The MerCruiser stern drive's shift cable connects the driver controls to the shift control plate. A shift interrupter switch is connected to the shift plate and stalls the engine when shifting in and out of gear to help prevent damage to the stern drive. This switch is directly centred in the assembly when the cable is in the neutral position and activates just as the shift cable places the drive into gear. MerCruiser stern drive shift cables can stretch and become misaligned after years of service.
Open the engine compartment and locate the shift control plate next to the exhaust elbow or near the left valve cover. Examine the cable for rust and corrosion. Replace the cable if necessary.
Examine the shift lever stud on the shift plate and make sure it is located at the edge of the opening next to the pivot point. Move it by loosening the stud nut with an open end wrench. Push it into place with your fingers and tighten the nut.
Slowly shift the cable into the wide open throttle position using the controls at the driver's station while your assistant rotates the propeller counterclockwise. Continue this action until the propeller shaft stops, indicating that the clutch is fully engaged. This action will pull the shift cable inward.
Pull on the shift cable guide to remove any slack from the cable. Remove the locknut holding the adjustment barrel in place. Remove the clevis pin with a set of pliers, and disconnect the brass adjustment barrel from the post. Adjust the barrel by rotating it until the slack has been removed.
Turn the brass barrel clockwise four turns, install it back on the mounting post, and reinsert the clevis pin.
Shift the controls into the reverse position and extend it to the wide open throttle mode while your assistant rotates the propeller clockwise until it stops. This will indicate full clutch engagement. Remove the cover from the controls and locate the shift lever adjustment stud if the clutch does not engage. Loosen the stud using an open end wrench and move it slightly upward inside the slot and test again. Repeat the process until the clutch becomes engaged.
Place the shift control lever into the neutral position. Check the engine shift interrupter switch and make sure that the roller or plunger is centred directly over the middle indentation of the switch frame. Investigate for cable damage or stretching if it is out of place.
Check the cable and tighten all connections.
Consult a Mercury Marine mechanic if you are unsure how the shifting system operates.