Tach failure only in one direction?

hey guys, working on a millar cvt with dc/dc generator. car keeps tripping overspeed on generator when going in the down direction. doesn't trip when traveling up. when going down i here the generator winding up as car is accelerating to top speed. Motor field voltage is correct in both directions.
i know that tach on machine can cause similar problems. but i never seen a tach fail in one direction. so my question is, has anyone ever experienced tachometer failure only in one direction?

If memory serves me correct CVT cars have an Up and Down tach feedback trimpots. The Up adjust is in the CVT cabinet and the Down adjust pot is in the rack above the main controller. I am pretty sure those should go to 10V to signal a highspeed run. It may be possible that is going too high?