What happens
A break in the secondary path prevents the control load from receiving power.
Typical symptoms
Normal input but no usable secondary output; downstream controls remain dead.
Checks that distinguish this cause
Separate the transformer terminals from the downstream harness in the documented isolated state. Test output and winding integrity using the manufacturer's procedure.
What can look similar
A loose secondary terminal or an open R conductor can imitate a winding failure. Output that returns when the load is disconnected suggests a load fault.
Repair direction
Repair external connections if they are the break; replace the transformer only when the internal failure is established.
Verify the repair
Verify output at the transformer and R-to-C at the remote controls during a call.
Equipment-specific limits
Read the primary taps, secondary rating, VA capacity and protection arrangement. A normal unloaded voltage can differ from the rated loaded voltage. There is no universal winding-resistance value.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Resideo: Universal transformer installation instructions (69-0251)Model-specific primary taps, output ratings and checkout; use your own transformer label.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
Editorial review: 2026-09-29. Verify the current manual for the installed model.