Mercury 90 optimax charging issue - stumped

Not charging. -New battery fully charged.
-Checked voltage at battery and at the output post of the alternator. 12.7v not running. 12.4v running.
-Checked ohms from alternator to common lug where power wire comes in and got none so fuseable link is still good there.
-pulled plug and got 12v in one wire and 0.5v in the second wire when not running.....11.5v in second wire when running.
-no change at higher RPM.
-pulled alternator and had it tested by a full time specific alternator shop. He hooked it up to his machine and plugged in his clip and it went to 14.2v. So alternator is good.

Tells me my exciter is not telling alternator to energize but everything checks out that it should on a tested good alternator. What am I missing?

My 2012 75 hp Optimax voltage at idle is just over 13 v. Off idle voltage is 14v, 12.4 v running seems a little low, maybe a bad connection ?

Check alternator GROUND.

also check for red wire disconnected at a bullet connector in the wiring harness

Well as I showed above everything is checking out as it should it just wouldn't energize. Tests good on the bench....

So I went thru it allllll again. And again it all checks good. So I had it tested again....same result tests out working perfect on the bench.

So I added this lil ground jumper from alternator frame to block. And like magic works as it should.... dont know why it isn't grounding with the mounting bolts but this fixed it immediately.

strange indeed , so what happened to the grnd wire that goes on the rear mounting bolt and connects to the back of the block , the alt has rubber mounting isolators for bracket that bolts to top of block

There is inline polarity fuse on the wire running from alt to power junction, if you so much as look at connectingit reverse it will pop the fuse. Hope it helps

Quote:

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Originally Posted by JOE54

strange indeed , so what happened to the grnd wire that goes on the rear mounting bolt and connects to the back of the block , the alt has rubber mounting isolators for bracket that bolts to top of block

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+1…. Either not connected, on wrong bolts, or paint on the contact surfaces.