Ref: 2001 F-150 Supercrew

I don't think my battery is too old; I'll pull my files to see.
Everything runs and starts fine. But if I romp on the gas, above 3500 rpm the battery light comes on and the rest of the electricals (esp the headlights!) go dim.
If it is the alternator, why would this just happen at that rev level?

Barry

 

When the brushes on your alternator wear down, the springs holding them against the slip rings have to expand in order to keep the brushes firmly against the slip rings. As the springs expand, the pressure against the brushes decreases and then the brushes tend to bounce on the slip rings at high RPMs. When the brushes are not in contact with the slip rings the alternator stops supplying current to the electrical system (for those small periods of time during each bounce) and the light comes on. The first thing I would do is to inspect the brushes for wear.

 

NKENN,

Thanks.

Is there any on-line instructions/diagrams for inspection/replacement of the brushes?

Certainly sounds cheaper than a dealer diag with a new alt, new batt, and so on as you know they'd do all that and more.

Barry

 

99.9% of the time it is the alternator as previously stated. Easiest repair is to replace the alternator.

 

Replacing the brushes is relatively simple. You may be able to find instructions with pictures on the net or in a Haynes or Chilton manual. Usually the brushes cost less than $15, which is a lot better than the price of an alternator. But, if you do, check the brushes for wear before indiscrimately replacing them.

 

My 97 Expedition started doing this , I waited too long and the brushes wore so much that the spring contacted the ring cutting a groove in it.. I replaced the brushes just to try and it didnt work.. Brushes were 12 bucks from napa, very easy to change.. Chilton manual tells how.

Alternator is easy to change also.

 

I know this is a repeat but my truck did the same thing and the alternator was the cause. Just backing everyone else up here. Pretty easy to change.