First let's determine whether you are getting power to the low pressure switch and compressor from the internal control unit--get a 12 volt test light and attach the alligator clip to a good ground. Then go to the low pressure switch on the drier pull off leads and test the wire end connections for power. If one lights up the test light, then you are getting power to the switch from the internal control unit. If not then your problem is electrical with the control unit or fuse. Then reattach the wires to the low pressure switch and do the same with the wires that go to the compressor. One should light up. If not, then you are probably low on freon or the pressure switch may be bad (less likely).

BTW, Autozone here will "lend" a set of gauges and a vacuum pump if you leave a deposit.

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead