Thats what i would do, just go to auto zone, they can test your **** for FREE and tell you if it is the alt or bat

 

A-zone's bench tester does not detect diode issues. I had my 130amp alt checked numerous times (light came on for 10 seconds, then go off & back on continuously), they replaced it with a 105amp (light stayed on constantly), went back to a new alt from O'Reilly (same result as the first). I bought a BRAND NEW Delco unit @ wholesale cost ($183) through dealer hookup, and have not had problems since. Service tech told me that problem requires alternator rebuild, and that alt will eventually die. My bad alt lasted for one month with irractic volt readings, and quit charging one Saturday. I got to the dealer at 10am, and didn't leave until nearly 2pm.

I was a REMAN-believer until this started happening. Electrical components will get replaced with NEW components from this point forward. Just my $0.02.

 

still happening. i notice the other day that with my truck in gear, stopped with the defroster on that my voltage went down again. i could actually see my headlights dim. once i turned off the defroster the voltmeter started to go back up. so now i am confused. is the problem still with the alternator or is the ac/heater?