FREE 310T Electrical and Electronic Systems 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?

Accepted Answer

A faulty engine crankshaft position sensor.

Answer

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?

Accepted Answer

Begin removing fuses one by one while monitoring the ammeter.

Answer

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?

Accepted Answer

One of the two terminating resistors is missing or has failed.

Answer

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?

Accepted Answer

It reduces the amount and complexity of wiring by allowing multiple electronic control modules to communicate over a shared data bus.

Answer

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?

Accepted Answer

A faulty alternator or voltage regulator.

Answer

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.

(310t) Truck and Coach Technician Certification Practice Test

(310t) Truck and Coach Technician Certification Practice Test

FREE 310T Electrical and Electronic Systems Questions and Answers