Hey guys,

I have a strange HVAC issue I’d like to hear some opinions on…

The AC on my 1995 has seemed to be a little weak lately, I put some gauges on it and I have about 65psi on the low side and the high side – same pressure on each side. The compressor is running (clutch engaged), and there is cool air coming from the vents (much cooler than ambient, but not as cold as it should be).

I had a can of 134a connected (but with the can valve shut off) when I checked pressures in anticipation of low pressure, I cracked the can slightly for about a second and the low and high side went up in tandem, i.e. 70 psi high/low while the can was cracked then settled back to approx. 65psi high/low once I closed the 134a can outlet valve. Also, during this check I had the gauges connected to both the low side and high side ports on the car, but only the low side valve was open on the gauge manifold.

A couple years ago I added a small amount of 134a, maybe 4-5 oz, when the AC was blowing about the same outlet temp (cool not cold). At that time, the system was slightly low on refrigerant according to the gauges, about 15-18 psi on the low side and correspondingly low on the high side (I forget what it was, but the spread between high and low was normal). After adding the refrigerant the pressures went up to normal for the ambient temp at the time, about 30-35psi low and about 160-170psi on the high side and the interior vent outlet temp was very cold.

It has been a long time since I did any AC diagnosis, I seem to recall that even pressures on the low and high side were representative of a stuck expansion valve. But the information I have read online over the weekend suggests a failed compressor.

The strange part of this is that the AC is working, just not as strongly as it should. When I found stuck expansion valves back in the day, the AC in those cars did not reduce air temp at all.

And if the compressor is indeed weak but still working (as I do have cool air coming out of the vents) it seems like the pressures would be different than they are.

I don’t have a problem with replacing whatever is bad, but I want to be sure of the diagnosis before tearing into it.

Any thoughts or suggestions would be appreciated!!

 

May want to read through this -

https://axleaddict.com/auto-repair/D...sis-by-Symptom

Also be certain that the compressor is actually turning on - the middle of the compressor is turning.

Dale

 

Thanks Dale.

This would seem to indicate an expansion valve problem. The compressor was definitely engaged.

 

I was going to say the same. Equal pressure means one of two things: the compressor isn't generating pressure or the orifice/expansion valve is not restricting flow enough to create a differential in pressure.

A theory regarding the cooling you are seeing... Even though the pressure measures equal, there could still be a minor restriction allowing the compressed refrigerant to expand somewhat in the evaporator. Not efficient, but still offering some cooling.

Still, it is troubling that you say it is much cooler than ambient and the pressures are equal. Something doesn't add up there Are you sure the high and low side gauges are connected properly on the manifold set? I think I have to crack the valves on mine to get a proper reading but will look when I get home.

 

I completely agree. I was stunned when I saw the pressures, totally off compared to my expectations. It's a 134a set, so the high and low connections are different sizes on the car so there was no issue there. I think I will need to take another look at it before ordering parts. Thanks for the feedback.