Watch the belt tensioner moving back and forth in this video. In this case, the movement comes from uneven loading by an alternator with an electrical fault. This alternator was nearly new and had been installed elsewhere. It was working to charge the battery, but the owner brought the car to us because they were noticing a weird rhythmic noise.

An electrical problem with a visible mechanical effect

The second video shows the alternator’s output on an oscilloscope. With the scope set to AC coupling, the steady DC voltage is filtered out so the voltage fluctuations are easier to see. The peak to peak voltage is 1.25V, which is way too high. A tiny amount of AC ripple can be normal, but only at a much higher frequency and much lower amplitude.

The voltage rises and falls rhythmically, matching the movement of the belt tensioner.

As the alternator’s load on the belt changes, the tensioner responds. It’s an interesting example of how an electrical fault can show up as movement in the belt drive.

This is Why we’re selective about alternators

We see too many low-quality aftermarket alternators fail prematurely. A lower purchase price offers little value if the repair soon needs to be done again.

That’s why we use original-equipment new or remanufactured alternators, or suitable alternatives from original-equipment manufacturers such as Denso, Delphi, Hitachi, and Mitsuba when available at a lower cost.

Having charging problems, belt vibration, or trouble after an alternator replacement? Learn about our electrical diagnosis and repair in Berkeley, or call us for an appointment.