If a rooftop unit has relays and contractors chattering, where should troubleshooting begin?

Troubleshooting relays and contactors that are chattering in a rooftop unit should ideally begin on the secondary side of the transformer. This is because a chattering symptom typically indicates an issue with power supply inconsistencies or low voltage. The transformer steps down the line voltage to a lower voltage suitable for control circuits, and problems in this area such as insufficient voltage can directly lead to chattering.

By inspecting the secondary side of the transformer, one can determine if the voltage is stable and sufficient to properly operate the control devices. If the voltage is inadequate, it may cause the relays and contactors to rapidly engage and disengage, resulting in the chattering noise. This step is crucial before investigating other components like the compressor, thermostat connections, or ductwork, as these areas may not be the source of the issue if the primary power supply is unstable.