A/C low side pressure erratic

[sandz71]

Sometimes my A/C blows hot air, then suddenly is cools down. I thought I'd go ahead and charge it but noticed the pressure on the low side was at 35 psi with the front and rear air running. After a bit the pressure began to increase steadily until it stabilized at about 70 psi. It would then drop to 20 and the compressor would shut off. After it kicked back on, it would climb to about 50 psi and then drop back to 20. Other times it goes back to about 70. Any ideas why I'm getting such a swing in pressure?


03 Z71 Tahoe

 

[1_8TTony]

The psi readings will fluctuate as the compresser turns on and off. This is normal. The important reading is the pressure indicated while the compresser is engaged.

The compresser turning on and off is an indication that the system has too much or too little amount or r134a in it. There are safety switches that turn the compresser off if the system has too low of a pressure and too high of pressure.

Could be that some r134a has leaked out of the system and the low pressure switch is momentarily cutting off power to the a/c compresser.......more specifically, the a/c clutch.

 

[retiredsparky]

Over time the compressor seals start to wear. This eventully causes a slow freon leak, giving the varying pressures you are seeing. If the leak is small, you can just add freon periodically. No permanent fix, except new compressor.
You could do a leak test around the components, but if it is a very slow leak, you may not detect any gas, especially if it is low on charge.

After getting your charge back to normal, run the engine with AC active for 30 minutes. Use an IR temp scanner to check cool air temp. Should be under 45*. Then check the temperature of the compressor clutch. If it is too hot to touch/well above 100* you should replace the clutch. When the clutch starts to short internally, the heat of malfunctioning clutch will damage the front bearing/seal on the compressor. The clutch can still function even as it starts to fail.