To double-check, your can is connected to the low side hose that goes into the compressor, right? The pressures in the high and low side will equalize when the system isn't running.
The gauge readings are irrelevent when the compressor isn't running because there is always some vapor and some liquid. When vapor leaks out, the pressure goes down. The lower pressure allows more of the liquid to vaporize and expand. That causes the pressure to go right back up again. The pressure will remain constant until all of the liquid has turned to a vapor.
When the system IS running, it will be real easy to pump the low side too low or into a vacuum. That's when the low pressure cutout switch stops the compressor. If your low side gauge is above the cutoff point, the compressor should at least cycle on for a second or two. If that isn't happening, the place to start is with the compressor relay. Bypassing the relay should make the compressor run so you can see what happens to the gauges. The easiest way to bypass the relay is to pop its cover off, reinstall it, then squeeze the contact. If the compressor runs, that proves the electrical circuit to the compressor, including the fuse, is ok.
If that works, use a piece of wire or a stretched out paper clip to jump the terminals in the low pressure cutout switch connector. If that makes the compressor run, the system is low on charge or the switch is defective.
Thursday, April 8th, 2010 AT 5:27 PM