Boat's gounding system has 12volts

[STARCRAFT63]

Hello all,
I have been a long time reader and decided to join recently.
I have a Searay Seville 17' bow rider 2.5 mercruiser 120hp.
I have 12volts on my grounding wires and engine metal. Recently installed a shift interrupt switch and new alternator. Boat ran fine since last work done.
Went to start engine in drive way and heard a click when key was turned. I tried several times, still getting a click then it finally started and ran well at idle.
Yesterday went to run boat after sitting for a few days got one click and lost power to all my gauges and trim motor. There is no power under the dash at all. While testing with a test light hooked to the negative side of battery I touched the case of the alternator and the test light lit. I also touch the engine main grounding bolt and the light lit again. I do have dual batteries with a switch.
I am not sure where to start.
Sorry for the long read, I know people will have questions so, I tried to include all I can think of. All help will be greatly received. Thanks in advance.

 

[GA_Boater]

To begin with, disconnect both batteries. Do you have a meter? A test light won't help tracing shorts with the batteries disconnected.

With the meter on a low ohms scale, measure between the plus and minus battery connectors on both batteries. If the meter shows a short, check the other battery leads for the same short. If both are shorted, start disconnecting circuits by pulling fuses and any connectors until the short goes away. I would start with the alternator since that was the last electrical work. The shift interrupt shouldn't be causing this, but for elimination, disconnect it too.

 

[Bt Doctur]

Your missing the ground cable connection from the engine to the neg side of the battery

 

[achris]

Your missing the ground cable connection from the engine to the neg side of the battery

+1

 

[Bondo]

Your missing the ground cable connection from the engine to the neg side of the battery

Ayuh,....^^^^^ If it's missin', or so corroded it can't pass power,.....

 

[Grandad]

Ayuh,....^^^^^ If it's missin', or so corroded it can't pass power,.....

Yup. Bad continuity at a negative connection. Clean and repair that termination and you'll be back in business. - Grandad

 

[STARCRAFT63]

Thank you all, you were spot on. I found a little corrosion under the negative cable and on top of the battery post. cleaned it up and I was back in business. I love IBOATS