NAPA - National Automotive Parts Association Certified Mechanic Electrical and Electronic Systems Questions and Answers — Questions and Answers
Question 1: A technician is diagnosing a no-communication issue on a high-speed CAN bus. Using a multimeter with the battery disconnected, what is the expected resistance measurement between the CAN High and CAN Low lines?
- Approximately 120 ohms
- Approximately 60 ohms (Correct answer)
- Infinite resistance (OL)
- Zero ohms
Correct answer: Approximately 60 ohms
A standard high-speed CAN bus uses two 120-ohm terminating resistors at each end of the network, wired in parallel. When measuring the resistance across the entire bus, the multimeter will read the equivalent resistance of these two resistors in parallel, which is 60 ohms. A reading of 120 ohms would indicate one of the terminating resistors is missing or the bus is open.
Question 2: A customer's vehicle has a dim headlight and the corresponding taillight is completely out. All other lights function normally. Which of the following is the MOST likely cause?
- A faulty alternator
- A blown fuse for that specific circuit
- A poor ground connection for that lighting circuit (Correct answer)
- A failed lighting control module
Correct answer: A poor ground connection for that lighting circuit
A poor or high-resistance ground connection is a common cause for multiple lighting issues on the same circuit. The high resistance in the ground path will cause a voltage drop, leading to dim lights or preventing them from illuminating at all. A blown fuse would cause both lights to be completely out, not dim. A faulty alternator would affect all vehicle electronics, and a failed lighting control module would likely cause more widespread or different symptoms.
Question 3: When performing a voltage drop test on the positive side of a starter circuit, a technician connects a voltmeter between the positive battery post and the starter's main power terminal. While cranking the engine, the meter reads 0.8 volts. What does this reading indicate?
- A faulty starter motor
- A fully charged battery
- Normal operation for the starting circuit
- Excessive resistance in the positive-side starter cable or connections (Correct answer)
Correct answer: Excessive resistance in the positive-side starter cable or connections
A voltage drop test measures the amount of voltage lost due to resistance in a circuit. In a high-current circuit like the starter circuit, a reading above 0.5 volts generally indicates excessive resistance. This resistance could be in the cable itself, at the battery terminal connection, or at the starter solenoid connection, and will impede current flow, causing slow cranking.
Question 4: Which of the following is a critical safety precaution a technician MUST take before working on the high-voltage systems of a hybrid or electric vehicle?
- Disconnecting the 12-volt auxiliary battery only
- Wearing standard leather work gloves
- Disabling the high-voltage system by removing the service disconnect/plug (Correct answer)
- Keeping the vehicle key fob within 5 feet of the vehicle
Correct answer: Disabling the high-voltage system by removing the service disconnect/plug
Before any service is performed on or near high-voltage components, the system must be depowered. This is accomplished by removing the high-voltage service disconnect plug or switch, which isolates the high-voltage battery from the rest of the vehicle's electrical system. Simply disconnecting the 12V battery is not sufficient, and proper high-voltage insulated gloves, not standard work gloves, are required personal protective equipment (PPE).
Question 5: A vehicle's check engine light is on, and a scan tool reveals a code for a malfunctioning Mass Airflow (MAF) sensor. To test the sensor's signal, a technician back-probes the signal wire. What should the technician expect to see on a multimeter or scan tool as the engine RPM is increased from idle?
- A steady voltage of 5.0 volts
- A voltage or frequency that decreases as RPM increases
- A voltage or frequency that increases smoothly as RPM increases (Correct answer)
- A voltage that cycles rapidly between 0.1 and 0.9 volts
Correct answer: A voltage or frequency that increases smoothly as RPM increases
The MAF sensor measures the amount of air entering the engine. As engine speed increases, more air is drawn in. The sensor reports this change to the ECU by producing a signal whose voltage or frequency increases in direct proportion to the airflow. A good sensor will show a smooth, linear rise in its output as the engine is revved.
Question 6: A vehicle's charging system is being tested. With the engine running at 1500 RPM and all accessories off, the voltage at the battery terminals is 14.2 volts. When the headlights, blower motor, and rear defroster are turned on, the voltage drops to 12.5 volts and continues to fall. This MOST likely indicates:
- A faulty voltage regulator or failing alternator (Correct answer)
- A sulfated battery
- High resistance in the battery ground cable
- Normal charging system operation
Correct answer: A faulty voltage regulator or failing alternator
A healthy charging system should be able to maintain a voltage typically between 13.5 and 14.5 volts even when under a heavy electrical load. A significant voltage drop that continues to fall when accessories are turned on indicates the alternator cannot keep up with the demand, pointing to a failed diode, worn brushes, or a faulty voltage regulator within the alternator.
NAPA - National Automotive Parts Association Certified Mechanic Electrical and Electronic Systems Questions and Answers
A technician is diagnosing a no-communication issue on a high-speed CAN bus.
Using a multimeter with the battery disconnected, what is the expected resistance measurement between the CAN High and CAN Low lines?