My 2002 Ranger FX4, has something weird happening with the alternator. When I accelerate above 3000 rpm, the battery light on the dash comes on, and the alternator gauge will drop to a 1/4 of the way between the L and H. (Normally it is right in the middle of L and H). Then once the rpms drop below 2000, the battery light goes off, and the gauge returns to normal.

I have never seen one behave like this. I am assuming the alternator is dying, but most I have seen, just quit working. Any suggestion would be appreciated.

Thanks,

 

check voltage regulator first, it's cheaper than the whole alternator.

 

I did do a little testing this evening. First serpentine belt looks good and tight. No oil on it to cause slippage. All pulleys are running true, and belt tension is good.

I place a volt meter across the battery, when the truck running at idle, I had 14.4 Volts. Once I revved the engine up to 3000 rpm, then volt meter dropped to 12.5, which is what it reads when the motor is not running. So, it appears that the alternator completely drops out at 3000.

Thanks for the voltage regulator suggestion. One question, to save me a bit of scavenger hunt, can someone tell me where it is located? My truck has the 4.0 liter in it.

Thanks

 

The regulator is inside the Alternator.

Run this puppy by your favorite autoparts store for a no cost, in vehicle electrical system check up, via their portable electrical system tester. Properly used, it can check out the vehicles electrical system & can likely sniff out your problem.

If it's the alternator, have the new one bench tested before leaving the store with it.

Let us know what you find.

 

How many miles on this alternator. if it is above 125,000 miles I would suspect the brushes are getting short and skipping on the slip rings. The voltage regulator is on the back of the alternator it also holds the brushes. It is the part with the three wire plug that plugs into the alternator 4 screws takes it off. If the brushes are short it usually means at least one of the slip rings is grooved pretty badly.

 

wtroger,

The truck has 89,000 miles on it. Thanks for all the info guys.

 

I changed the alternator this weekend and it fixed the problem. Friday morning when I went to leave for work it had completely quit working.

 

Cool, three day weekend.