Free 310T Electrical and Electronic Systems Questions and Answers — Questions and Answers
Question 1: A technician is diagnosing a truck with a 'cranks but does not start' condition. The starter motor turns the engine over at a normal speed, but the engine fails to fire. The batteries, starter, and fuel delivery to the injection pump have been verified as good. Which of the following electrical system faults is the MOST likely cause?
- A faulty engine crankshaft position sensor. (Correct answer)
- A failed alternator diode.
- An open circuit in the glow plug relay.
- High resistance in the battery ground cable.
Correct answer: A faulty engine crankshaft position sensor.
The Engine Control Module (ECM) relies on the crankshaft position sensor signal to determine engine speed and position. This information is critical for timing fuel injection and ignition. Without a valid signal from this sensor, the ECM will not command the fuel injectors to fire, resulting in a 'cranks but no start' condition even if fuel pressure is present.
Question 2: While performing a parasitic draw test on a heavy-duty truck, a technician measures a steady current of 450 milliamps (0.45A) after all modules have entered sleep mode. What is the next logical step in this diagnostic procedure?
- Begin removing fuses one by one while monitoring the ammeter. (Correct answer)
- Replace the batteries as they are likely failing.
- Perform a voltage drop test on the starter circuit.
- Load test the alternator.
Correct answer: Begin removing fuses one by one while monitoring the ammeter.
An acceptable parasitic draw for a heavy-duty truck is typically under 50 milliamps. A reading of 450 mA indicates a significant unwanted load on the battery. The standard procedure to isolate the source of the draw is to pull fuses one at a time. When the fuse for the problematic circuit is removed, the ammeter reading will drop significantly, identifying the circuit that requires further investigation.
Question 3: A technician is troubleshooting a communication failure on a J1939 CAN bus network. Using a multimeter, they measure the resistance between the CAN High (Pin C) and CAN Low (Pin D) terminals at the diagnostic connector and get a reading of 120 ohms. What does this reading indicate?
- One of the two terminating resistors is missing or has failed. (Correct answer)
- The CAN bus wiring is shorted to power.
- The CAN bus wiring is shorted to ground.
- The CAN bus network is operating correctly.
Correct answer: One of the two terminating resistors is missing or has failed.
A J1939 CAN bus network requires two 120-ohm terminating resistors, one at each end of the backbone. These resistors are wired in parallel. The total resistance of a healthy, unpowered J1939 network, when measured between the CAN High and CAN Low lines, should be approximately 60 ohms. A reading of 120 ohms indicates that one of the two resistors is not being seen by the circuit, either due to an open in the wiring to that resistor or a failure of the resistor itself.
Question 4: Which of the following BEST describes the primary advantage of a multiplexed electrical system in a modern truck?
- It reduces the amount and complexity of wiring by allowing multiple electronic control modules to communicate over a shared data bus. (Correct answer)
- It increases the amperage capacity of individual circuits, allowing for more powerful accessories.
- It provides a dedicated, hard-wired circuit for every switch and component, improving reliability.
- It eliminates the need for fuses and circuit breakers by using self-resetting electronic components.
Correct answer: It reduces the amount and complexity of wiring by allowing multiple electronic control modules to communicate over a shared data bus.
Multiplexing's main purpose is to reduce the number of wires needed in a vehicle. By allowing different electronic control units (ECUs) to send and receive messages over a common data bus (like a J1939 network), it eliminates the need for miles of dedicated wires for every single function, which simplifies the harness, reduces weight, and saves cost.
Question 5: A truck's headlights are dim, and the battery voltage with the engine running at high idle is only 12.5 volts. The battery itself has been load-tested and is in good condition. Which of the following is the MOST likely cause of these symptoms?
- A faulty alternator or voltage regulator. (Correct answer)
- A parasitic draw draining the battery.
- High resistance in the headlight ground circuit.
- A failed starter solenoid.
Correct answer: A faulty alternator or voltage regulator.
The alternator is responsible for charging the battery and powering the vehicle's electrical systems when the engine is running. A healthy heavy-duty charging system should produce a voltage between 13.5 and 14.8 volts. A reading of 12.5 volts indicates that the alternator is not producing sufficient voltage to charge the battery and run accessories, leading to dim lights and eventual battery discharge.
Question 6: A technician observes that the exterior lights on a trailer are not working correctly when connected to a specific tractor. The lights on the tractor itself function properly. Before checking the trailer wiring, what is the MOST appropriate first step to diagnose the issue at the tractor-trailer connection?
- Use a commercial trailer light tester to check the output signals at the tractor's 7-way connector. (Correct answer)
- Measure the resistance of each circuit in the trailer's pigtail cord.
- Bypass the 7-way connector by using jumper wires between the tractor and trailer.
- Replace all the bulbs on the trailer as a precaution.
Correct answer: Use a commercial trailer light tester to check the output signals at the tractor's 7-way connector.
The most efficient first step is to isolate the problem to either the tractor or the trailer. Using a dedicated tester on the tractor's 7-way (J560) connector allows the technician to quickly verify if the tractor is sending the correct power and ground signals for each lighting circuit. If the tester shows proper output from the tractor, the fault lies within the trailer's wiring or components. If the tester shows no output, the problem is on the tractor side.
Free 310T Electrical and Electronic Systems Questions and Answers
A technician is diagnosing a truck with a 'cranks but does not start' condition.
The starter motor turns the engine over at a normal speed, but the engine fails to fire.
The batteries, starter, and fuel delivery to the injection pump have been verified as good.
Which of the following electrical system faults is the MOST likely cause?