Electrical and Electronic Systems Flashcards

A truck's ABS warning lamp stays on after the vehicle exceeds 6 mph. What is the MOST likely cause?

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.

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?

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.

A technician finds excessive resistance in a truck's starting circuit. Which tool is BEST suited to locate the high-resistance connection?

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.

On a multiplex (J1939 CAN bus) system, a technician notices multiple modules going offline intermittently. The MOST likely cause is:

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.

A coach's air conditioning compressor clutch engages but immediately disengages. Which electrical component should the technician check FIRST?

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.

What is the purpose of the data link connector (DLC) on a heavy-duty truck?

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.

A truck's fuel gauge reads full immediately after filling, then quickly drops to empty. The MOST likely fault is:

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.