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Electrical Systems Flashcards

6 cards from real DFFT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 Electrical Systems flashcards as text

  1. A vehicle's cooling fan does not run when the A/C is switched on. The fuse is good. What should be tested?

    Answer: The cooling fan relay and the PCM output signal to the relay

    Most engine cooling fans are relay-controlled; if the fan runs in other conditions but not with A/C, the relay or PCM command signal is the most likely fault.

  2. An alternator bench test shows correct output voltage but the battery repeatedly goes flat in the vehicle. What should be investigated?

    Answer: High resistance in the alternator output cable or connections causing voltage drop

    Even a correctly functioning alternator cannot charge the battery effectively if the output cable has high resistance that drops voltage before it reaches the battery.

  3. A scan tool shows a MAP sensor reading of 100 kPa at idle with the engine running. What does this indicate?

    Answer: No vacuum signal is reaching the MAP sensor, indicating a disconnected or split vacuum hose

    At idle, a healthy engine creates significant intake manifold vacuum, so MAP pressure should be well below atmospheric (100 kPa); a reading of 100 kPa means the sensor sees no vacuum.

  4. A vehicle starts fine when cold but refuses to start when hot. What component is most commonly responsible?

    Answer: A crankshaft position sensor that fails due to heat soak

    Crank sensors are sensitive to heat; after the engine is shut off, residual heat can cause the sensor to fail temporarily until it cools down, preventing the PCM from detecting engine rotation.

  5. What does a negative fuel trim value (e.g., -15%) indicate about the engine's air-fuel mixture?

    Answer: The mixture is running rich and the ECU is subtracting fuel

    Negative fuel trim means the ECU has detected a rich condition and is reducing injector pulse width to bring the air-fuel ratio back to stoichiometric.

  6. A vehicle's PCM sets a lean misfire code with high positive short-term fuel trim. What should be inspected?

    Answer: Vacuum leaks and MAF sensor contamination

    High positive fuel trim with lean codes indicates the ECU is adding fuel to compensate for unmetered air entering the engine, which is most commonly caused by vacuum leaks or a contaminated MAF sensor.