310T Electrical and Electronic Systems Questions and Answers — Questions and Answers
Question 1: A technician measures the resistance between the CAN High and CAN Low pins (6 and 14) on a standard J1939 diagnostic connector with the key off and batteries disconnected. The multimeter reads 120 ohms. What is the MOST likely cause of this reading?
- A short circuit between the CAN High and CAN Low wires.
- One of the two terminating resistors is missing or has an open circuit. (Correct answer)
- Both terminating resistors are functioning correctly in parallel.
- An electronic control unit (ECU) on the network is internally shorted.
Correct answer: One of the two terminating resistors is missing or has an open circuit.
A standard SAE J1939 CAN bus network uses two 120-ohm terminating resistors at each end of the datalink. When measured in parallel, their combined resistance should be 60 ohms. A reading of 120 ohms indicates that the measurement is only seeing one of the resistors, meaning the other is either disconnected, has failed open, or there's a break in the wiring leading to it.
Question 2: A coach is brought to the shop with a complaint of the right-side marker lights flickering intermittently. The bulbs and light housings appear to be in good condition. Which of the following is the MOST likely underlying cause?
- A faulty flasher module.
- A poor ground connection for the lighting circuit. (Correct answer)
- An alternator that is slightly overcharging.
- The body control module (BCM) needs reprogramming.
Correct answer: A poor ground connection for the lighting circuit.
Intermittent flickering in multiple lights on the same circuit is a classic symptom of a poor or corroded ground connection. The circuit loses its stable reference to the chassis, causing the voltage to fluctuate and the lights to flicker. While other options could cause lighting issues, a bad ground is the most common and direct cause for these specific symptoms.
Question 3: What is the primary advantage of a multiplexing (MUX) electrical system in a modern truck compared to a conventional wiring system?
- It increases the voltage available to components for brighter lights.
- It allows for the use of less expensive, lower-gauge wiring.
- It significantly reduces the number of wires, simplifying diagnostics and reducing potential failure points. (Correct answer)
- It eliminates the need for fuses and circuit breakers by using smart switches.
Correct answer: It significantly reduces the number of wires, simplifying diagnostics and reducing potential failure points.
Multiplexing allows multiple electronic control modules to communicate over a shared data bus (like a CAN bus). This dramatically reduces the number of individual wires needed to control various functions, which simplifies the wiring harness, reduces weight, and provides fewer points of potential failure like corrosion or breaks.
Question 4: A technician is performing a parasitic draw test on a truck that fails to hold a charge overnight. After connecting a multimeter in series with the negative battery cable, the reading stabilizes at 450 milliamps (0.45 A). What is the next logical step in the diagnostic process?
- Replace the batteries as they are likely failing to hold a charge.
- Start the engine to test the alternator's charging output.
- Begin pulling fuses one by one while monitoring the multimeter reading. (Correct answer)
- Disconnect the starter motor power cable to check for a short.
Correct answer: Begin pulling fuses one by one while monitoring the multimeter reading.
A reading of 450 mA is significantly higher than the acceptable parasitic draw for most trucks (typically under 50-100 mA). The next step is to isolate the circuit causing the draw. This is done systematically by removing one fuse at a time and watching for a significant drop in the amperage reading on the multimeter. When the reading drops, the fuse for the offending circuit has been found.
Question 5: During a charging system analysis on a heavy-duty truck with the engine running, a technician notices the headlights are dim and the system voltage is 12.5V. Which of the following conditions is the MOST likely cause?
- A faulty voltage regulator or failing alternator. (Correct answer)
- A fully charged battery with high internal resistance.
- An open circuit in the starter solenoid.
- Excessive resistance in the battery ground cable.
Correct answer: A faulty voltage regulator or failing alternator.
A healthy charging system should maintain a voltage between approximately 13.5 and 14.5 volts to charge the batteries and power the vehicle's electrical loads. A voltage of 12.5V is essentially surface charge from the battery and indicates the alternator is not producing adequate output, which is most often caused by a failed internal voltage regulator or other alternator component failure.
Question 6: The SAE J1939 standard is a high-level protocol used for communication and diagnostics between ECUs on a commercial vehicle. On which underlying network technology is J1939 built?
- Ethernet
- LIN Bus
- FlexRay
- Controller Area Network (CAN) (Correct answer)
Correct answer: Controller Area Network (CAN)
SAE J1939 defines the communication language and data structure (the software protocol) that runs on top of the physical Controller Area Network (CAN) bus (the hardware). CAN provides the physical and data-link layers for the J1939 protocol to operate.
310T Electrical and Electronic Systems Questions and Answers
A technician measures the resistance between the CAN High and CAN Low pins (6 and 14) on a standard J1939 diagnostic connector with the key off and batteries disconnected.
The multimeter reads 120 ohms.
What is the MOST likely cause of this reading?