my tachometer is not workin right .checked the teeth for the crank sensor and cleaned them and the sensor. the crank sensor is the sending unit right? tach shows 200 rpm at idle should be 750rpm it still moves up and down but shows different rpms goin down the road sometimes 2200 sometimes 2700 at 70mph. i have a 2006 1 ton big horn dually 6 speed. I would appreciate any help on this situation 
