I am having a problem with the tachometer on my tractor. The tach is electronic and gets its signal from a magnetic pick-up. The problem is when the tractor has not been used for a day; the temperature is less than 50 degrees outside. Without starting the tractor just turning the key to the run position, the tach needle begins to move slowing and will continue to move sometimes as high as 2500 rpm before falling back to 0. The colder it is outside the higher it moves. This will occur even when the only wires connected to the tach are power and ground. If additional information is needed let me. I am guessing some component is leaking voltage, my question is what will do this only when cold? Thanks