Rick's Free Auto Repair Advice

Repeat alternator failures

What causes repeat alternator failures

The factory alternator usually lasts about 125,000 to 150,000 miles. But some owners replace their alternators over and over again. They wonder what causes repeat alternator failures when the first one lasted so long.

Alternators can fail due to worn bearings or rotor brushes but most fail due to burned-out diodes or a burned-out internal voltage regulator. In other words, the electronics fail before the mechanical components fail. So the question is; what causes the electronics to burn out early?

Reason #1 Replacement alternators can fail early 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. Causing an 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.

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.

Reason #2 High electrical resistance between the alternator and battery causes repeat 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.

Reason #3 High internal battery resistance causes repeat alternator failures

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.

How to test for internal corrosion

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.

Reason #4 Belt problems cause repeat alternator 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

Posted on by Rick Muscoplat

Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN