[2000CAYukon]

Disconnect the pressure cycling switch on the accumulator. Jumper a wire across the electrical connector. If the compressor comes on, then it could be:

1) not enough r12/r134a
2) the pressure cycling switch is bad

Before adding r12/r134a check the pressure on the low side. If it is below 25 psi for r12 or 21 for r134a, then you are low on freon.

If the pressure is higher, the switch is probably bad.

//2000CAYukon