[TerryW]

should have a well pressure switch where the air compressors are.  if you take the cover off it and take a wood stick and pressure the relay contacts down and it starts, then the pressure switch is bad. 

tom

Yep, at least that's the way it is on my 86. The switch is right on the 120V pump and it looks identical to a water pressure switch on a well. Plastic cover with a nut about center. Be careful once the cover is removed. If the power is on, then one side of the contacts will be energized and if the pressure drops below the switch's set point it should close and energize the pump. If it doesn't, it's an easy fix to just replace the pressure switch. You can test it by manually pressing the contacts together, but use a reasonable amount of caution here. Some pumps are 240 volts and can give you quite a shock.