What condition is necessary for checking the heater on the air dryer for resistance?

The need to check the heater on the air dryer for resistance during extreme temperature conditions, both below 40 degrees Fahrenheit and above 90 degrees Fahrenheit, arises from the way heaters function in air dryer systems. At temperatures below 40 degrees Fahrenheit, the heater is typically activated to prevent moisture from freezing in the air dryer, which can impact its performance and reliability. Conversely, when temperatures exceed 90 degrees Fahrenheit, it is essential to ensure that the heater is not malfunctioning or remaining on unnecessarily, as excessive heat can lead to component failure.

By checking the heater's resistance in these temperature extremes, a technician can determine if the heater is functioning correctly and ensuring the air dryer operates effectively. This thorough approach helps maintain optimal performance of the braking system and overall vehicle safety. Thus, the need for dual conditions—both below 40 degrees and above 90 degrees—emphasizes the importance of monitoring the heater's functionality throughout different temperature ranges.