Help-what Amperage Alternator To Get

Have a 2005 Mustang Gt (stock) & just replaced the battery 2 weeks ago. Don't drive the car much in the winter up here, except to start it, drive it a bit, then park it. Since the battery replacement, noticed a huge drop in RPM's from what used to be around 900 RPM's to 600-700 RPM's at idle, drops even more when using heat. Now the battery warning light just came on yesterday. Was told by my mechanic (without testing alternator) that my alternator might be going south for the winter. He also said the drop in RPM's could be because the computer has to relearn with a new battery? The alternator is original, so I am thinking this might be the problem but will go get it tested. What amp alternator should I get? And what other things should I check also-ie terminals leads.

racerraj said:

Take it by an autoparts store and have them test the alternator before you replace it. The idle is probably off just because the car needs to re-learn. Take it out on your day off and put some miles in the city on it. Driving around and stopping at a few traffic lights, it will probably settle back to what you are used to. The battery light will come on it the idle it too low, it isn't spinning the alternator fast enough. So drive it a bit, let it idle here and there, once it settles I'd bet all is well and an alternator test won't even be needed.

Click to expand...

I checked the two cables on the positive post of the battery. One seems to go directly to the alternator. I moved the cable down a bit, and the idle went really low. Moved the cable back up and idle came back and battery light on the dash went away. So I guess a set of new cables is in order. Cheaper than an alternator. In addition, the mechanic who installed the battery didn't tighten down both negative & positive cables enough. The idle is back where it should be. Thanks!