Automotive Air Conditioning Information Forum

My AC was cooling ok, but not great and then went to cooling not at all. I put on a set of gauges and it showed low charge. Added freon and back to cooling OK. Two days later, it was not working and the gauges showed low charge again. I replaced the evaporator, expansion valve, and filter-drier. I added 45mL of PAG 46 oil. Spec says 35mL for evaporator replacement and 25 when fixing a leak. However, there was very little sign of oil on the old evaporator or the evaporator housing so I arbitrarily decided to put in a smidge less. Evac and recharge and the system holds a charge and cools good when in the garage. I varied the rpm from idle to 2500rpm in the garage and at 1500rpm (ran it at 1500 for 10 minutes to equalize), the low pressure is around 30psi, high is around 225psi and temperature drop of around 30 degrees F. Outside temp is around 90-95 degrees F. When in the garage, the compressor is on continuously and both radiator fans are running.

I cruise down the road with the engine speed in the same idle to 2500rpm range and the compressor cycles on and off and the system blows cold then warm then cold, then warm, etc. for a while. Then it blows only warm. Stopping, turning off and restarting the car makes it work again (with the warm-cool cycle). I can hear and feel the compressor cycling on and off with the temperature rises occurring when the compressor is off. I realize the engine computer may shut off the compressor during acceleration, but this is not the case. It will occur randomly under constant speed, acceleration, or deceleration. All of this occurs with the system on recirculate and blower on high. Any suggestions of why the issue presents itself when driving, but not in test conditions in the garage? I had a suggestion of evaporator or expansion valve icing, but why would this not occur in the garage? And if there is evaporator icing, wouldn't there be reduced air flow from the vents? My next thought is an issue with the evaporator temperature sensor or a pressure sensor. Maybe they have issues when driving due to road vibrations? Any help is greatly appreciated.

Might be overcharged a little. 30PSI @ 1500 on the low side seems high. Expansion block system.

I measured the weights of the cans during recharge so I believe it is correct. Also, the service manual indicates 30 psi is in the range considering the ambient temperature of around 95 degrees F. One odd thing I noticed is when I put the gauges on it again with the car off for 20 minutes or so, the low and high side pressure were not equal. When I replaced the evaporator, I also replaced the expansion valve with the thought that after 10 years and since I was in there anyway, why not replace it. Now I am wondering if I replaced a good expansion valve with a defective one. Any thoughts on these odd pressures when off would be greatly appreciated. Thanks.

Check the compressor clutch gap.

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bohica2xo wrote:Check the compressor clutch gap.

Bohica is wondering if the AC clutch is slipping, causing your intermittent cooling. Visually check to see if the center of the AC compressor is turning when this lack of cooling is observed.

Two weeks ago I had similar issue on my 1998 Frontier - the AC clutch was not always engaging when it got 12 volts positive - and I removed a thin AC clutch shim to narrow/adjust the clutch gap, and it has worked fine since.