It won't kick on at all if you are too low on coolant.

 

If you read the thread, you would notice that the amount of refrigerant(not coolant) is not the problem..

So the low pressure switch I got from Napa and what I'm taking of the accumulator don't seem the same. Their connection and basic build is the same, but the one I take off the accumulator has a metal collar with a nut-like part that contains the shrader valve.

The collar/connection point seems like it's crimped around the switch part(that I just purchased) and there's no obvious way to decouple them. Am I supposed to get a switch that has the shrader valve attached? If not, how am I supposed to replace this properly?

As a reference, the one I got(and all the ones I see listed) look like this photo.

 

Oops, that's a high pressure cut out switch, (located on the back of the compressor). The clutch cycling switch is located on the accumulator and looks like this: Buy ACDelco Air Conditioning Low-Pressure Cut Off Switch 15-5643 at Advance Auto Parts

The schrader valve is in the accumulator and there's a pin molded into the cycling switch that depesses the schrader.

Edit: The description says low pressure cut out switch, that is not what it is, or what it does. It cycles the compressor clutch according to pressure in the accumulator.

 

Yep, the problem was the low pressure cycling switch. Got that installed but I had another leak at the compressor from the tube connection coming loose at the top AGAIN. Tightened that back up, filled it up and everything is working great now.

Thanks to everyone who provided useful information.

Too bad it'll probably have to come apart for a heater core replacement now...

 

Refrigerant does NOT need to be discharged to replace the heater core.

 

keep trying to charge the system. check the guage when the compressor comes on and see where it goes. When the compressor is running it should not drop below the green markings on the guage. when the compressor is off the pressure will read higher giving a false indication that you are in the green zone.