Mercruiser Alpha One Gear Shift Problems

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If you have a Mercruiser Alpha one with gear shift problems and you are showing any of these following symptoms, it might be worth you having a read of our guide

Drive Does not shift gear properly
Engine stalls when shifting into gear
G ear shift lever feels stiff
Engine stalls when shifting into neutral
Clattering or grinding noise from the alpha drive when engaging gear

If you have any of these symptoms the first thing to suspect is the Shift cable

The Shift cable is the gear cable that runs from the alpha leg through the gimble housing and terminates on a shift interrupt switch bracket, usually mounted somewhere on the engine (rocker cover or exhaust riser).

To understand why the shift cable is usually the issue you have to understand how the mercruiser alpha one gear shift mechanism works.

The forward and reverse gears are in the lower leg behind the propeller, they are both revolving all the time, one clockwise and one anticlockwise and the propeller shaft runs through the reverse gear and sits in the forward gear at the back of the casing.
Both gears are free to revolve independantly of the prop shaft and the shaft direction is only determined by the position of a sliding 'dog clutch' which slides along the shaft and engages with either the forward or reverse gear.

This 'Dog' is a cylindrical piece of hardened steel which has 'teeth' in either face and slides along splines in the prop shaft, mating up with similar 'teeth' in either the forward or reverse gear.

These teeth are slightly 'undercut' which means they are sloping backward and this is good in that there is much less chance of them ever jumping out of gear whilst under load, however when the engine is running and there is loading on the prop (ie it's in the water) These teeth do not want to disengage AT ALL, so heaving on the poor old gear lever would just stretch the cable and achieve nothing.

Mercruiser came up with a solution to overcome this

They split the gear cable into two separate parts, being the 'shift cable' (from the leg to the engine mounted bracket) and the 'gear cable' (from the control lever to the engine mounted bracket)

Mounted on the bracket is a mechanism which senses the tension between the two cables and when it reaches a certain level it operates a micro switch, this switch kills the ignition circuit and this in turn takes the pressure off the 'Dog clutch' allowing it to disengage with whichever gear it is in.

All this hapens in a split second and should be quick enough that the flywheel momentum allows the engine to continue running.

Imagine what happens when the 'shift cable' starts to get old and sticky, now you're getting the picture, it holds the microswitch over too long and stalls the engine, usually when you're just coming into a marina.

It Also causes problems when shifting into gear, as the 'stickiness' causes the mechanism to throw the microswitch over, however this mechanism should not actually flinch at all when shifting into gear.


Some people think that this is related to a 'start in gear protection' mechanism but this is not the case. If you are having shift p.roblems, check that the shift cable is not sticking first, it should slide very smoothly when removed from the bracket, but DO NOT TAMPER WITH THE ADJUSTMENT BARREL ON THE CABLE UNLESS YOU UNDERSTAND WHAT YOU ARE DOING, THIS SHOULD ONLY BE CARRIED OUT WITH MECHANICAL KNOWLEDGE AND A MAINTAINENCE MANUAL

Shift Problems can be caused by other factors which can only be assesed with the drive removed,

Sticky slider
Intermediate shift shaft sticking in its bushes
mud or debris around mechanism
Bent shift shaft
To name but a few, however, suspect the shift cable first.

To replace the shift cable you should be armed with a workshop manual and a replacement shift cable kit

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