Forgive me if this is been pointed out already, but it takes a different kind of sensor to serve a gauge than to power a warning light.. The warning light sensor is just a switch; the gauge sensor is kind of rheostat.
So if your dummy lights work and your gauge doesn't the problem may lie with either the gauge itself or the sensor that drives it. There's clearly no problem with the sensor that drives the warning light.
sensors are pricey. You could use an automotive sensor instead, but they use American (NPT) threads, while Yanmars use the British standard (BSP). Adapters are available: Google
Pegasis Automotive to find them. They sell them to Mazda owners looking to add a gauge to their cars.
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