Random loss of power to dash

If you have already disconnected the large black connector and have been able to repeatedly start the engine with a make shift ("jumper/switch rig how ever you have done it") at the slave solenoid then the problem is between the dash/helm and the big black connector.

You now need to find the bad/weak connection in the wire harness now.

What you could try is to run a heavy gauge wire from Battery + to the B+ key switch position (removing the red wire there now before testing).

No fuse just a wire. Not sure what you have available but try to use any type of wire as long as it is 10 ga or larger, meaning thicker.

If this works then you know what the source of the issue is. Pin 6 from connector to helm. INPUT to helm.

If it does not work then the source is something else, OUTPUT from helm/key switch. (purple and yellow with red) We know the yellow with red goes thru the neutral safety switch and then to the slave solenoid. The purple goes to the ignition at engine and also powers all gauges in dash when key is on. All inputs and outputs most likely as i have said go thru at least one multi pin connector in the harness. Most large boats do. Why? because they pre-wire the dash electrics with its own wire harness and with a multi pin connector and they do the same with the mercury wire harness for quick and easy final assembly of the boat. Other wise someone would have to connect every wire by hand during final boat assembly. To time consuming/costly so it is all done before hand. Then they just plug and play.

Remember, the fact that you loose everything (knowing the key switch is brand new and works) tells me you are loosing INPUT at this juncture in time.

I would suggest you are seeing 12 volts at red key switch wire (#6) with your meter but it has no amperage......This is where a test light comes in as the brightness would tell you this. Bright=high amperage, dim=low amperage

So with this plan your next actions..........

Perfect
10ga wire is cheap enough, I’ll pick some up.
I have a test light. I’ll use it at the switch to see how it lights up.

Then I’ll try the B terminal directly to battery.
Then the yellow with red at “S” to the slave solenoid.
How about the purple? Should I try running a direct wire from the switch? To which contact? There’s also a red/yellow/black wire on that terminal. The factory schematic indicates it’s for stereo service.

The purple typicall has two leads attached at switch.
One goes all the way back to coil+ by way of a couple other shared connections.
The second lead would feed gauge cluster that activates when key is in on position.

Not at all sure of that other wire, i would say disconnect till u sort this out.

Ah Jack you ****ing rock!

Test light showed nothing on the red wire to switch. Test light showed nothing at the fuse panel

A little jumper wire directly to the battery and the b connection... instruments on, ignition on, and a 7.4l just purring away now.

You were right. Not enough amperage.

Thank you. Summer for me can now begin.

One question though. I'm going to have to find the break in the line. Is it safe to run that temp 10ga wire directly from the battery to the ignition breaker until I find it?

I would say yes but it must be fused!!!
I believe 20 amp is good.

Also trace the original wire feeding pin 6 from engine side black connector and disconnect from point of origin.

If ur not sure, i will look at schematic later.
At work now so no access to files

snip...rehash

No worries.
I ended up taking my time and splicing in around midpoint of the harness and running to the 20amp breaker.

Took a cruise and all appears good.
Thank you again