Hi Matt!!! Sorry you haven't fixed it yet. It should be real easy.
#1, you don't have an Amp-meter (ammeter), you have a VOLT-meter. Amps is flow, and Volts is pressure. The gauge in your dash is showing you the VOLTs on your battery.
If your voltage (VOLTage) is dropping, your alternator isn't charging.
When you start, 13V is a good indicator that you have a fully charged battery. They are 12V at almost dead. 12.5 to 13 is usual for a full battery.
When your alternator is running, you should see 14V or more on the VOLTmeter in the dash. If not, your alternator, or the connection from it, is bad.
If you have a bad alternator, it won't be the first time I've seen a dead one out of the box. Just because it's new don't mean it's good. Really... Sometimes people return a dead one and say they didn't install it. We see that around here.
If you're within a few miles of your auto parts store, take the alternator to them and have it checked.
Your alternator isn't discharging the battery. It simply isn't RE-charging it. Your heater fan motor draws a ton of current. That'll bring her down fast. Right now, all your alternator is doing is giving the belt a place to run.
If your alternator checks out, your fusible link is GONE, toast, fried, dead. If the alternator is good, and spinning fast enough, it's creating about 14V or more on the output. If that output is connected via a wire to the battery, it WILL charge the battery. If the wire is either open or damaged, it won't be able to charge the battery.
If you have a voltmeter, measure the V at the alternator output lug. When running, it should be +14V or more. When not running, it should be the same Voltage as your battery since it is all the same circuit. If you measure 0V on that lug/stud, your fusible link is wide open, burned out.
Hope this helps!
Jim
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