Don't know what motor but will take another WAG
Probably SB and have enough current supplied to start, but there is corrosion between starter and Bat post so sense is not seeing actual voltage

Check cables and Bat switch

Engs are 468's with 10.25:1 compression. It takes some juice to spin them. Don't see any corrosion at any of the connections I've seen so far. Haven't removed the batt switch yet. May be something going on there.

The bad alt ground theory made sense. If the alt ground was a couple of volts above batt ground, that would explain things. However I'm pretty sure I can rule out the bad alt ground theory now.

Still having trouble understanding how I can read 16.7V at the SENSE terminal of verified good alternator with a good ground and why jumpering from alt output to pos batt terminal helps lower the alt output from 16.7V down to 15V.

Next steps are to install jumper in parallel with the remaining batt cables (one at a time) and batt switch.

BTW, thx for the assistance!!

Found this.
https://www.thehulltruth.com/boating...rcharging.html

See post #4 saying some Mondo VRs have a dummy SENSE connection and the voltage is sensed on the EXCITE terminal. I haven't checked the EXCITE terminal when eng is at higher RPMs.
If the EXCITE voltage is coming from a source that has higher current draw at higher rpm, that might be the issue. I know the MSD box pulls significantly more current at higher RPMs. So if EXCITE is tapped into that wire....humm.

Edit: Just remembered that during bench testing of the voltage regulator, I connected EXCITE terminal to a variable bench power supply and set to 12V. SENSE was connected to another variable power supply and as I sweep voltage from 12V to 15V, the brushes would de-energize after SENSE terminal went above 14.6V. Doesn't sound like I have a dummy SENSE terminal on my voltage regulator.

A brief follow-up. Made a new jumper with correct terminals and 10ga wire. (Still have old additional jumper installed between alt output and starter solenoid). Installed new jumper between starter solenoid and batt switch. Voltage dropped from 16.7V to 16.3V @3k rpm. Removed new jumper and re-connected it between batt switch and batt, no change. Disconnected cable terminals at batt switch and bolted them together bypassing switch, no change. Re-measured alt ground post to batt neg post and got 30mV at 3k rpm. Re-checked batt ground at block, all good. No corrosion seen anywhere.

Decided to winterize and pick this back up next season. May try swapping the alternators between engs. The port side is the orig old school style with adj voltage regulator.

Missed last season and just getting back around to this.

Port batt died over the winter so got 2 new Interstate marine batts. Thought there was an outside chance it would fix starboard alt issue but no. With new batt, seeing 14.5V at idle but 16V (and climbing) at 3k rpm. Since I'm on the hose can't hold it there long. 2-3 secs at most. Measured 16V between alt SENSE lug and alt ground at 3k rpm. Connected hand held meter between alt ground and batt neg post. Just a few millivolts diff at 3k rpm.

Also re'tried connecting alt output to directly to pos batt post terminal with a 10 ga jumper with correct ring terminals. Last time it seemed to help but this time batt voltage at 3k rpm was 15.6V and rising.

Think new batt is able to absorb more current so it dropped max voltage a little. I was seeing 16.7V two yrs ago.Every indication points to bad voltage regulator but this is the third alternator. Took "bad" alt back to alt shop and asked them to spin it higher than normal speed and apply a load, they say it's fine. Grrrr..

Needed to to winterize engs so figured while I was messing with it, I'd try some more tests in regards to the over-charging alternator issue.

Disconnected the Sense wire from the alternator, started eng and run up to 3k rpm. 16.1V and climbing.
Re-connected the Sense wire and disconnected the Excite wire, 16.1V at 3k rpm.
Disconnected both Sense and Excite wires, stayed at batt voltage (12.6V) up to 2500 rpm. Went to 3k rpm and voltage jumped up to 15.1V and was climbing!!

Appears the alternator is somehow self-exciting. Guess I'll have to purchase something else next season and try again. Maybe an Arco??

Or get current Reg in ALT replaced
Could probably find a new Reg for under $25

Update; got a 55A Arco installed this evening and all is well as far as I can tell on the trailer. 14.6V at idle and 14.8V at 3000 rpm.. Need a test drive to confirm.