Was cruising around the other day in my truck (99 5.4) and layed into the throttle to pass someone. The battery light in the gauge cluster momentarily lit up and caught my attention. I've since found that anything above 4200-4300 RPM causes the battery light to come on until the RPM drops again. The battery gauge never moves, and the truck still runs fine. All the power accessories work, and no other wierd symptoms are present. I hooked a voltmeter up to the battery and got a steady 14 1/2 volts from idle to slight revving. This probably doesn't mean much, because i didn't test it under load above 4 grand.

Does this sound like a bad alternator? Maybe not dangerous bad but going South? What puzzles me is why the truck would need one? From my experience, alternators are not usually components that wear out. I've seen them go for 20-30 years.

Is this something I should worry about? Maybe keep a close eye on it for more bad signs? Is this a common problem? Anyone else ever had it happen? Thanks for the advice
Brian

 

Yup. Sounds like your alternator is on the way out. My 97's went at about 100,000 miles. Very easy to replace.

 

My truck only has 55000 miles. I wonder what caused it to go out? Where you having the same high-rpm problem?

 

Yup. I was. If you do a search, you will find the same. As to why it failed, who knows. Just gotta suck it up and replace it. If you're not careful, you'll also be replacing your battery.

 

Your battery light is coming on at high rpm due the alternator regulator brushes bouncing. Causes for the brush bounce is:
A) worn brushes due to mileage (55K is low so doubtfull) or worn due to contamination (ie spilled oil, transmission, or brake fluid into alternator)
B) High engine vibration: rear alternator mounting bracket loose or engine mount problem.

The battery light will come on more often as the brushes get shorter.

 

If you want to take a quick look at the battery charging voltage while driving, run a set of wires into the cab, and hook up the meter to it.
-> Have a passenger watch the meter when you stand on it. ( legal disclamier, don't try driving and watching the meter )

The other maybe common thing Mike_2005 did not bring up, is a sliding belt.
When did you change the belt last ?
Do you have the stock belt on with a tune on the truck ? ( also will get sliding from the stock belt, even when new at the top of the RPM band ).

The battery light normal range :
8.5-10.7 volts, Normal band start
15.8-18 volts, Norm band end

Any thing above 18 V or below 8.5 will cause the battery light to come on, so I guess finding out which side of the band you are on, to see what is the cause might be in order.

Good luck

 

Those are several things I hadn't thought about trying. I'll give everything a closer look and get back. Thanks for the info and suggestions

 

my truck has 57000 miles on it and I am on my third alternator.