My mom’s AC stopped working in the middle of this heat wave and I’m trying to fix it. The clutch engages but it doesn’t seem to be pumping.

I had 60 PSI on both low and high side when it’s off. When I turn the AC on, low drops to about 55 and high goes to 65. I thought it might no be enough so I charged some more into it. Now it’s 90 on both when it’s off and 80 low/100 high when it’s on. So it’s pumping a bit but not nearly enough to cool.

From my research, I think it’s either the expansion valve stuck open or a bad compressor. Neither is a quick job so I’d like to narrow it down before I throw parts at it. Are there any easy ways to check if it’s one part or the other? Car has 90k miles and as far as I know, all original AC parts and factory freon charge.

 

Pinch off the low side hose and see if you get a pressure differential?

I don't think you will be able to pinch off the high side hose as the high side service port is after the rubber hose and you would have no way to monitor pressure.

 

I’ll see if I can reach any of the rubber hoses. If there is a big pressure differential when I pince, that would mean my expansion valve is stuck open?

 

That was my thought.

If you get a good pinch on either hose with no change I would bet the compressor is worn out.

 

A refrigerant static pressure of 60 psi means, for all intents and purposes, that the refrigerant level in your A/C system is reading EMPTY.

Locate the leak and fix it.

 

Compressor is shot. Or charge is still too low. You need to do a proper evac and recharge first.

 

not true. a low charge would show in the pressures as a very low low side and a low high side. For example, 60psi static, and then 13psi low side and 88 psi high side when running. The compressor is compressing, its just that there isnt much in there TO compress.

In his case it's almost certainly a compressor issue. an expansion valve on a car being "open" is still enough of a restriction on it's own to produce a higher pressure differential. If you wanna test this find the expansion valve. I'm not sure if yours is a orifice or some sort of TXV. An orifice doesnt get stuck open, so if you have an orifice you can immediately condemn the compressor. If you have a txv put the sensing bulb into a glass of ice water. does your pressure differential change closer to where it should be? If not, condemn the expansion device.

But like I said, compressors with failed reed valves do exactly what you described in the OP, i've seen it plenty of times.