Engine Electrical Systems Flashcards

A technician performs a voltage drop test across a battery cable and finds 0.8 volts with the engine cranking. This indicates:

Answer: Excessive resistance in the cable causing a voltage drop concern

Acceptable voltage drop across a battery cable should be no more than 0.1–0.2 volts; 0.8 volts indicates significant resistance causing a power loss.

What is the purpose of the alternator's voltage regulator?

Answer: To maintain charging voltage within a safe range regardless of engine speed

The voltage regulator controls alternator field current to maintain output voltage between approximately 13.5–14.7V regardless of RPM or load.

A battery load test shows the voltage drops to 9.2 volts under load. This result indicates:

Answer: The battery failed and should be replaced

A battery that drops below approximately 9.6 volts during a load test has failed and cannot supply adequate current for reliable starting.

An alternator output test shows only 12.8 volts at 2,000 RPM with lights and accessories on. Normal is 13.5–14.7V. This indicates:

Answer: Low alternator output requiring further diagnosis

Alternator output should be higher than battery voltage to charge it — 12.8V under load at speed indicates insufficient alternator output.

A starter motor cranks the engine slowly. Battery tests good. The NEXT step is to:

Answer: Perform a starter current draw test and voltage drop tests on starter circuits

Slow cranking with a good battery requires testing starter current draw and voltage drops to distinguish between a faulty starter and high circuit resistance.

When checking charging system output, the technician should perform the test with:

Answer: The engine running at 2,000 RPM with a moderate electrical load applied

Testing at 2,000 RPM with a load applied reflects real-world demand and ensures the alternator is tested under the conditions it must perform.