will not start

[jeanmike]

2007 Gulfstream Yellowstone DP, Workhorse R32 chassis, Cummins ISC, Allison 3000 6 speed. Parked for the night and everything worked fine. Got in the seat to pull out, turned the key and nothing. No lights, no clicking,no starter turn over, no engine revolution, nothing. Battery Isolation light blinking indicating low battery voltage according to Workhorse manual, so try aux start finally got lights and some engine try to turn over but not enough power to get even one or two revolutions. 2006 chassis, maybe batteries are finally gone, jump in toad, off to autozone for two new ones. Replaced batteries, same as before. Put charger on new batteries, try 50 amp jump feature on charger plus aux start, still got lights but no start (same as above). Bang on starter with hammer, jumper power directly to starter, still no joy. Call coach net, talked to tech, no start. Tow truck finally came, he tried jumping around solenoid and 12 volts direct from tow truck with aux start-nothing. Reading voltage at new batteries gives 12.5 volts. i think that is low but with all the "extra power" from jumping that should have taken care of that. What have I missed? Sitting at Poole Equipment in Garner, NC parking lot (Workhorse Chassis approved) on their shore power for the night so they can try in the AM. What have I missed?

[94-Newmar]

Well bypassing everything and jumping directly at the starter removes anything else from the equation - sure sounds like a bad starter motor to me - and that's where I'd start (no pun intended) with a replacement.