Sea Ray

Thank you so much for the response, I tried the emergency start yesterday and nothing different happened.
When you mention port engine contactor, do you mean the solenoid located in the master DC panel?
I can tell you that the port engine switch in the main DC panel under the floor is lit up just like the other two switches and when I turn off the port and re-engage the switch, I can hear the solenoid kick in.

The original post includes an inquiry about the 10A fuse.

The 2008 340DA Owner's Manual is linked below.

https://bbg1.my.salesforce.com/sfc/p/#E0000000Z21E/a/E00000000ULc/iDGWbLr70vHnCxxoICS.vLIU4C1BrHPUy90rMkcLI_Q

On Cabin DC Wiring Diagram (1 of 2), page 27 or 31, the over current protection shown next to the switch, is the switching circuit breaker, on the salon DC panel.

The dash switches are shown on DC Wiring Diagram (2 of 6), page 17 or 21 in the middle.

Those dash switches then go to DC Wiring Diagram (6 of 6), page 21 or 25, connector M11, with incoming power on the violet wires.

It is a convoluted path for that power.

Starting on DC Wiring Diagram (6 of 6), connector M5, pins 1 + 2, RED/VIO wires, to sheet 5 of 6, connector F5, pins 1 + 2, RED/VIO, to engine harness M33.

Then to DC Wiring Diagram (6 of 6), connector F33, pins 1 + 2, RED/VIO, to plug 30 and Cap 31, both pins 2.

Those then go to Cabin DC Wiring Diagram (1 of 2), plug 31 and cap 30, both pins 2, RED/VIO. The incoming power is on pins 1, RED.

Back to DC Wiring Diagram (6 of 6), to plug 30 and Cap 31, both pins 1 RED, that are each supplied by the 50A fuse (fuse on my engines, but shows as circuit breaker on this wiring diagram) on each engine starter solenoid.

So, using a DC voltage multimeter, turn on both battery banks and turn on power to both engines at the salon panel and at the dash, start at the starter solenoid lug. Connect the black meter lead to a solid DC ground. Measure with the red meter lead.

On DC Wiring Diagram (6 OF 6), is 12VDC power present at the starter lugs? Is power present on the (fuse circuit breaker) protected side, on the RED wire?

Then to the salon DC panel, Cabin DC Wiring Diagram (1 of 2). Is 12VDC power present on the incoming RED wire to the circuit breakers? Is it present on the outgoing RED/VIO wires?

Sorry for the looong reply...

The Factory Service Manual for my Mercruiser engines specify that those starter battery cables be insulated w/ brush on liquid insulation, underneath the rubber boot.

Assuming yours has the same requirement, then removing every battery negative terminal first for safety, followed by disassembly of those starter main battery cable terminals, then inspection, cleaning, reassembly, proper tightening, then application of the liquid insulation, then the large boots reinstalled would be good for long term protection.

Thank you so much for the manual link. I thought I had every manual ever made for this boat but this one from an electrical perspective is much easier to navigate. You have shared a number of potential remedies and I will let you know the outcome.
Thanks so much and I'm an engineer who likes and respects detail!
As for the cables, I do not see any brush on liquid insulation but that would be a great idea.

Following the wiring diagrams and doing some of the action items you sent, the problem is solved. It was the fuse on the DTS power wire attached to the battery. I probably would have looked at the fuse earlier if I could see the wire but the wire fell behind the port side battery bank and was hidden.
Thank you so much for your help and I notice that you help many other boaters. I read most of the posts and have picked up many of your suggestions.
Thanks, be safe and happy Holidays

Great news that the problem was identified and resolved. Thanks for reporting the status.

Note that the Mercruiser Digital Throttle & Shift (DTS) controls are not shown in that linked Sea Ray wiring diagram.

In my case I created supplemental wiring diagrams to document OEM omissions / errors, plus include additions / modifications that I've implemented over the years. Here is an example:

https://live.staticflickr.com/65535/48111080478_7c691a046f_c.jpg

My preference is to NOT attach ANY extra wiring to the battery lugs and instead make electrical connections, like this DTS power, properly according to ABYC on the OEM power distribution system.

The ABYC standard limits the battery lug wire quantity, to three maximum, to my recollection.

All of the OEM wires, plus everything I've added, are labeled at both ends, to aid in diagnostics, such as in this linked example. Everything is properly secured along the run, within sheathing, according to ABYC.

https://live.staticflickr.com/4666/38460422820_a9355d1716_c.jpg

If this DTS existing wiring is ever updated at a later time, then usage of an existing or additional fuse panel would be better than a dangling fuse holder.