Insight regarding those alternator failures

August 1, 2017 | By Staff


In the March issue Peter Harding described his serious alternator problem and it may be mechanical rather than electrical. (Peter’s 1985 Volvo, equipped with a Ford V-8 and an automatic overdrive transmission, has experienced repeated alternator failures.)

Perhaps the alternator is running too fast or too slow, or the belt wrap around the alternator pulley is insufficient.

As for speed, he should determine the rpm operating range of the alternator and then measure the alternator rpm of his installation at idle, at red line and at the normal speed that he travels on the highway for long periods of time.

For example, suppose the alternator comes from a Volvo 740 and the damper and alternator pulleys are the same size…the alternator would idle at 700 rpm, red line at about 6000 rpm and run at 2500 rpm for long times on the highway.

If his damper was twice the circumference of the alternator, the alternator would run at twice the speed of the engine…like 1500 rpm at idle and 12,000 rpm at red line and 5000 rpm on long trips. The alternator speed might be too low if the pulley ratios were the opposite situation.

As for belt wrap around the pulley… Because the belt wrap was very short, our ’86 Malibu would discharge at 70 mph on a rainy day!

In short, he should find out the rpm operating range of his alternator, and adjust his pulleys if his engine does not drive the alternator at the proper speed. This should be calculated for any set-up that is not completely stock. I would be interested in your thoughts on the matter.


Those are all valid points, Jim. Thanks for your input. I am sure readers will find it helpful. I did quite a bit of shopping myself to get the pulleys right on my 1958 Chevrolet Pickup also featured in the March issue, and I know they are critical for the alternator as well as the water pump and the air conditioning compressor if there is one.