What do I need to check next?

Car is a 1986 with a Delphi compressor in it (model 4346) installed by the previous owner. I cross checked the compressor number and it is for a 1992 Chevy pickup. I have owned the car for two years and the system worked properly (compressor, blower, fan), it didn't blow very cold but did a good job defogging the windshield.
I just had the system evac'd and recharged with Freeze 12 (looking for a temporary fix). The mechanic said the system was not leaking, pressure was good, and the vent temp was around 50 degrees. Soon after I left the compressor quit working. The compressor doesn't cycle at all when I hit the A/C. The blower comes on and blows air out the vents. The aux fan is also engaging when I engage the A/C. Going through the previous owner's receipts he replaced the control module in 2000.
After researching threads I disconnected the battery cable to reset computer (for 10 min). This did not solve the problem.

- Low pressure switch reads 5.11v
- Jumping the LPS with a paper clip did not engage compressor
- Voltage at the compressor switch harness is 5.5v
- I get no voltage reading at the compressor switch terminals when the harness is plugged in.

Is my compressor shot?

 

Putting Freeze 12 in on top of R12 can lead to compressor failure. Freeze 12 is 80% R134 and since the old charge has been leaking out, some of the lubricant has gone with it. Remove the belt and see if you can turn the compressor shaft (outer hub) by hand. If the compressor is ok, it will turn over with minimal resistance. Assuming it is ok, the low pressure switch needs 50 psi to close the contacts. A system with a half pound of refrigerant in it should have that much (though it won't run very long before it starts cycling on and off quite a bit).
Remove the connector and check resistance across the low pressure switch contacts. If it's open, either your non leaking system developed a leak, or the switch is bad. Only way to tell would be with a manifold gage set and it isn't a great idea to try and jumper the harness if those switch contacts are open.

I'm assuming - by your voltage at the Low Pressure Switch - that this is Electronic Air. If so, the compressor is grounded by the Blower module as long as the Low Pressure and High Pressure Switch contacts are closed. Always a possibility that the High Pressure Switch opened because mixing gases can lead to extremely high pressures. Circuit though is through the High Pressure Switch and then onto the Low Pressure Switch, so if you have voltage at the Low Pressure Switch, the other is closed. And as long as that circuit is completed, the Control Panel sends the "on" signal to the Blower Molule which then grounds the compressor, so follow the lead from the compressor to the module and jumper it to ground. The clutch should engage. If it does, you probably need a new module, though the Dash also likes to see a completed ECM circuit - that runs from the Panel to Pin B7 at the ECM and it should have 5 or so volts on it when the a/c is requested, so you could check that before running out and buying a new module.

Your Year only communicates with the ECM for idle and WOT control and the few troublecodes (there's none for low or no gas) that can be stored within the Processor (dash controls) will not prevent the a/c from running. So disconnecting and reconnecting the battery or removing memory power from the dash isn't necessary, but it doesn't hurt anything.

 

Thank you much for your detailed response. I saw you other posts while researching this problem and was hoping you would see my question. I'll follow your recommendations and PM you if I have any further questions. Thanks again.

 

Duh - just noted you have no voltage at the compressor. Circuit is from the a/c (25 amp) fuse - through the coil and onto ground at the Module. With the key on, you should get 12 volts at the connector. Check the fuse. If blown, check the diode too (should be in the connector) and if it shows continuity in both directions, the Module may not be working any longer either. Also, check resistance across the coil leads. If none, it's shorted and needs to be replaced.