Currently driving through Utah into Reno and at around 3500-4000 rpm under load up a hill my battery light is coming on. Volt gauge reads steady.

This is a 99, v10, 4x4, lariat edition, extended cab shortbed.

I replaced belt, it was getting hard and tensioner cuz I was in there. Has alt tested at advance auto and they claimed it was fine. I don't have much faith in their test though.

At this point I am thinking alt or battery. Your thoughts? Anything I can test to rule it out? Thanks

 

How old is your battery?

 

May 2011, not terribly old. Cranks over just fine and I keep it on a maintenance charger. I wonder if it is sulfered(sp?) up.

I think I am okay getting into Reno, will have it tested there.

 

5 years and 3 months is GAME OVER for automotive batteries.

I'm not saying this *is* your current issue but you need a battery for sure.

 

I guess I am used to the days of 10 years. A shame they don't make them to last. I will give that a shot first. Thanks.

 

Get a VOM-meter and measure your alternator output . I t should be 14-15 Volt .

 

Did you replace the belt and tensioner because of this problem? Or before this problem?

I'm thinking it's unusual to have battery light only at high rpm. .. So no battery light at regular rpm?

 

I had the same thing happen to my 2005 E250 and it ended up to be my alternator

 

It just didn't like the high rpm?
This one has me puzzled. Curious for more details.

 

Sounds like the alternator. The difference is between room temperature when they tested it, and under the hood doing 3000-3500 RPM with a load on it.

There are all sorts of reasons an alternator starts to go - from the diode bridge to the brushes, and then the regulator, when they get hot any of these things can start to become intermittent.

Let them cool down, and everything's fine.

 

I must confess to a song filter in my head.. EVERYTHING gets routed through the filter, so...

"Let them cool down" reminded me of this tune:

 

Okay so yes at idle measures 14.2 ish. Problem only at higher rpm as mentioned. Replaced belt and tensioner because of this issue. Theory was belt was slipping. Tensioner replaced for cheap insurance.

New clue discovered last night. When this happens headlights dim. Light goes off headlights get bright. Sounds like alternator to me. Strange that dummy gauge needle doesn't move, but I guess that is why they are not to be trusted.

Thanks for all the quick responses guys. I made it into Reno last night so this should be an easy fix.

 

Flashing battery light means the diode in the alternator is going. Mine did that and it was the alternator.

 

I had the same problem with my alternator on the way home from Canada a few years ago. The battery light came on as soon as I got to 4000 rpm and over. It waent off when I got below 4k rpm.

I tore the alternator down 2 weeks after I got home. Pic below is what I found. One of the brushes was worn down really far. My guess is once the rpm got up the brush started to lose contact with the slip ring. My slip ring was worn too but still had enough copper on it to function properly. Since I had it apart I rebuilt the entire alternator, rectifier and all, though really the slip ring and brush/brush holder is really all that needed to be replaced.

 

Problem is fixed. It was the alternator. But unfortunately or fortunately my battery died as well. Stopped at autoparts store to test things and after the battery test it wouldn't crank. So after replacing battery, new terminals, cutting back wires, alt tested bad. Replaced that and all good. While I hated to replace so many components that battery would have likely left me stranded in the desert. Thanks again for the help.