First are you sure the power is on?
Next confirm that the relay is pulling in when the thermostat is calling.
Check for voltage on the output side of the relay.
Check that the compressor windings are not open after floating the compressor and using an ohm meter.
If all is good to this point it may be the starter cap.
You will have to open the shroud for some of these tests
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles