Hello all, just registered here today, but I'm having a couple issues. Never owned a Mustang before, but I now have an 03 GT Convertible. Recently started having an issue where the alternator wasn't charging and killing the battery. Traced it out to a bad ground and replaced it. Alternator is charging fine, had it tested 3 times, and had the battery tested as well. The two issues I am having are:
1. After driving for about 30 minutes, when I brake or use any lights, my battery guage will drop to about 1/4 but comes back up to half or better when I let off the power draw. (if I shut the car off shortly after it gets to this point, it turns over slower than usual when I restart, but after that it's fine.

2. I've read several issues on here about Idle Hang, and I think that might be whats going on, but it started about the time the electrical problem started... When I let off the gas and push in the clutch, the idle will sit right around 1000RPM, then after a few seconds drop to normal (650?) If I am sitting at a stop light in neutral with the clutch let out, it does the same.

It sounds like either another ground problem or the battery is on its way out..Any thoughts? And could the idle hanging be related to the charging system?
Thanks in advance,
Adam

If the alt is good and the batter is good. You might have an overload problem. Do you have aftermarket sound system?

Here's some information on how to trouble shoot hanging idle and IAC issues.

https://www.allfordmustangs.com/forum...9.html#2260649

However, if the idle hangs around 1k RPM and then drops a few seconds later, this is normal. Also, the idle will tend to be higher if the car is still moving.

It does sound like the charging system is not in tip top shape. The usual cause is poor or dirty battery terminals.

Bottom line. Today's cars simply will not run right without a strong battery and charging system. Every electrical problem should begin with an inspection of the charging system. Cut corners here at your own risk!

Thanks for the replies,

It does have an aftermarket sound system, just a deck though, the rest looks like stock Mach audio components. Looks professionally installed, it has the Metra harness and everything else done right from what I've been reading about it. I replaced the Sony Xplode deck with a JVC deck a month or so ago, but it was a simple plug swap, same general specs on the deck as far as voltage, OHMS and all that so I don't think the load changed much. Checked all the wiring that I changed and everything is clean and not grounding out on anything.

The idle might have always been that way then, and maybe I'm more sensitive to things that don't seem right since it died on me a week ago. (This lead me to the negative cable being swapped, the alternator just quit charging randomly but has since worked fine with the new ground)

As far as cables and wires, I replaced the negative battery cable, it was pretty corroded and had the thin lug on the battery end, so I used a heavier duty terminal. Did resistance readings on several spots and didn't get over .2ohms on anything. The positive cable is clean, no corrosion or anything as far as I can trace it. I have no idea how old the battery is, the date was never scratched off, and the alternator was replaced by a shop in 2011 according the maint. records I got with it. I'm suspecting the battery might be nearing the end of its service life, or possibly an intermittent alternator issue. I hate to just throw parts at it, but a battery might not be a bad investment?
Thanks again!

This is similar to the issues I have seen in the past / currently see. I am opening a post on it now.

I think it is a load problem at this point..If I am driving for a while, and notice the voltage drop, I turn the radio off (Only happens it seems like when I listen to it at high volumes) and it jumps right back up. Would the solution be a new battery or a higher output alternator?

Yes to those 2 questions... Also if your sound system has an high power amplifier for a subwoofer or something, you can get a capacitor which will help even out the power load on your battery / alternator

Finally got to the bottom of the problem. It died on me on the way to work last week, I started poking around under the hood with a multi-meter hooked to the battery and the car running, no charging at all. Grabbed the two wire plug on the alternator and all of a sudden it started charging again....Pulled the plug off, checked and cleaned everything, plugged it back in and been driving for over a week without a problem. The idle hang went away as well as soon as I reattached the plug. Thanks to everyone for the tips!