ASE School Bus Technician Brakes Certification (S4) Practice

To determine the functionality of the heater on the air dryer, it is important to measure voltage while the system is powered on. Measuring voltage with the key OFF will yield no power to the heater circuit and therefore won't provide any useful information regarding the heater’s functionality. The heater must be energized to assess whether it is receiving adequate voltage for operation.

Checking resistance and observing the physical condition of the heater can be useful diagnostic methods, as they evaluate the component's integrity and operational capacity when it is not energized. However, without voltage measurement while the system is switched ON, one misses critical operational data that ensures the heater is functioning properly.