Repeat alternator failures — What causes multiple failures

Here’s what causes repeat alternator failures

A factory alternator usually lasts about 125,000 to 150,000 miles. But some owners replace their alternators over and over again. Most repeat alternator failures are not caused by multiple defective alternators.

Here are the 4 most common reasons for repeat alternator failures

1) Replacement alternators can fail if you use it to charge a dead battery

An alternator can recharge a battery after a start, but it’s not designed to recharge a completely dead battery. That’s why they make battery charges. Forcing your alternator to run at maximum output for long periods is the #1 cause of diode and voltage regulator failure. Trying to charge a completely dead battery by letting your engine idle is even worse. You’re forcing the alternator to put out lots of current at a low speed. That’s the best way to fry the diodes. Using it to regularly to charge a dead battery is the #1 cause of repeat alternator failures.

If your battery is dead, charge it with a battery charger. A charger costs less than $75, while an alternator costs well over $200-$500.

2) High electrical resistance between the alternator and the battery causes a constantly undercharged battery and early alternator failure

High electrical resistance in the connections at the alternator or corroded battery terminals will cause the alternator to work harder all the time. That shortens the life of the alternator. The fix? Clean the battery posts and terminals when you see corrosion.

3) High internal battery resistance forces the alternator to work harder

As batteries age they develop internal resistance. A voltage and load test tells you what’s coming out of the battery, but it doesn’t tell you how power is flowing into the battery. In addition, a single bad cell in the battery can also cause repeat alternator failures.

Test for a bad cell

A bad cell will show up as battery voltage dropping about 1.5-2 volts within an hour after charging.

How to test for high internal resistance

1) Use a battery tester with a resistance/capacitance feature

2) Connect an amp probe to your fully charged battery and start the engine. You’ll see approximately 40-50-amps for about 5-mins. Then, the amps should drop down to below 10-amps by the 5-min mark. If the amps remain high and all the cables and posts are clean, the battery has too much internal resistance.

A current draw of just 28-amps for longer than 5-mins due to internal battery resistance can kill a replacement alternator in as little as 3-days.

4) Belt problems cause repeat failures

No engine produces a smooth rotation. Every time a cylinder fires the drive belts encounter a power pulse that tensions the belt and then a relaxation of the belt. That constant pulse/relaxation causes the belt to vibrate and that vibration transmits to the alternator bearings. An automatic belt tensioner includes a dampening mechanism to reduce belt vibration. The tensioner dampener has a limited lifespan and when it wears out, it stops dampening the pusles.

©, 2021 Rick Muscoplat