My original Mercruiser/Mando alternator maintained 14.7V (+/-0.1) from idle to 5000rpm. Sierra 18-5960 is supposed to be an exact replacement to Mercruiser 807653, but it varies from 14.5V at idle to 16V at 5000. Is that bad enough to declare it bad? It is brand new. The voltage was measured with a Fluke 117 multimeter. These alternators have internal regulators.
---rick

PH1971 had the same issue after replacing his alternator, seems he needed to attach a wire that was terminated by the alternator. Try contacting him as he chased this for a few weeks and 2 alternators on his Formula. Good Luck

That is too much voltage and will fry the battery

have to stay away from Sierra electrical and ignition and fuel injectors stuff period. If you have an EFI engine that is one way to knock out the ECM. and that is correct - about the battery hopefully you did not already ruin it. If the battery end sides are pouched out its been over charge already and you will need a NEW BATTERY.

sometimes the cheap junk marine stuff online and or thru whomever is not cheap at all. It ends up costing way more after its installed. This is one of those examples. Then you have your time involved as well plus lost down time on the water.

And do not buy DB electrical alternators either. I have an ECM in right now that was knock out from a huge voltage spike that the alternator caused as it let go. It was less than 2 years old.

What about their starters ?

Not A sierra fan but with your ALT going that high says your since lead (purple wire) is not connected, or the regulator has blown. Recheck the 2 small wires which come out the back of the ALT

That is the fix PH1971 found also (purple wire) He attached and all was fixed.

plastic gear instead of metal compared to the heavy duty like API marine has for the starters but the price is way more for the api vs DB. I have been told by a few that they had a bad DB starter right out of the box but DB took care of it with a return first ---- before they sent out another. IMO I would not use the the DB starters for big inch engines. IMO Lower comp ratio engines and small blocks and down prob for the price is fine using a DB starter. I WOULD NOT USE THEIR ALTERNATORS PERIOD. I hear more bad than good with those plus the handful I see yearly -- they do not last.

I was thinking the voltage regulator gave out for the high voltage readings because I seen that a couple of times with the Sierra Alternator in the past couple of years. Also had a few the other way that discharge the battery of voltage. Bad regulator for those few Sierra as well.