Mine does the same when the engine is running. Works fine on trucks just every cub cadets 've tried testing it goes nuts

my meter did that until I found the coil was sending voltage into my frame and back into the Neg side of the battery.Make sure your high voltage is not leaking off. Get all it's componants,condenser and it's wire includeing sparkplug wire mounted correctly.Make sure the wire insulation is good the entire length and check the coil for any damage.I finally got a steady reading on my meter after chaseing down those issues.