310T Electrical and Electronic Systems 4 — Questions and Answers
Question 1: A truck's electronic engine control module (ECM) stores a fault code for a MAP sensor but the sensor tests good. What should the technician inspect NEXT?
- Replace the ECM
- Inspect the wiring harness and connector for damage, corrosion, or chafing (Correct answer)
- Replace the MAP sensor anyway
- Clear the code and return the vehicle
Correct answer: Inspect the wiring harness and connector for damage, corrosion, or chafing
Intermittent faults are commonly caused by wiring harness issues such as chafing against the frame or corroded connectors rather than a failed sensor itself.
Question 2: Which type of truck lighting circuit uses a pulse-width modulated (PWM) signal to control brightness?
- Stop lamp circuit
- Marker lamp circuit
- Dash dimmer-controlled interior lighting (Correct answer)
- Hazard flasher circuit
Correct answer: Dash dimmer-controlled interior lighting
Dash dimmer controls vary LED or incandescent lamp brightness by modulating the duty cycle of the supply voltage via PWM.
Question 3: A technician is diagnosing a no-crank condition on a truck. Battery voltage is 12.6 V. There is a click at the starter solenoid but the starter does not crank. What is the MOST likely cause?
- Faulty neutral safety switch
- High resistance in the solenoid hold-in winding
- Discharged battery or high resistance in the heavy-gauge starter cables (Correct answer)
- Open ignition switch circuit
Correct answer: Discharged battery or high resistance in the heavy-gauge starter cables
A single click from the solenoid that does not engage the starter motor points to insufficient current delivery, caused by a weak battery or high resistance in the main starter cables.
Question 4: On a coach with a multiplexed body control system, a technician wants to verify that a door switch input is being received by the BCM. The BEST method is:
- Backprobe the switch with a test light
- Use a scan tool to monitor the BCM's live input data stream (Correct answer)
- Measure voltage at the BCM harness connector with a DVOM
- Replace the BCM and retest
Correct answer: Use a scan tool to monitor the BCM's live input data stream
A scan tool's live data stream shows exactly what the BCM is seeing from its inputs, confirming whether the signal is reaching the module without intrusive probing.
Question 5: What does a resistor in series with a trailer marker lamp circuit indicate when used in a conventional (non-LED) hookup?
- It protects the BCM output from overload
- It reduces voltage to prevent lamp burnout
- It simulates the load of a burned-out lamp to prevent false turn-signal hyper-flash (Correct answer)
- It filters electromagnetic interference from the circuit
Correct answer: It simulates the load of a burned-out lamp to prevent false turn-signal hyper-flash
When LED trailer lamps replace incandescent lamps, load resistors are added to mimic the higher current draw so the truck's flasher unit maintains the correct flash rate.
Question 6: A truck's electronic parking brake (EPB) system activates the service brakes rather than the spring brakes on shutdown. Which component is MOST likely faulty?
- ABS modulator valve
- EPB control module or its solenoid valve circuit (Correct answer)
- Air dryer purge valve
- Foot brake valve
Correct answer: EPB control module or its solenoid valve circuit
The EPB module controls the solenoid that exhausts the spring brake chambers; a fault in the module or solenoid circuit can cause incorrect brake activation.
Question 7: A technician measures 0.8 V drop across a battery-to-starter cable during a crank attempt. SAE specification allows a maximum of:
- 0.1 V
- 0.2 V
- 0.5 V (Correct answer)
- 1.0 V
Correct answer: 0.5 V
SAE J541 specifies a maximum 0.5 V drop across any single heavy-gauge cable in the starting circuit; 0.8 V indicates excessive resistance.
310T Electrical and Electronic Systems 4
A truck's electronic engine control module (ECM) stores a fault code for a MAP sensor but the sensor tests good.
What should the technician inspect NEXT?