18/06/2026 6:12 am
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Automotive Q&A
Is 0.82 V AC at the battery enough to condemn the alternator?
Review the evidence, add a focused answer, and distinguish confirmed measurements from assumptions.
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A 2009 Accord has intermittent dimming at idle and a faint whine in the radio. Charging voltage is 14.1 V with headlights and blower on, but a handheld meter shows 0.82 V AC at the battery. Battery terminals and main grounds pass loaded voltage-drop tests.
I know some meters respond poorly to switching noise. What waveform or comparison would confirm a rectifier problem rather than electrical noise from another load?
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19/06/2026 6:30 am
Use a scope AC-coupled across the battery with known bandwidth and repeat the test at idle and raised RPM under a controlled electrical load. A healthy three-phase pattern has evenly spaced, low-amplitude ripple. A missing phase or repeated deep notch at alternator frequency supports an open diode or stator fault. Compare the pattern at the alternator B+ stud and battery to rule out cable-induced noise.
Temporarily switching suspect loads off can identify a separate noise source. Do not disconnect the battery while running; that can damage modules and removes the battery's filtering effect.
Topic starter
20/06/2026 10:49 am
The scope showed a repeating missing-phase notch at both the alternator and battery. Ripple was 0.76 V peak-to-peak under load. The replacement alternator produced an even pattern at 0.09 V peak-to-peak, and the lamp and radio symptoms disappeared.