In this troubleshooting problem, it’s the middle of July, and you are the follow-up technician on a callback. The equipment is a standard 3-ton package unit, a/c with gas heat, 208/230, single-phase residential installation. The original complaint was a “no cooling at all” call, and the technician who preceded you on this job (we’ll call him Technician #1) has only limited experience, but he reported correctly that he found a failed transformer and replaced it.

He’s only been gone for two hours, and the customer is calling back to say that the unit is out again, and the house is warming up. When you arrive, you find the following conditions:

1. An irate, overheated customer.
2. The thermostat is set to the cooling mode, and the fan switch is in the auto position.
3. No air from the discharge registers.

Escaping from the irate customer and going to the roof, you find:

1. The outdoor fan motor, compressor, and indoor fan motor are not running.
2. The unit is sitting quietly, making no attempt to start.

Removing the access panel and using your volt-meter, you find:

1. A 230-volt reading at L1 and L2 of the contactor.
2. The contactor is not pulled down.
3. No voltage reading (24-VAC) at the contactor coil.

At this point in your troubleshooting procedure, even though it’s not something you would normally do, you perform a manual test of the contactor. When you press down, the unit operates normally, and when you release the pressure you exerted to overcome the spring on the contactor, everything continues to run. Upon close inspection of the transformer wiring (see our simplified illustration in Figure 1), you realize what Technician #1 did wrong.