310T HVAC and Cab Systems 3

Question 1

A truck's cab heater produces warm air only at idle but blows cool air at highway speeds. What is the most likely cause?

Accepted Answer

Low coolant level causing air pockets at higher RPM

Answer

Low coolant can cause air pockets that disrupt heater core flow under the increased demand of highway operation.

Question 2

When recovering refrigerant from a 310T-compliant truck A/C system, the technician must:

Accepted Answer

Use EPA-certified recovery equipment and store refrigerant in approved containers

Answer

EPA Section 608 regulations require certified recovery equipment and approved containers when servicing any stationary or mobile A/C system.

Question 3

A coach A/C system uses a thermostatic expansion valve (TXV). The evaporator is frosting over. What is the most probable cause?

Accepted Answer

TXV stuck closed restricting refrigerant flow

Answer

A TXV stuck closed starves the evaporator of refrigerant, lowering evaporator pressure and temperature below freezing, causing frost buildup.

Question 4

What does a sight glass with bubbles during A/C system operation typically indicate?

Accepted Answer

The system is low on refrigerant

Answer

Bubbles or foam in the sight glass during normal operation indicate insufficient refrigerant charge in the system.

Question 5

A truck cab blower motor runs only on high speed. Which component is most likely faulty?

Accepted Answer

Blower motor resistor or speed control module

Answer

The blower resistor (or PWM control module) provides resistance for lower speeds; if it fails open, only the direct high-speed circuit works.

(310t) Truck and Coach Technician Certification Practice Test