Where is the plane located when you try to take a reading? If it is in a garage that may explain it. The tach needs to see a light source through the prop as it spins and it counts the number of ties it sees the light. Even out in the open on a sunny day the angle of the sun may prevent the tach from being able to get a good reading. If you have to run the engine in a garage placing a lamp in front of the prop and then taking a reading from behind the prop should work. Hope this helps. [8D]