2001 S-series A/C pressure switch

The air conditioning on my SL1 is not working. In troubleshooting it, I have no power to the compressor. From there I found the pressure switch has no continuity. So, the next thing is checking the pressure on the high and low sides. Both sides showed 30 PSI. I'm pretty sure the problem lies there. What I can't find is what PSI each side should be. My knowledge of A/C systems is fairly basic, but if I remember right, the high side should be substantially higher pressure than the low side. Assuming I am correct, what's my next step?

 

I think I goofed up when testing the system. I used the A/C testing kit from AZ under their rent a tool. The directions didn't have any instruction to have the engine running during testing, so I didn't have the engine running. Perhaps that gauge set doesn't require the engine to be running?

 

If you haven't solved this ac problem, I think I can help. With an old car, ac problems are usually due to one major issue; the leak no one wants to find but will readily run to the auto store for the refill kit with sealer to attempt refilling...........a leaking system. As a 21yr owner of an L300. I'm familiar with ac problems. Don't use sealer unless you want to spend more money on repairs that may require rebuilding from contamination from sealer use.

The ac pressure switch is the safety device to prevent compressor damage when loss of refrigerant occurs. In general, system pressures mimics ambient temperatures; 75F/75psi, 85F/85psi, etc. 30 psi is a hint of loss of refrigerant..............from a leak; fittings, service valves, compressor, etc. Normal ac pressure sensors are set for around 40 psi; this corresponds to 40F in cold weather when ac is attempted and won't allow compressor operation. Some think they need cold ac air when it's already cold outside. Refrigerant moves lubricating oil throughout a system, returning to lube the compressor. Loss of refrigerant means oil won't circulate, resulting in compressor damage so the pressure sensor sends a disable signal to the ecm to prevent compressor operation. Repair the leak, evacuate the system and refill to restore factory ac cooling. Most GM ac systems using r134a already have dye from factory installation. It glows greenish/yellow when using an inexpensive uv blacklight in shade or dark garage/night time inspection. Look inside the two service valves for dye and oil for examples. The valves may be worn and leaking and easily replaced.

There's a simple bypass test to see if the compressor runs; disconnect the ac pressure switch and insert a paper clip or wire into the two terminals and startup the engine and turn on ac. If you see the compressor turn on, clutch plate engage and spinning, stop. This verifies the compressor works. Do not run the compressor this way unless you want to destroy it from lack of oil lubrication. Once repairs are done and the system holds a vacuum after evacuating the system, refilling with r134a with the first can (12oz, no sealer) should pressurize the system above 70 psi, well above the pressure switch setting to allow ac operation. The first can should allow ac operation with the second can filling the system for cooling.

 

I haven't addressed the issue since my post. A major issue with my wife's car and a family issue put it on the back burner. Since then, another issue arose with the car that I'll need to start another thread for. I'll try your suggestion after I get this issue fixed. Thanks for your suggestion.

 

Nice description.

With the gauge set attached and the can closed off, you should see about 225 on the high side and 30 on the low side.

I think the low pressure switch triggers between 20 and 25.

 

You're welcome