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Alternator slowly falling asleep

Hello folks-

Any help on this would sure be appreciated. I have a 1996 4 cyl Ranger. Issue is, I was driving to work and the ammeter indicator started wiggling around. It started dropping, and when I revved the engine, it would pop back up- but eventually, even that wouldn?t work and it died. I was stuck without my tools, so I got a ride to the local Walmart and dropped a new battery in. Made it home, but the same thing happened as I pulled into my driveway.

Charged the battery, waited a ? day, got out the voltmeter, battery was at 12.4 V when not running, engaged the engine and started at 14V, then slowly over 3 minutes with lights, AC and radio on, dropped down to 11V. Turned off engine. Took alternator out and had it bench tested at local auto parts store where they sell wine (get it?). They said it was fine. As a test, went to a Ford salvage yard and picked another one up that was also tested fine, put that one in. Same exact issue.

Checked all my fuses, the box in the engine and the panel in the cab, everything checks out fine. Seems that the alternator charges fine, until it starts to run for a bit. At the top of the morning, I can drive for about 20 minutes before the voltage drops.

Any thoughts? I don’t mind buying a new alternator, I’m just not sure if that’s the issue.

Thanks in advance for the help!


Measure the voltage at the alternator itself. Does it fall off there as well, or does it stay steady. If it stays okay, then you have a problem in the wiring between the alternator and the battery or the engine ground strap.

You could have a slipping alternator drive belt.

Alternators have brushes that transfer the field current to the field coil. If you have over 100k miles on this Ranger, it is likely that the brushes have worn so short that they are no longer making contract with the slip rings. One way to check this is to tune the AM radio to an unused station. If you hear a buzzing that rises with the speed of the engine, that is EM evidence that the brushes are sparking as they break contact with the slip rings.

You could take the alternator out and have it tested at an auto parts store or just bite the bullet and get a replacement. I don’t know if the Chilton or Haynes manuals have rebuild instructions for alternators. If they do you could be adventurous and rebuild this one. You might only need brushes.

Hope this helps.

Two possibilities come to mind, first check and clean all CABLES and CONNECTIONS and belt tension as mentioned previously, second replace the voltage regulator, as the spare alternator displays the same symptoms I assume it is not the alternator