310T Electrical and Electronic Systems 3 — Questions and Answers
Question 1: A truck's ABS warning lamp stays on after the vehicle exceeds 6 mph. What is the MOST likely cause?
- Low brake fluid level
- Faulty wheel speed sensor or damaged tone ring (Correct answer)
- Worn brake pads
- Incorrect tire pressure
Correct answer: Faulty wheel speed sensor or damaged tone ring
The ABS ECU monitors wheel speed sensors continuously; a faulty sensor or damaged tone ring will prevent the self-test from clearing and keep the warning lamp illuminated.
Question 2: When testing a truck's charging system, the technician measures 13.2 V at the battery with the engine running at 2,000 RPM and all accessories on. What does this indicate?
- Normal charging output
- Undercharging — alternator output is insufficient (Correct answer)
- Overcharging — voltage regulator is faulty
- Battery is fully charged and alternator is resting
Correct answer: Undercharging — alternator output is insufficient
A fully loaded charging system should maintain 13.8–14.5 V; 13.2 V indicates the alternator is not keeping up with electrical demand.
Question 3: A technician finds excessive resistance in a truck's starting circuit. Which tool is BEST suited to locate the high-resistance connection?
- Ohmmeter across the open circuit
- Ammeter in series with the battery
- Voltmeter performing a voltage-drop test (Correct answer)
- Test light across the starter solenoid
Correct answer: Voltmeter performing a voltage-drop test
A voltage-drop test with the circuit under load reveals resistance losses across connections without needing to break the circuit.
Question 4: On a multiplex (J1939 CAN bus) system, a technician notices multiple modules going offline intermittently. The MOST likely cause is:
- Low system voltage
- An open or shorted CAN bus terminating resistor (Correct answer)
- Faulty ignition switch
- Corroded battery ground strap
Correct answer: An open or shorted CAN bus terminating resistor
The CAN bus requires two 120-ohm terminating resistors (240 ohms in parallel = 60 ohms) for proper signal integrity; a missing or shorted resistor causes bus-wide communication failures.
Question 5: A coach's air conditioning compressor clutch engages but immediately disengages. Which electrical component should the technician check FIRST?
- Blower motor relay
- High-pressure and low-pressure refrigerant switches (Correct answer)
- Thermostat sensor
- Condenser fan motor
Correct answer: High-pressure and low-pressure refrigerant switches
Pressure switches open the compressor clutch circuit when refrigerant pressure is out of range, causing rapid engage/disengage cycling.
Question 6: What is the purpose of the data link connector (DLC) on a heavy-duty truck?
- To connect the trailer ABS module
- To provide a standardized interface for diagnostic scan tools (Correct answer)
- To supply battery voltage to electronic control modules
- To ground the chassis to the engine block
Correct answer: To provide a standardized interface for diagnostic scan tools
The DLC (typically a 9-pin Deutsch connector per TMC RP1226 or SAE J1939) provides a standard port for diagnostic tools to communicate with vehicle ECUs.
Question 7: A truck's fuel gauge reads full immediately after filling, then quickly drops to empty. The MOST likely fault is:
- Faulty instrument cluster
- Open circuit between the sending unit and gauge
- Sending unit float is stuck at the top of its travel
- Shorted fuel level sending unit circuit to ground (Correct answer)
Correct answer: Shorted fuel level sending unit circuit to ground
A short to ground in a resistance-based sending unit circuit pulls the signal wire to zero ohms, initially reading full on many gauge designs before the gauge responds to the short.
310T Electrical and Electronic Systems 3
A truck's ABS warning lamp stays on after the vehicle exceeds 6 mph.
What is the MOST likely cause?