The power goes to the gauge and then earths through the sensor, not the other way round, therefore if you disconnect the wire at the sensor and touch it directly to the block or any other earth point, the gauge should read Max. If this is the case, then the sensor is at fault. These sensors are extremely prone to misread when damp or wet but are not expensive and easily changed.
Another thought is whether or not you need a voltage stabiliser for the gauge.
Cheers, Clive
(If I'm not here, I'm either in my new workshop or on the golf course!)
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