Mercruiser intermittent electrical / no start / no dash power

[Baba1]

2005 Searay 300DA 350 MPI MAG
Engine Serial No P: OW061015
Suddenly no electric power at port helm gauges/ignition / no start on port engine ignition. Stbd engine is fine. We were on the hook for about 6 hours with fridges running and stereo on. But voltage was ok and now boat has been on a smart charger for a couple of days
When I tried using the combination battery switch, the starboard engine switch breaker in the cabin would trip and the starboard engine would also die. Reset the breaker and then the stbd engine will work normally, still no elec. power to port engine. I noted the green indicator light on the halon system was out or weak. Had to come home 15 miles on one engine.
12.4v batteries and tight connections
50amp breaker on top of engine not tripped and power thru it
I have voltage at the orange wire on alternator so believe 90 amp starter fuse is good.
Start switches swapped and now I can hear fuel pump come on so i think switches are good. (day before there was nothing) Boat is in slip with batt charger

The port batteries are 5 yrs old load tested earlier this summer and were good. And I am showing 12.5v resting ( isolated both port batteries to voltage test - no shorted cells)
Question. Could it still be a weak or dying battery condition? Even though there are 2 batteries in parallel and also the combo switch was used?
Not sure where to look next

 

[alldodge]

Don't know if this is day ja voo but I've seen this same post here or on another marine web site

Deleting previous post to make it clearer for those following

12.5V resting means Bats are at 90% power

With starboard motor OFF. What is port Bat voltage at starter prior to starting? What changes when key is moved to start with Bat voltage?

What do you mean "there are two Bats in parallel"?

 

[Baba1]

Don't know if this is day ja voo but I've seen this same post here or on another marine web site

Deleting previous post to make it clearer for those following

12.5V resting means Bats are at 90% power

With starboard motor OFF. What is port Bat voltage at starter prior to starting? What changes when key is moved to start with Bat voltage?

What do you mean "there are two Bats in parallel"?

The problem on this boat is there is no access to the starter without removing the exhaust riser/elbow so I am doing what I can before getting there. I cant even see or reach the starter without doing that.
There are two battery banks of two batteries each.
The battery bank for the port engine is 2xbatteries wired in parallel.
I removed all wires isolating the port batteries and they showes 12.4
V each,

Im wondering if its time to replace the port batteries anyway even if that isnt ths issue as they are going on 6 yrs old

 

[Lou C]

No power to the helm gauges and ignition switch with battery voltage OK suggests a fault in that engine's ignition wiring circuit, could be a fuse (blown, oxidized contacts) ignition switch, or wiring. Start at the fuse box, does the tab for the ignition fuse have battery power? Move on to the switch, you can pull it out and make sure the red wire (battery) has battery volts at the B terminal, when you turn the ignition on, (purple wire, I terminal) you should have battery volts between that terminal and a ground. I had a similar problem some years back, no gauges or engine crank, but tilt n trim worked. Turned out to be an oxidized fuse for the ignition circuit. Same thing happened to my blower motor this year, this year, I think it's time to replace that old fuse box with the old style glass fuses!

 

[alldodge]

There is a short somewhere, finding will be hard

Need a helper, but place meter on Bat, turn key ON and then turn to start and see if voltage changes

 

[Baba1]

There is a short somewhere, finding will be hard

Need a helper, but place meter on Bat, turn key ON and then turn to start and see if voltage changes

Im thinking Ill pull the batteries and bring to auto parts store for checking anyway