[ap0001]

Silverado Z71 5.3L. A/C did not cool at all. Noticed the compressor did not engage whe a/c was turned on. Bridged the low pressure swithch with gator clip to check if the compressor would kick on. Compressor did kick on. Checked the coolant pressure on low side and found no pressure. Added refrigerant (R134a) and a/c worked pefectly for about 3 days then stopped cooling.
Rechecked pressure on low side with vehicle off. Pressure checks normal (about 45psi). Started vehicle the rechecked pressure.... here's the confusing part: (note - all pressure is checked on low side)

- Compressor kicks on for about 5 seconds and pressure reads low - 20psi
- Then compressor turns off and pressure gauge climbs to about 80psi
- 5 seconds later, compressor kicks back on and pressure rapidly decreases
to about 20psi
- 5 seconds later, the compressor turns off and pressure gauge rapidly
climbs back up to 80psi
- repeat
- repeat
- etc......

Anyone know what may be causing this? Suspect blockage in system somewhere. Did not find any leaks with dye an uv light. ANy help is greatly appreciated.

[Denny]

Sounds like either the receiver dryer, or a expansion valve problem.
You didn't mention what the high side pressure was ?

[ScoT Frenzel]

First you have a leak. That is why it works a few days then quits.
Second the low pressure switch kicks off at 28 psi.
you need more juice.

[econoaddict]

Originally Posted by ScoT Frenzel

First you have a leak. That is why it works a few days then quits.
Second the low pressure switch kicks off at 28 psi.
you need more juice.

Second this one.
Leak and low freon causing the fast cycling

[Denny]

That could be a third it, but, without knowing the high sides. you can't be sure. Many have a high side pressure safety also. But you guys probably have it correct.

[ScoT Frenzel]

Let's not forget to mention that most new vehicles do NOT have a receiver dryer nor an expansion valve, that is why they have a low pressure cycling switch on the low pressure/suction side of the system. I'm fairly sure this is an accumulator/orifice tube system.

Rethinking this: If the static pressure is 80 then there must be 134 in there. 80 might be fine depending on the ambient system temperature. What's the high side? Adding 134 to keep the low side above cut-off might make the high side extreme. 134 is very temperature sensitive.

Consult a temp/pressure chart for 134 by google.com. http://dodgeram.org/tech/repair/HVAC/R134a.htm
80* ambient system temperature would mean 87* static pressure. You say yours is 45 psi static? Definitely low on gas.

Remember, a hot engine compartment will make the system temp very hot, so static pressure might need to be way higher,, as you have seen 80 over 45, etc.

This is where an infarred gun is really handy for charging.

Fix that leak. and charge to proper amount. Remember overcharging hinders cooling. So does too much oil. Don't add oil unless you find an obvious oily leak. And remember, a little goes a long way.

Further thought indicates that a restriction is possible depending on high pressure readings.