Bill, using my ohm meter I checked the resistance between the engine blocks and the ground bus in the fuse cabinet -- there was zero resistance, telling me that ground bus in the fuse cabinet is a good grounding point. So I just fished a new ground wire from the ground bus up to my tachs, which was only about 3 feet. I was doing this while at the blues festival, so didn't take the time to run down what was going on with the factory ground.
In any case, about half the time when electrical systems do weird things, there is a grounding problem involved. So that is always a good place to start.
Occasionally when you find voltage on a ground wire, it doesn't adversely affect many pieces of equipment (lights, pumps, etc) because the path where the voltage is leaking in from is high resistance, so no significant current really will be flowing in the ground circuit (your volt meter won't tell you that). But if the ground has voltage on it and you are using it on a delicate instrument like an electronic tach, even a very small amount of current can make the instrument go nuts.
If you wanted to trace down where voltage could be getting onto a ground wire, it may be easy or quite difficult. Normally someplace in the ground wire there will be a high-resistance connection (perhaps from a lose wire, or an oxidized connection). If the ground wire in your instrument circuit has a high resistance connection in it somewhere, voltage from the instrument light circuit can't flow back to the main boat ground, so some of it may go to other devices (like instruments) that are also hooked to that ground.
Wiring colors are funny. In residential and commercial building wiring, green is always ground, and white is always neutral (basically, also grounded back in your main panel). But in automotive wiring (and also RVs and boats) often white or black is the ground -- at least for your auxilliary equipment (lights, etc). But there is a real lack of standards. For example, on our bilge pump, black is ground and brown is hot. On some other devices, black is hot and white is ground. On our fuel pumps, red is hot and black is ground. I have been trying to consistently use green or while wires for ground runs on our boat. But the old owner used a lot of green wire for hot wires. It sure is confusing trying to sort all that mess of wiring out, but we are slowly getting it.
I hear you about ribbon cables -- makes it harder to track down what is happening.
Good luck -- that stuff can be frustrating.
Best wishes, Curt...
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1967 fiberglass 38' Chris Craft Commander Sportfisher with twin 427 CID 300 HP engines.
