Rick's Free Auto Repair Advice

How to test for alternator AC ripple

Perform an alternator AC ripple test yourself with a multimeter

Modern alternators/generators produce an AC voltage

that’s converted into DC voltage using diode rectifiers. A diode rectifier, in

normal AC ripple pattern

Normal AC ripple pattern captured on a scope

simple terms, is a one-way valve that allows AC current to flow one way (it’s more complicated, but let’s go with this explanation). The rectifier “bridge” contains 3-8 diodes that allow only the negative portion of the AC sine way to pass to ground. Even when working properly, the rectifier bridge passes a small amount of AC current to ground. This is referred to as AC ripple. Too much alternator AC ripple can cause problems with other sensors.

The best way to test for normal AC ripple is with a scope since you’re really looking for an abnormal pattern not just high AC voltage. But most DIYers don’t have scopes, so you can still get some idea using a multimeter.

Here’s how to test your alternator to see if it’s producing too much AC ripple.

Set up your test

• Your battery must be fully charged — Check battery voltage; it must be at or around 12.6 volts. If you perform an alternator AC ripple test on a discharged battery, you’ll get false results.
• Set your multimeter to AC volts
• Connect the multimeter leads to the positive and negative battery terminals
• Start the engine and reve to 1,500 RPM
• Turn on at least one large electrical load on the alternator (headlights on, for example)
• The maximum allowable AC ripple is 50mV-100mV, depending on the vehicle specs. Anything over 0.1 volts is too much.

bad ripple pattern

Here’s an abnormal AC ripple pattern caused by an open diode. This alternator will show that it’s charging, but will constantly undercharge the battery.

What to do if your AC ripple is too high

High or irregular AC ripple can be cause by one or more open diodes in the rectifier bridge or by a fault in the stator windings. Since it doesn’t really make sense these days to replace internal components, replace the alternator with a new unit

The effects of excessive AC ripple

Excessive AC ripple is a sign of a failed diode or shorted stator. However, besides undercharging your battery, AC ripple can really screw up the electronics in your vehicle. It can cause interference on the digital communication circuits in your computer system’s CAN bus networks.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat



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