WTF? Low battery warning only at high rev?

[AltarCrystal 10+ year member]

I'm not sure WHY this is happening, but when my car goes over 3.5k-4k RPM or so (I'm just going off of an estimate, I never checked the gauge while the light was on), my low battery warning light comes on. After shifting or the lowering the RPM, the light immeditally goes off.

I'm not really runnign anythign stressful. Just the one amp in my sig (powering the coaxals), a power inverter (150w) and a hard drive.

Any clue what's wrong? I had my alternator tested awhile back, and it was fine, I forget what it was poutting out (I've been out of that stuff for a good 6 months now, memory is horriable) but it was putting out what it was rated at. Car is a 2004 Mustang.

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[Ferendon 10+ year member]

It's not putting out enough at high RPMs. Check the cables first, just to be sure. O'Reilly and Autozone only test alternators to 3k rpm, which is considered "normal driving".

 

[AltarCrystal 10+ year member]

It's not putting out enough at high RPMs. Check the cables first, just to be sure. O'Reilly and Autozone only test alternators to 3k rpm, which is considered "normal driving".

I've checked the wires running from the battery, and they were all secure and no breaks or anything, I'll go though more throughly tomorrow if I can.

 

[joelowrider 10+ year member]

look at the belt it might be sliping at highr rpms

 

[Ferendon 10+ year member]

This is true. Check the belt tensioners.

Also a cable can be bad, even with a flawless appearance.

 

[joelowrider 10+ year member]

the cable would be bad all the time wouldnt it anyways try to ohm out the wire but to me it sounds like the belt pully or if you have a clutch in you alt it could be going out

 

[Ferendon 10+ year member]

I've seen cables have RPM-related issues, but they are not very common. I'd be willing to bet it's the belt tensioner.

 

[AltarCrystal 10+ year member]

Hm, yeah, seems very possible. I'll check the belt tomorrow.

 

[Ferendon 10+ year member]

Not the belt... The tensioner. Pull on the belt and see how hard it is to draw some slack out of it.

 

[joelowrider 10+ year member]

?? you lost me but look at the belt and see if it looks burt from sliping also the belt should move in about 1/2 in when you push on it

 

[SPYDERTUNE]

Tensioner & belt definitely. Look at the underside of the belt that touches the pulleys. It should be farily dull in appearance not shiny. If it is, it may be the culprit.

 

[Ferendon 10+ year member]

A dull or shiny belt is something to be noticed by a trained eye, with the experience to make that comparison.

Here's what I'd do... Get some STP Belt Dressing... and a buddy. Have him gradually get on the gas until the light comes on. Spray a few quick spurts of belt dressing on the belt, and see if the light goes off. You should notice an ever-so-slight drop in RPMs when it grabs hold of the pulleys. If that doesn't work, take it down to idle, and repeat.

Also, a failing belt will usually squeal when you hit the gas quickly. Might also try a few drops of water on it while it's running to see if it will squeal.