I know this is an old thread, but it serves to indicate how useful this forum is.
I had the issue of the Onan not cranking over the last few days.
Press the start button and it would prime, it would try to crank and then stop.
I checked all the power connections, cleaned and retightened...same issue.
then I went in the forum and saw this post.
Hmmm...so I started the main engine, and the Onan cranked no problem.
So it's the start battery?.no, well it is and it isn't.
I hadn't been on the boat for 4 months but i have a small 12v maintenance charger to keep the start battery up.
And that's what the problem is - the charger had died!
I've since charged up the start battery (1008CCA) and load tested it (perfect) and the Onan cranks every time.
So big thank you to everyone on the forum for all the input, it truly is a marvelous resource.