DFFT Electrical Systems 3

Question 1

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

Accepted Answer

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

Answer

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.

Question 2

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

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

A crankshaft position sensor that fails due to heat soak

Answer

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.

Question 5

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

Accepted Answer

The mixture is running rich and the ECU is subtracting fuel

Answer

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.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Electrical Systems 3