Not long ago, I had a contactor problem on a Manitowoc rooftop condensing unit. The control circuit is 208 volts, fed from the line side of the contactor, and a low pressure switch is used for de-energizing the contactor when the unit pumps down due to a closed solenoid down in the ice maker head.

The low pressure switch contacts started to cause a chattering, then each time the contacts closed, the high compressor starting current pulled the voltage down which made the chattering even worse. Because the voltage drop was different each time the pressure switch operated, the unit could be making ice normally for up to a week or two.

The fix was to replace the damaged contactor and the pressure switch.

In this instance, the fact that line voltage was used for the control circuit was a key element in the problem, and it was difficult to find the cause because the switch did not always behave in the same way.