// Waveform & Charging

How to Read Alternator Ripple on an Oscilloscope

Published by DiagEdge · 8 October 2026 · 5 min read

An alternator produces AC internally and rectifies it to DC with a diode pack. When one of those diodes fails, the charging output develops excessive AC ripple — and that ripple causes a surprising range of symptoms, from dim flickering lights to intermittent network communication faults that look like something else entirely. A scope finds it in two minutes.

The setup

• Set the scope to AC coupling (so you see only the ripple, not the ~14 V DC it rides on).
• Connect across the battery terminals, or better, at the alternator B+ and a good ground.
• Use a small volts-per-division setting — you're looking at tenths of a volt.
• Load the charging system: headlights, blower, rear demister. Ripple shows up worse under load.

What good looks like

A healthy alternator rectifying with all diodes produces a small, even, regular ripple — a tight repeating pattern with consistent peaks, typically well under 100 mV peak-to-peak. Every hump is the same height and evenly spaced.

What a failed diode looks like

When a diode fails, one phase of the AC no longer gets rectified. On the scope you'll see a large, irregular sawtooth — the regular small humps are interrupted by a big dip or a missing hump at a regular interval. Peak-to-peak voltage climbs dramatically, often to several hundred millivolts or more. That repeating "one tooth missing" pattern is the classic signature of a failed rectifier diode.

Why it matters beyond charging

Modern vehicles are full of sensitive electronics and data networks. Excessive alternator ripple is AC noise injected straight onto the vehicle's power supply, and it can:

• cause CAN bus and sensor communication errors that throw confusing, intermittent codes,
• make lights flicker or gauges behave oddly,
• interfere with audio and comfort systems,
• and give a battery/charging warning that comes and goes.

Technicians chasing a phantom network fault for hours have, more than once, found a single failed alternator diode as the real cause. If you see ripple like this, replace or rebuild the alternator — cleaning up the power supply often makes a pile of unrelated-looking faults disappear.

Quick reference

• Tight, even, low ripple → charging system healthy.
• Large sawtooth with a repeating missing/irregular hump → failed diode, replace the alternator.
• Always test under electrical load — a failing diode can look fine at idle with nothing switched on.