Over 4k RPM-Voltage drops, battery light goes on

[Mystic1350]

as it the title says, when i floor it or go over 4k rpm, the batter light comes on and the battery indicator goes way down

any possibilities of what this could relate too????

thanks!

 

[nyuk98gt]

Mystic1350:

Disclaimer: my electrical skills are quite limited.

If the battery is older (say +4 years) and/or the vehicle does not get driven much and/or a large sound system is installed then the [stock] alternator cannot charge the battery fully. Belt slip is another possibility but this is unlikely on an alternator pulley (imho). Underdrive pulleys make this problem a little worse but this is not a 'root cause' (imho).

You can have the battery and the alternator tested (separately).

You guys have had some sustained cold weather in NJ this winter and this is hard on batteries. Could be time to replace it.

HTH,

Chris

 

[GDawg]

Yeah, I would start by having the battery and alternator tested at an auto parts store for free. If they tell you either one is bad have them show you or get it tested at another store so they don't just tell you they are bad to sell you new ones. If they both come out good then we'll dig deeper...

 

[flstang65]

Sounds like the alternator to me.

 

[Mystic1350]

you think this would just happen out of the blue??? alt is under 2 years old and battery is under 3 years old....
i just had header leak fixed and guy who worked on car accidently crossed up 02 wiring, which i fixed this morning....problem as described didnt happen beofre i fixed the wiring, but i wouldnt think correcting the problem would lead to this issue

 

[nyuk98gt]

Mystic1350:

Interesting additional info. Being electrically-challenged, I need to know what you mean when you sayd "he crossed up the wiring". Did he short something out sufficient to blow a fuse or just plug in the harneses to the wrong sides (i.e.--left on right, right on left)?

If there has been a short then there's a diode (I think that's what it is) and a voltage regulator in the alternator that *might* have suffered damage from that wiring issue. Check the Haynes manual (or your choice of Mustang fix-it books) for the wiring schematics and see if there is an electrical path from the O2 wiring to the alternator.

I mentioned cold weather and not driving the car much and these two things will sap the battery and then the alternator can not charge the battery (because the initial voltage is too low). A "Battery Tender" (smart charger/trickle charger) is very handy and will extend the life of the battery significantly.

Chris

 

[Mystic1350]

He just crossed the harness to the wrong side--i wouldnt think that this would do anything, especially after i switched them back over to the right harnesses

i hear ya on the cold but im not 100% convinced that out of the blue it would do that, especially it since it only happens over a certain rpm

i also recently 'deleted my antenna' and by that i mean i put a plate over where the antenna comes out--i left the antenna wire just hanging loose under the fender well...i dont think that would cause any problems though

just trying to think of everything i can!

 

[trinity_gt]

Check your belt tensioner and accessory drive belt for glazing.

 

[yellerstang]

light on dash

Mine had a bad alternator, same symptoms exactly.

 

[Tim2002GT]

Sounds like the alternator to me.

Mine had a bad alternator, same symptoms exactly.

^ this!

Take the alternator off and have it tested at a local parts store.

 

[Mystic1350]

alternator was tested and ended up being shot
got myself a PA performance one and just got done installing it tonight and ripping my car around
all good to go
the PA performance appears to be a real nice quality as opposed to the remanuf crap ones i usually buy

 

[nyuk98gt]

Mystic1350:

Nice work. FWIW, I have a PA Performance alternator on my GT and I think it does a much better job at charging the battery at idle than the stocker did. I splurged and got it powdercoated for a little bling under the hood.

Thanks for posting an update.

Chris