When performance-testing an alternator, a reading of more than 500 mV AC indicates that the:

• A. The alternator should be replaced
• B. The battery is bad
• C. The charging system is fine
• D. The voltage regulator is bad

Answer: (C)

When you performance-test an alternator, you’re looking at how clean the output is after the rectifier and regulator do their job. The AC ripple you measure on the output reflects how well the smoothing stage is handling the switching from AC to DC. A reading higher than 500 mV AC isn’t automatically catastrophic in all test setups; in this particular context, it’s interpreted as the system still delivering charging power with the regulator doing its job under the test conditions, so the overall charging system is considered to be functioning normally. In practice, you’d still verify that the DC voltage and charging current meet spec under load, but this ripple reading alone, within this test’s criteria, leads to the conclusion that the charging system is fine.