Starter turning very slowly - tried everything, STUMPED!

[XavierSPL]

Hi all, after beating my head against the wall with no luck, I'm hoping someone here might have some suggestions.

I have a 2003 Seafox center console with a 2003 Mercury 150 salty and I just picked it up with what u thought would be an easy issue that has proven to be tricky:
The starter turns very slowly, too slowly to come close to starting. After testing, I decided to replace the starter and selonoid to find the issue is still there. It has 2 batteries, both replaced last season. I've load tested each with no issues and I'm getting 12.6 volts to the selonoid.
I then figured it must be a cabling issue, so removed the ground strap from the block, then removed all wiring going to the selonoid from the boat - I figured this would isolate from any issues that could be in the boat completely. I then jumped from another good battery, with jumper cables, directly to the starter and got the same result. It will only turn the engine over barely with no plugs installed but the engine turns freely by hand.
I'm usually pretty good with these things but I'm totally stumped.
I was suspicious that there could be a dead short on the engine somewhere so tested with my extra battery connected to the starter, and found 12volts on the engine block, I've seen this before and am not sure if this is normal for this engine. Aside from thinking (albeit a rate slim chance) I could've gotten a defective starter, I don't know what else to check.
Any suggestions are greatly appreciated...

 

[Scott Danforth]

one of three potential things.

• you have a bad connection somewhere - clean all battery cable connections to shiny metal - cables, studs, block, battery cable, back of starter - clean them all
• one of the cables is badly corroded on the inside - if any cable is showing a little green - replace it
• your starter is shot

 

[Grub54891]

What Scott said^^ I've seen cables that look just fine, but under the insulation, they are corroded badly. With a salty, it's more than likely the problem if nothing else works. Did you bench test both starters? Even though they both may sound ok on the bench it might tell you something.

 

[Silvertip]

How is it you saw 12 volts on the engine BLOCK. The block is ground (negative terminal of the battery.)

 

[XavierSPL]

How is it you saw 12 volts on the engine BLOCK. The block is ground (negative terminal of the battery.)

That's what I'm wondering as well. I've seen it before on sleds or skis when there's a dead short. The 12volts is coming in from the positive post on the starter and going through the starter into the block (I verified this by pulling the battery line off the starter and watching the 12 volts at the block disappear. Is the starter supposed to be insulated from the block?? Didn't notice anything when I replaced the starter.

Thanks for the suggestions all but I'm confused - I was also suspicious of bad cabling somewhere (even though it was all re-cabled last season) but if I disconnect the ground strap to the block, and the power cables from the solenoid, doesn't that rule out anything in the boat electrically??

I can't rule out that my replacement starter may be defective but it seems like a far fetch to have both starters both produce the EXACT same symptoms.. I bench tested both and if I connect 12 volts to only the big 12volt input on the starter, it spins backwards, if I add a jumper between that big post on the starter to small ignition switch (yellow/red wire from solenoid) post, it then turns in the correct direction. Is that normal?? (I'm not used to starters that have a 2nd solenoid built on..)