I'm gonna jump in on this one and reinforce the comment about connections.
They must be clean. And by clean I mean bright and shiny and wire brushed and NO chemical residue. If you are gonna use chemicals then use them first and then mechanically clean the surfaces through which the current will pass. Or the current won't pass. At least not as well as it should.
One way to test a connection is to put a voltmeter on each side of it. Take the battery post for example: one probe on the post, and dig it in until you get a good clean lead surface, and one probe on the cable, again through the insulation if you have to and into good clean copper. Then try to start it. Anything more than a half volt across that post-to-clamp interface is too much.
Lead should look bright and shiny like clean silver - almost like chrome.
Brass should look like gold - yellow and bright.
The same surface that reflects light conducts electrons.
Clean, shiny, and smooth is the key.
I've said it before but it bears repeating - most electrical problems are NOT shorts. Short circuits are self correcting and obvious - they blow up, release smoke, and open up all by themselves. No. Most electrical problems are opens or "poor" connections, or otherwise high resistance connections.
I know a guy from my distant past who replaced his starter 3 times before I found the loose ground wire that was the actual problem. Chemicals, sandpaper, and wire brushes (in that order) are your friends. Meaty, beaty, big, and bouncy can be easily taken down by a grain of sand between the ground cable and the chassis or block. And it's not just the surface between the ring terminal, at the end of the cable, and the chassis - the bolt will conduct almost half the current so make sure it's contact surface is clean too.
Hope this helps.
Bogy
"Hardware eventually fails. Software eventually works" - Michael Hartung
