I had a 93 Dakota that was eating batteries. At first, a mechanic tested the alternator and said it was faulty and was drawing a current when the truck was off. It actually turned out to be a bad voltage regulator. I replaced it, for $12 and all was good.

 

start the truck and disconect the battery...if it dyes then your alternater is bad...if it keeps running then its good...

 

Do not disconnect the battery with the engine running on a computer controlled motor, you could see a power spike that will fry the ecm. I have seen this before on vehicles and also seen them fried because of jumping other vehicles. Removing the alternator will only tell you if it is charging not draining the battery, remove the alternator and bring it into a shop to be tested.

 

it wont spike...if it did then youd have a write off everytime someones battery was bad...car works off alternater not battery...and the only time your gonna fry battery jumpin is if u cross the cables

 

Actually Mitchalvin is right, but my expirience shows that it's possible to run a car without battery if you give some load to the electrical system and will not raise RPM above 2000. In this case system voltage is within normal range. To check an alternator i usually let the engine warm up until it drops RPM to normal idleing, turm on the headlights and the heaterm and disconect the (+) terminal of battery. This way worked fine for me so far.