Alternator not working

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. Disconnecting the battery and running the vehicle to check alternator output should never be done. It doesn't really tell you anything. Your vehicle's starting and charging system requires a known good fully charged battery in order to check components for their operation. An alternator should be tested with a known good battery with the engine running at 2,000 revolutions per minute -- checking for amperage output and voltage. Yes, you'll be looking for north of 13.8 volts and over 65 amps. How is O'Reilly's checking the alternator output? See above for proper alternator and charging system testing. My guess is you may have a bad diode in the alternator. But properly test it to find out what is going on and verify a known good battery that can sustain a load is in place first. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.