310S Electrical and Electronic Systems 4 — Questions and Answers
Question 1: A technician notices the battery warning light flickers at idle but goes off at higher RPM. What should be checked first?
- The battery state of charge
- The drive belt tension and alternator output at idle (Correct answer)
- The dashboard bulb socket for a loose connection
- The fuse for the charging circuit
Correct answer: The drive belt tension and alternator output at idle
Flickering at idle suggests the alternator is not producing sufficient output at low RPM, which could be caused by a slipping or worn drive belt, or an alternator with a failing stator winding or diode.
Question 2: What is the purpose of the diode trio in an alternator?
- To convert AC to DC for the vehicle's electrical system
- To supply DC current to the rotor field coil from the stator output (Correct answer)
- To protect the battery from overcharging
- To regulate alternator speed
Correct answer: To supply DC current to the rotor field coil from the stator output
The diode trio taps a small portion of AC output from the stator, rectifies it to DC, and feeds it to the rotor field coil. This makes the alternator self-exciting after initial field current from the battery.
Question 3: What is the correct procedure for testing a fuse without removing it from the fuse box?
- Visually inspect the element through the transparent housing
- Use a test light or voltmeter to check for power on both sides of the fuse with the circuit energized (Correct answer)
- Shake the fuse and listen for rattling
- Measure resistance across the fuse with the circuit off
Correct answer: Use a test light or voltmeter to check for power on both sides of the fuse with the circuit energized
Testing both exposed test points on top of a blade fuse with a test light or voltmeter while the circuit is on confirms power is passing through. If only one side has power, the fuse is blown.
Question 4: What causes alternator whine in the audio system?
- A faulty radio antenna
- AC ripple from a failing rectifier diode coupling into the audio signal wiring (Correct answer)
- Speakers that are too large for the amplifier
- A worn drive belt squealing at high frequency
Correct answer: AC ripple from a failing rectifier diode coupling into the audio signal wiring
A failing diode in the rectifier bridge allows AC ripple to pass into the DC electrical system. This AC noise couples into audio signal wiring, producing a whine that increases in pitch with engine RPM.
Question 5: A vehicle equipped with daytime running lights (DRLs) has one DRL that does not illuminate. The headlights work normally. What is the most likely cause?
- A blown headlight fuse
- A faulty DRL module or relay controlling that specific circuit
- A burned-out DRL bulb or LED unit (Correct answer)
- A discharged battery
Correct answer: A burned-out DRL bulb or LED unit
Since the headlights work normally, the bulb wiring and ground are functional. The most likely cause is a burned-out DRL-specific bulb or LED unit, as many vehicles use separate elements for DRL and headlight functions.
Question 6: What does a waveform with missing humps on an alternator AC ripple test indicate?
- A worn drive belt
- One or more failed rectifier diodes (Correct answer)
- A shorted rotor field coil
- Normal three-phase output
Correct answer: One or more failed rectifier diodes
A normal three-phase alternator produces a smooth ripple pattern. Missing humps indicate that one or more rectifier diodes have failed, preventing that phase's output from contributing to the DC supply.
310S Electrical and Electronic Systems 4
A technician notices the battery warning light flickers at idle but goes off at higher RPM.
What should be checked first?