AC still work the same after your observation as before? You did not add any refrigerant?

You really need gauges for both the high and the low side to service a modern AC system. If you don’t have a cabin airfilter, you might have an accumulation of debris in the evaporator coils. On some cars, you can remove the blower motor and peer into the evaporator compartment through the ductwork. You need a mirror and small or flex head flashlight.

Definitely should take it to a specialist who can hook up a proper set of gauges to check both low and high side of the system. Did you notice if the radiators fans kicked on? Maybe the fans for some reason aren’t turning on causing excessive build up of pressure .

The problem is most likely a stuck expansion valve.

If you connect a set of manifold gauges to the system and the high side pressure is low, the expansion valve is stuck.