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Discussion Starter #1
This makes me feel dumb.

I had one bad battery - one good battery. I replaced the bad batt with a good one but it didn't make a difference in starting.

Has over 12V just sitting there, cranking it gets down to near 10V. Keep cranking a bit and it drops into the high 9s.

But it sounds like the batteries aren't keeping up. Seriously sounds like batteries that can't handle a load.

So I put in two new Motorcraft batts as a test. Same problem.

Could this be a bad starter? It's only 8 months old - from ebay.

Is it possible to have a problem with the cables?

This is getting ridiculous. Is it possible there was a problem with the install?
 

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have you cleaned both ends of all the batt cables, see what the volt drop is from the batts to the starter.
 

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Discussion Starter #4
Cable ends at the batteries are OK. Havent checked the other end of the cables.

When you say check the "condition" of the cables, how can I check more than the visible ends?
 

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When I first bought my truck they had just put new batts in it. It didn't seem like it cranked very fast though. I put a voltmeter directly on the battery post when the engine was running and sure enough one battery was reading 14+ volts and the other was reading 12.5. The problem was a dirty connection. Make sure the inside of the terminals are very clean (they sell a little wire brush tool that is made to clean both the cable and the post). Cleaned the terminals and cranks much nicer now.
 

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One trick I like is use a clamp-on ammeter on each battery ground while cranking. That tells you which battery (or its cabling) is not puling its weight.
 

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Make sure the connections are good from the batteries clear down and including the starter. These engines require massive amounts of amperage to roll good and it has to flow with very little restriction. One other thing is check that the battery terminals are actually tight, they can appear tight but sometimes they're not like -----> C but more like ------> O this happened to me and caused a lot of head scratching until I spread the terminal apart and filed the two inside surfaces down then they would tighten down all the way.
One other thing that can cause grief is that darn solinoid. If the internal brass bar becomes pitted it won't make good contact when it pulls in.
 
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