You can load test the batteries first off all - load them for ten seconds and if the voltage drops below 10 volts then you need to replace them. Load test them individually, not together. Also, make sure you connections are good and you have good grounds. If all goes well there, do a voltage drop test (while cranking) to the S post on your starter. You should have battery voltage within a few tenths going to the starter before cranking. Make sure your starter connections are good, clean and tight. If you have good connections and good voltage - you will need a starter more than likely. Contact me if you want, I have a good used one if you need it.