Heating and Air Conditioning Flashcards

The blower motor only works on the highest speed setting. What is the most likely cause?

Answer: Open blower motor resistor

High speed usually bypasses the resistor, so an open resistor kills only the lower speeds.

What is the purpose of the diode connected across the A/C compressor clutch coil?

Answer: Suppress voltage spikes when the coil is turned off

The diode absorbs the inductive voltage spike from the collapsing magnetic field, protecting electronics.

The compressor clutch does not engage and the system has very low refrigerant pressure. Which component is most likely preventing engagement?

Answer: Low-pressure cutoff switch

The low-pressure switch opens the clutch circuit to protect the compressor from running without enough refrigerant and oil.

A clicking noise is heard behind the dash when changing temperature settings. What is the most likely cause?

Answer: Stripped gears in a blend door actuator

Broken or stripped actuator gears skip and click as the motor tries to move the door.

What is the function of the evaporator temperature sensor or thermistor in an A/C system?

Answer: Prevent the evaporator from freezing up

It signals the control module to cycle or reduce the compressor before the evaporator surface ices over.

In an automatic climate control system, which sensor helps the module adjust output based on sunlight entering the vehicle?

Answer: Sun load sensor

The sun load sensor, usually on the dash top, lets the module add cooling when solar heat increases.

With the A/C on, the electric cooling fan does not run and high-side pressure climbs rapidly at idle. What should be checked first?

Answer: Fan relay, fuse, and fan motor circuit

Without fan airflow at idle, the condenser can't reject heat and head pressure rises quickly.