On my twin diesel Tollycraft with an upper and lower set of gauges (so, 4 sets of gauges in all), I'm trying to track down an electrical gremlin.
Here's what I know:
Each of the lower gauges has TWO sensor wires going into it. For example, the water temp gauge has a 12v+ and a ground, and then it has two sensor wires going into it. When I follow these sensor wires, I see that they split off into two sensor wire bundles - one that is labeled LOWER and one that is labeled UPPER.
My original (possibly incorrect) assumption is that in the case of the LOWER sensor wire, that wire is coming directly from the water temp sensor, and then into the water temp gauge, and then the second sensor wire goes out along the UPPER sensor wire bundle to the water temp sensor up top.
I ran with this theory, and unplugged the upper sensor wire bundle, and all of my gauges downstairs continued to operate correctly. Well, they operated correctly with the exception of the tachometer. Dead tach. So I plugged in the upper wire bundle again, and then the tach started working. I checked the wires behind the gauge, and it looks like the tach on the port side does *not* have two wires. Just one. But the starboard tach DOES have two wires.
I believe there's a broken connection somewhere, so I should easily be able to find the lower signal wire and get it connected to the tach. However... the question remains: How is my tach getting a signal when ONLY the upper wires are connected?
If the upper and lower sensor wires are simply run in parallel (sensor connected to lower tach; sensor connected to upper tach, and tachs connected to one another), the lower tach should work regardless of which wire bundle is plugged in. But that's not the case.
If it's run in series (sensor connected to lower tach; lower tach connected to upper tach), the upper tach should have no signal if the lower signal wire is disconnected. Also not the case, on account of the tach working when I have just the upper wires connected.
How else could this be connected?
I mean, I can spend some time tracing it and figuring out exactly where it goes, but I won't be back to the boat until this weekend, and if somebody else is familiar with how this is typically wired up, that would help me solve this mystery!