Merc 7.4 Alternator/Voltage

[711KMD]

Aye. Long story short. Ran a 92 Carbed 7.4 Bravo 1 for 14 years. The voltage at the gauge ran pretty much at 14V steady at ALL times. I'm now running a 2000 7.4 EFI Bravo 3. It only has 220 hours but my voltage is all over the place. On average, I start her and she's only at 12.5v, once I rev to around 2k it wakes up and the gauge jumps and runs normal enough just under 14v. (most times). I can go for miles with the gauge reading right, but then there are times after a stop/shutdown, on restart it drops and stays pegged at 12v and bounces lower when I trim. It's almost like the alternator only works when it needs to, but I'm pretty sure that's not how it works. Thoughts? Batteries were installed new when I bought used this winter and the alternator is Merc original with 220 hours.
I've tested on occasion at the battery with a meter and it always matches the gauge, I've ruled out the gauge.

 

[alldodge]

Without more testing, its just loose wires and a cheap gauge installed by the manufacture.
To find out what its actually doing, use a hand held meter and compare

 

[Bondo]

I can go for miles with the gauge reading right, but then there are times after a stop/shutdown, on restart it drops and stays pegged at 12v and bounces lower when I trim.

Ayuh,..... Welcome Aboard,....... Sounds pretty normal to me,.....

The alternator don't kick in til ya hit 'bout 1200 rpms, 'n then it charges til ya shut it off,.....

If ya restart, 'n don't rev it abit, the volt gauge is showin' ya the trim is usin' power,.....

 

[achris]

Sounds like either it's an aftermarket single wire alternator, or one of the internal diodes might be open circuit. Remove the alternator and take it to an auto-electrician for testing (and repair if necessary)....

Chris.......

 

[711KMD]

Thanks! Dug deeper into it, the orange wire post was junk and corroded. It was actually smoking a little, ordered a new one.