When you arrive and test cycle the unit by setting the thermostat to call for heat, you discover that the burners ignite, but after a delay, the system shuts down on the high limit switch due to the fact that the indoor blower motor did not operate.
When you consult the wiring diagram (click here for a PDF of the diagram), you note that the motor receives its 230-volt power supply via the wiring shown from T1 and T2 on the contactor. You also note that the replacement contactor that was installed in the previous service call is a two-pole component, and that the technician who performed the repair maintained the wiring as it is shown on the diagram.
Your troubleshooting question: What is the underlying cause behind this situation that is preventing the blower motor from operating in the heating mode?
Compare your answer with ours by clicking on the PDF link below.
Publication date: 03/05/2012
For information on Jim Johnson’s HVACR technician training DVDs, go to www.technicaltrainingassoc.com/HVACRVideoStart.htm.