DFFT Electrical Systems 2 — Questions and Answers
Question 1: A vehicle's horn does not work. The fuse is intact. What should be tested next?
- The horn relay and the horn switch continuity (Correct answer)
- The battery state of charge
- The alternator output voltage
- The main body control module
Correct answer: The horn relay and the horn switch continuity
After confirming the fuse is good, the relay and switch are the next most common failure points in a simple horn circuit.
Question 2: A vehicle's headlights flicker at idle but are steady at higher RPM. What is the most likely cause?
- Low alternator output at idle not meeting electrical demand (Correct answer)
- A loose headlight bulb socket
- A failing headlight switch
- A discharged battery with a dead cell
Correct answer: Low alternator output at idle not meeting electrical demand
Alternators produce less current at idle; if electrical demand exceeds output, voltage drops and lights may flicker until engine speed increases alternator output.
Question 3: During a DFFT electrical test, you find 0.8V voltage drop across a battery-to-starter cable under cranking load. Is this acceptable?
- No, maximum allowable voltage drop is typically 0.5V for high-current cables (Correct answer)
- Yes, 0.8V is within the standard acceptable range
- It depends on the battery's cold cranking amp rating
- Yes, as long as the engine cranks normally
Correct answer: No, maximum allowable voltage drop is typically 0.5V for high-current cables
Excessive voltage drop in the starter cable reduces voltage available at the starter motor, causing slow cranking; maximum acceptable drop is typically 0.5V.
Question 4: A vehicle's battery goes flat overnight despite no obvious loads left on. What is this fault called?
- Parasitic drain or key-off current draw (Correct answer)
- A faulty battery with reduced capacity
- An alternator that overcharges the battery
- A short circuit in the ignition circuit
Correct answer: Parasitic drain or key-off current draw
Parasitic drain refers to excessive current draw from a component that remains active after the ignition is off, slowly discharging the battery.
Question 5: What is the correct method to test for a parasitic drain on a vehicle?
- Connect an ammeter in series between the battery negative terminal and the cable (Correct answer)
- Connect a voltmeter across the battery terminals with the ignition off
- Use an ohmmeter to measure resistance across all fuses
- Start the engine and measure alternator output current
Correct answer: Connect an ammeter in series between the battery negative terminal and the cable
An ammeter in series monitors the actual current flowing out of the battery with everything off, allowing the technician to identify which circuit is causing the drain.
Question 6: A vehicle's check engine light is on and a scan tool shows a P0300 random misfire code. What sensor data should be examined first?
- Live misfire counter data per cylinder and ignition system output (Correct answer)
- Fuel trim short-term and long-term values only
- Engine coolant temperature sensor voltage
- Throttle position sensor sweep data
Correct answer: Live misfire counter data per cylinder and ignition system output
Cylinder-specific misfire counters help pinpoint whether the misfire is truly random or concentrated in one cylinder, guiding diagnosis toward ignition, fuel, or mechanical causes.
DFFT Electrical Systems 2
A vehicle's horn does not work.
The fuse is intact.
What should be tested next?