During alternator performance testing, a reading greater than 500 mV AC indicates the alternator should be replaced.

• A. The battery should be replaced
• B. The alternator should be replaced
• C. The charging system is good
• D. The vehicle should be towed

Answer: (B)

When testing alternator output, you’re looking for clean, DC current with only minimal ripple. A reading of more than 500 mV AC means there’s excessive AC ripple coming from the alternator, which usually happens when the rectifier diodes or voltage regulator inside are failing. That ripple shows the DC output isn’t being properly rectified and regulated, so the charging system won’t behave reliably and the battery can’t be charged correctly. Because this is a sign of internal failure, the appropriate action is to replace the alternator. The other options don’t fit because the issue isn’t a bad battery, a good charging system, or a condition that requires towing based on this measurement alone.