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Cage Alternator Question

Ken H.

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Can an alternator slowly die?


Vehicle – 2000 Ford Ranger 3.0l V6

Symptom – All lights dim significantly at idle, instantly back to full bright upon RPM increase.


Battery – 3-years old but passes a load test with flying colors. Terminals cleaned bright, shiny and tight.

Serpentine belt – new, no squeaking, no suspicion of slippage.

Alternator – No noise, or unusually hot, terminal there also cleaned bright, shiny and tight. Is not the original though, has a “rebuilt” sticker on it. (We didn’t buy the vehicle new.) This is the type with an internal regulator.


This has been a gradual onset getting worse over time thing.


Most of my thoughts have been leaning toward battery despite the load test ok, but I’m not sure.


Thoughts, suggestions on where to look next?


(We’re heading toward our 16 hours of daylight time of the year, so I have bit of time for now to pounder this!)


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Sure. The voltage regulator can stop doing its proper regulating job and things can start to fluctuate. The brushes can also wear down and not be making good contact to the bars on the commutator.


Most auto parts stores should have a test rig for alternators. You can somewhat check it out yourself as well. Check your battery voltage at the battery with the motor off, motor at idle, and again when at a moderate RPM (say 4k rpm on a vehicle with a 7k red line). The battery at rest should be ~12V and the other two should be ~14V. If you see a fluctuation between idle voltage and higher RPM voltage, then you most likely have a fried voltage regulator, or at least one that is on its way out.

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You state the the belt is new with no suspicion of slipping. Did you check the tension of the belt? I've seen belt tensioners that are siezed, or rusted enough that they stick and don't put the proper tension on the belt resulting in belt slipping, which will cause exactly what you are experiencing, low output at idle, but plenty of amperage when reveed up. Grab the belt and tug on it. The tensioner should allow the belt to move, though it will be very firm. If the tensioner doesn't move, or if it moves too easily or is notchy in it's movement, it needs to be replaced. If the tensioner checks out ok, then I would suspect the alternator, as the regulator is internal (inside it).

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How many miles on the truck. Often the brushes wear out or get oiled. This alternator has the regulator on the back, it is easily removed with 4 torx screws. Take a flash light, and look at the slip rings, they can be worn into the plastic. If not and it looks OK the brushes can be replaced.


If the slip ring is worn out the alternator will need rebuilt.


This is pretty common on fords.



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