I was about 3 hours in route and all of a sudden the radio goes dead and I noticed the battery light is on and battery indicator needle is at the bottom. I get home and grab my older battery and installed it. I noticed the battery light is still on but the needle is in the middle (where it should be). I take it out for a drive and after about 15miles the needle drops to the bottom immediatly.
Can I assume the alternator is dead? Would the needle all of a sudden drop to the bottom?

 

Battery light usually means alternator problem.

 

I would say either a bad alternator or a slipping accessory belt.

 

Take the alternator off and have it tested if its bad buy one.

 

Voltage regulator on the alternator probably took a dump.

 

You don't even have to take the alternator off to test it - drive it over to a parts store, they have in-vehicle testers.

 

All you need is a simple volt meter to tell.. Start up the truck, attach volt meter, what's it reading? If only around 12v or so, your alt is not doing anything. If it's over 13.5v, it's doing something, but not much.. If it's over 14v, it's working as it should.

Turn on the lights and see how much the voltage drops.. If the alt is working, it would pick up this extra load and still keep the volts in the 13.5v + range..

Good luck!

Mitch

 

Anything less than <13.4v means there is something wrong with the charging system or you have bad cells in your battery. Bad cells in a battery can ruin an alternator, because they act like a ground.

However, look at your running voltages carefully at a NO LOAD state:

~13.5v - ~13.8v and above it's charging, but usually means it's being temperature compensated.

Anything above ~13.8v, it is charging full rate. When you have a full battery and the alternator is trying to charge at full rate, it should be around ~14.2 - ~14.4 volt mark.

 

Yup alternator, new alternater installed, $266 out the door...