Truck charging system

1998 F150 with a 4.2 V6
Think it still has the original alternator on it as truck only has about 70,000 miles.

Charging system works great and battery is always full.
Then I had to floor the accelerator to merge with interstate traffic and the charge light came on.
As soon as I let off accelerator the light went off.
I very seldom; maybe once in a blue moon floor the accelerator so it may have been like this for years I just never noticed it.

Thinking that the serpentine belt may be loose I checked and it is tight.
Belt is about 2 years old and I have never heard any sound that would make me believe the belt is slipping.
Checked battery with volt meter and charge rate seems normal at idle in drive way.

So I just blew it off as a fluke and have been driving truck for several weeks and the charge light has not come on once.
So today I got to thinking about it. Took it out for a drive.
Sure enough; you drive like a normal person should and charge light never comes on.
But if you floor the accelerator; the light comes on every time till you let up on it.

Any ideas on what might be going on?
I check the belt again today and it looks good and is tight.

Those alternators tend to wear out the slip rings that the brushes ride on. They start out with just a small spot worn through to the plastic then get worse. Sometimes when it is just starting to wear in the faster the alternator turns it gets the brush jumping on it across the plastic spot. It is fairly low mileage for that to happen but I would still take it to a rebuilder and get it checked out. hth, jstpa

Some hi performance Ford engines built in the 60's had a full throttle cut out for the alternator; thus, any power saved by not loading the alt. could be used for acceleration.Maybe yours has something similar???

Ben

If you had the a/c on at the time, a common issue is the wot overide relay has kittens....ain't technology great?

It was already mentioned but I'll verify its a pretty common problem to see batt light at higher engine speeds.

As alt brushes wear, they loose spring tension and ability to maintain contact on the slip rings. At higher RPM the brushes are skipping across the rings and loose contact. Needs brushes or alternator.

dupe

Under heavy load it is suppose to shut off the Alternator to make all power available for acceleration.

(quoted from post at 00:23:56 09/17/15) Under heavy load it is suppose to shut off the Alternator to make all power available for acceleration.

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I'm not sure of any vehicle that does that, especially ford products from at least the last 30 years or so. I just have never heard such of thing, and it really makes no sense to kill an alternator under heavy engine load when in fact the electrical demand is still there and if anything increasing. Doing that would also create unstable electrical signals adversely affecting engine controls, sensors, modules etc.

Could be sticky, arcing brushes, get pushed away from the rings at high RPM. Could be a broken solder connection or a wire in the field going to ground from centrifugal force when the rotor gets up to high speed. Could be a loose wire, bad connection showing up at certain vibration frequencies or from the fan blowing the harness.

Whatever it is, could last forever, or could progressively get worse until it fails. If the external wiring checks out, I would leave it until it shows it's self, could be difficult to find otherwise.