The cause of the failure: the control part has too large clearance or the adjustment of the lever is not large, the sliding sleeve and the high and low speed gear wear and tear are too small, the high and low speed gear bushing and the output shaft are too large, the plug is deformed or the plug shaft positioning spring is broken.Elimination method: adjust the gap of the relevant tie rods, replace the damaged sliding sleeve and gear, replace the gear bushing, and adjust the gap to replace or repair the plug and replace the spring.