Rick's Free Auto Repair Advice

Alternator AC ripple test

How much Alternator AC ripple is too much?

What is Alternator AC ripple?

Modern generators actually produce AC voltage that’s converted into DC voltage using diode rectifiers. A diode rectifier, in simple terms, is a one-way valve that allows AC current to flow only 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. However, as the diode rectified ages and begins to fail, it allows more AC ripple.

How much AC ripple is too much?

In order to test for AC ripple, the car battery must be fully charged. If you perform an AC ripple test on a discharged battery, you’ll get false results. With a fully charged battery, the engine running at 1,500 RPM and 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.

At that point, you must either replace all the diodes or replace the alternator

The effects of excessive AC ripple

Excessive 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.

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Posted on by Rick Muscoplat

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