fearthisinc

I have a guy that is extremely knowledgeable with mustangs working on the car as we speak but I was curious and thought I would bring my question here.. It's an ongoing issue and was having some other stuff installed and the guy told me that for some reason the battery is not charging while the car is idling.. He said that if you rev it up it will start to charge but at idle there is no charging of the battery.. Just curious on what might cause something like this to happen..

3.0Taurass

Do you have a big stereo system or something? Stock alternator?

jthorn9

* Short inside the battery itself.
* Faulty alternator
* Faulty or correded battery terminals
* Shorted or faulty alternator to battery harness

fearthisinc

I do have a system but its nothing major... And we just installed the hi output alternator.. I was having charging problems prior to this and when I had this guy install it he said he tested everything and for some reason it is not charging while at idle.. When you rev it up though it shows that its charging.. I just remembered that when I first got the car and did alot of work we installed an under-drive pulley.. Wondering if that might be part of the problem..

fearthisinc

New battery, new and tested hi output alternator, checked all terminals and battery cables..

Fink

try a larger pulley on the alternator

Chromeshadow

Yes, the under drive pulley will make a difference but a good charging system should be fine with the UD. it's most likely a corroded cable or one bad diode in the alternator.

Jeff Ball

+1 on it being the pulley, depending on how small of a pulley you have on there it could cause it. The pulley could not be spinning fast enough at idle to charge it.

jthorn9

Yeah that would of been my next question is do you have under drive pullies. You said yes, and there likely lies the problem. The alternator is being under driven causing it not to charge at idel. You need to put the larger diameter pulley on the alternator.

mustang99v6

I say No on the pulley idea. I've got the much maligned 42% motoblue, been running it for 5 years now. Still on my stock 99 alternator. Never had problem 1. I say corrosion on your wiring. Some fibers are dead inside the shrouding, that's why at idle not much juice is getting through. When you rev lots of extra energy is available and jumbling it's way through your limited available fibers.