Electrical and Electronic Systems Flashcards

A truck's J1939 CAN bus network shows intermittent communication faults only when the ambient temperature exceeds 35°C. Voltage on the CAN High wire measures 2.8V and CAN Low measures 2.2V at idle. What is the MOST likely root cause?

Answer: A terminating resistor with a positive temperature coefficient is drifting out of spec under heat, causing impedance mismatch

The differential voltage (CAN H 2.8V, CAN L 2.2V) is within spec at rest but the temperature-dependent behavior points to a passive component failure. A terminating resistor that drifts above 120Ω under heat raises bus impedance, causing signal reflections and intermittent faults. Reversed wires would cause constant faults; a module in thermal shutdown would set its own DTCs; bus overload is unrelated to temperature.

While diagnosing a no-crank condition on a heavy truck with a 24V electrical system, you find 0.8V voltage drop across the positive battery cable under cranking load. The specification is a maximum of 0.5V. Which test would BEST pinpoint the exact location of the excessive resistance?

Answer: Perform a milli-volt drop test across each individual segment of the positive cable circuit, including connections and junction points, while cranking

A voltage drop test performed under load (while cranking) across each segment of the circuit pinpoints where resistance is excessive because resistance shows up as voltage drop only when current is flowing. An ohmmeter test on a disconnected cable lacks load current and may miss intermittent connections. Individual battery load testing addresses cell condition, not cable resistance. Starter draw confirms the starter is working hard but does not locate the resistive segment.

A coach's electronically controlled air suspension system raises the rear axle but fails to lower it on command. The height control valve solenoids test correctly with a direct 12V supply. The ride-height sensor signal reads 4.7V at all times. The ECU ground measures 0.04V drop to chassis. What does this pattern MOST indicate?

Answer: The height sensor has failed in a high-voltage clamp, causing the ECU to interpret the coach as perpetually below ride height

A height sensor producing a fixed 4.7V output (typically near the top of a 0.5–4.5V range) signals to the ECU that the chassis is far below the target ride height, so the ECU continuously commands the inflate/raise solenoid and never commands lower. This is a sensor failure (signal clamped high), not a valve or power fault. An open circuit would give 0V or an implausible reading; an ECU supply issue would affect all functions; a stuck exhaust valve would cause uncontrolled lowering.

A 310T technician is using a lab scope to diagnose a suspect alternator on a Class 8 truck. The AC ripple on the charging circuit measures 350mV peak-to-peak. What does this reading MOST likely indicate, and what is the correct follow-up action?

Answer: A diode trio or rectifier bridge has an open or shorted diode, allowing AC to leak into the DC output; the alternator requires replacement or diode replacement

AC ripple exceeding approximately 200–250mV peak-to-peak on the DC output indicates a faulty rectifier diode. A bad diode fails to fully convert AC to DC, allowing AC ripple to contaminate the charging bus. This can damage sensitive electronics and cause erratic ECU behavior. Heavy-duty alternators are not exempt from this threshold; 350mV is abnormal. The voltage regulator controls output voltage level, not ripple; and while a weak battery can mask ripple, the correct diagnostic sequence checks the alternator with the batteries in circuit.

During a pre-trip inspection on a coach, the driver reports that the left-front turn signal flashes at normal rate but the right-front flashes at double speed. All bulbs appear to be working. A technician finds that the right-front marker light circuit shares a ground with the turn signal. What is the MOST likely explanation?

Answer: The shared ground has developed high resistance, causing the right-front turn signal bulb to see reduced current and the flasher to sense a low-load condition, interpreting it as a burnt bulb

Electronic and thermal flashers determine flash rate by measuring circuit resistance/load. A high-resistance shared ground reduces effective current through the bulb. The flasher interprets the reduced current as a missing or burnt-out bulb and increases flash rate as a warning. Because all bulbs illuminate (even dimly through the shared ground), the technician may not notice the fault visually. A faulty flasher would likely affect both sides; a high-wattage bulb would slow the flash rate, not increase it; software faults are a last resort diagnosis.

A truck's diesel particulate filter (DPF) regeneration is inhibited even though exhaust back-pressure is above threshold and all regeneration enable conditions appear met. The technician notices that the diesel exhaust fluid (DEF) quality sensor is reporting a fault code for low DEF concentration. How does this sensor fault DIRECTLY cause regeneration inhibit?

Answer: The aftertreatment control module inhibits active regeneration when SCR system faults are present to prevent injecting unburned hydrocarbons into a compromised SCR catalyst

Modern aftertreatment systems treat the DPF and SCR as an integrated system. If the SCR is compromised (detected via DEF quality, NOx sensor, or doser faults), the ACM will inhibit active DPF regeneration because the hydrocarbon dosing required for regeneration would pass through or over a non-functional SCR catalyst, releasing uncontrolled NOx and HC emissions. This is an intentional OBD/emissions protection strategy. The other options misattribute the control logic or describe non-existent circuit relationships.