motor stall and odd electronic behavior

[fisher91blue]

Boat: 20 foot Grady Overniter
Engine- 4.3 MercCruiser Thunderbolt IV

While returning to home dock from a recent outing, the boat stalled just near the dock. We were able to restart and power in to dock, but then we went away for 2 weeks. During the time we were gone there was heavy rain, so I am sure we had bilge pump running.

At the time of the stall, I was low fuel, I immediately thought the stalling was fuel related, so first trip back out yesterday we headed for fuel and filled it up with 40 gallons. Half way home at wide open throttle, motor just quit. Just before quitting, sounded like carb flooded. Motor would restart relatively easily but wouldn't stay running. Alarm on VHF radio indicated low battery.

I put one of the batteries on charger and went down today to check the other, starts easily at dock, brought up to about 1500-2000 RPM and metered battery and got reading about 13.07 and rising. Battery cold was about 12.6.

I submit a couple weird things for comment to the forum:
1) volt gauge at dash read <10 while multimeter at battery posts 12.6
2) prior to the engine issues and even on outings in previous weeks, I would notice random interruptions in the gauges
3) today, when testing battery when running, I applied a load and turned on everything, but when I turned on the running lights all the gauges crashed but the motor kept running

thanks for all the good work and advice I usually see on the forum and pages

 

[Scott Danforth]

start by cleaning the connections. you most likely have a bad connection or two. start at the battery, clean every connection to the helm and back. by clean, shiny enough your kids can eat off it. once the connection is made, seal it with marine electrical varnish.

if any cables have lumpy insulation, are green or are stiff, replace them.

gauge voltage at dash is a suggestion as the $3 gauges are not that accurate

 

[alldodge]

brought up to about 1500-2000 RPM and metered battery and got reading about 13.07 and rising

Either power is not being applied to the alternator or your ALT has gone bad.
With key OFF
Should have 12V on the Orange and Red/Purple wires.

With key ON
Should have 12V on purple wire and the other 2 previously listed.

If voltages are correct and you do not see 14V or more on the orange wire with motor running, and above 1200 rpm, then the ALT is not working