2004 rust bucket F150 with 6 cyl. engine. My coworker showed up at work with flickering battery light. Battery is below 12V. Voltage at battery, engine running is below 12V. Directly out of the alternator output cable is reading 11 volts while running. Is this an accurate test of bad alternator? Solenoid? He's dumping the truck this year. Which local parts store alternator might be o.k.? Advice appreciated. Thank you!!!!!!