A/C troubleshooting

Hey guys, lost A/C in my 05 Chevy Suburban last week. Been looking into it today.

Here’s what I’ve found so far. A/C compressor doesnt kick on at all. Checked fuses and relays, they seem fine. Checked voltage at the low pressure switch. Voltage is good there. Checked voltage going to the compressor clutch. No voltage at all there. Cant seem to locate the high side pressure switch to check voltage going there.

I also hooked up the manifold guages and I’ve got 40psi on the low side at ~80° ambient temp. Got 40psi on the high side too, cuz uhh… no compressor action lol.

Not sure what this points to or where to look next. Suggestions?

The system is low on refrigerant.

At 80 degrees ambient the static pressure should be about 85 PSI.


80°F* that might be the case at 80 celcius but I’ve consulted two r134a charts and both say it should be between 40 and 50 psi at 80°F

Did you check the low pressure switch for continuity? I don’t believe that the mere presence of voltage at the switch indicates that the switch is working. You will find voltage present at a light switch, but if the switch is broke open, the light will not turn on. Just something to think about.

That’s close- if the system is running as it should. You say your compressor isn’t engaged, so 40 psi on the low side is way too low. You found the low side switch, so I would jump it (connect the two wires inside the connector,) and see if you can force the compressor to activate. If it does, I bet you find WAY lower than 40psi on the cold side.

you can then get to work finding the leak.

What static pressure should I expect to see when I hook up my gauge set.

Each refrigerant has it’s own static pressure at every corresponding degree in temperature. The important thing to keep in mind is static pressure changes based on temperature. Any change of temperature brings with it a change of pressure. The greater the temperature, the greater the pressure. You can use a refrigerant pressure chart to find static pressures at various temperatures. Static pressure will not be used to determine if a system is fully charged. Using the chart below, if the R-134a system has a static pressure of 88 psi at 80 degrees F., we can then assume the system has some amount of liquid refrigerant. The system may be full -or - may not be. At the same temperature, if the system showed only 75 psi, we could say with confidence, the system is low. This is because static pressures shown on a temperature chart would show inadequate pressure for the presence of any liquid refrigerant.


If you have 40 PSI on both the low and high side then there is a lack of refrigerant. The static pressure of 40 on both sides is way too low. The static pressure should be in the 115.125 range.

There could still be a low pressure switch problem but the pressure (or lack of) is a concern.