Under very hard acceleration the battery light comes on on my 1998 F150 with 4.6L V8.

I just installed a new serpentine belt for maintentance and in the hopes the old belt was just slipping. Belt tensioner seems good. I did notice a just a little noise in the alternator while spinning the pully. Nothing major and no movement in the pully/bearing.

The truck always starts and runs fine. No apparent charging problems are eveident.

Any ideas?

Thanks, Jim

 

I would take the alternator off and have it checked.

 

Took alternator off expecting it to be bad. It tested good at two different parts stores.

Changed belt tensioner.

Cleaned battery posts and alternator connections.

Lightly sanded idler pully to remove and glazing that might cause belt slipping.

All pullys and accessories appear to tight.

Still getting a battery light under very hard acceleration. No light under normal acceleration and steady speeds. Battery is charging fine.

Any ideas?

 

Sounds like you've covered the easy and obvious stuff.

For grins, did you inspect/clean the ground cable connection at the block? Can the battery light be brought on by wiggling the various cables? Looking for a bad cable / connection that may open up only when the engine is torqued enough out of its resting position through the acceleration.

Does the new belt still look good - no oils or other crap in the serpentine grooves, and has it seated properly in the crankshaft pulley? If the belt itself were slipping, would think other accessories would be affected, such as the power steering perhaps becoming stiff.

With the belt off the alternator - does the alternator spin relatively freely? Just speculating here - but not sure that the tests done would get the rotational speed up to levels seen during the hard accelerations. Again, speculating, whether there is a higher-than-normal energy required to turn the alternator that only gets to the point of breaking traction with the belt at the higher RPMs.

Do you have a voltmeter on the gauge cluster? If so, does it do anything goofy during this period? Trying to determine whether this is an actual problem, or just an instrumentation problem.

Is the belt the correct size? Does the tensioner still have plenty of room to operate?

Apologies for not being of more help - the brain cells are still a little sluggish after the feed yesterday.

 

That sucks Jim. Here's a long shot but cheap to check; pull the black cover off that's on the firewall passenger side and check the 20amp fuse that's under the mega fuses. It could have a crack. Be careful, that area has a lot of juice. If I think of something else I'll post.

 

Thanks for all the tips guys!

Maybe some black electrical tape placed over the battery indicator light will do the trick. LOL

 

I thought I'd bring this back up today. It's still doing it.

Now, however, the truck seems to miss and sputter when the lgiht is on. Otherwise, it starts and runs like a champ.

Any more ideas?

 

Normally when the battery light comes on only at high rpm the cause is the brushes in the alternator are chattering on a worn rotor which lowers the output then once the rpm drops the brushes stop chattering and the output goes up and the battery light goes out. With that said the test was done at a low rpm and didn't pick up on the problem. If you have a alternator repair shop in your area they could test it. This could also be weakening the battery and causing the sputtering.

 

Thanks.

 

Here's an update:

Took alternator off and battery out and headed to Napa to get alt. tested and buy new one. Napa said alternator was good and battery was bad.

I took them to Advance to be tested. They said alt. was bad and battery was good.

I bought a new Delco Remy alternator and put it on. Problem solved. (hopefully)

Thanks for the info guys. The goofs at the auto parts stores sure don't help much.