Dolphin Marine Service

Old World Craftsmanship with New Technology

205 Pennsylvania Ave.  Fortescue, NJ 08321

Below  you will find information on adjustment of Mercruiser I/O Shift Assist Switch and adjustment of the Lower Shift Cable.  We do not claim to be experts in all fields.  Nor can we promise to be correct on all issues.  Nor can we address the content of these pages.  (Please read our legal disclaimer page)

But, I hope you will find this page useful.  We want to thank Aftermarket Marine for their technical assistance with this page.  Aftermarket Marine is a supplier of high quality aftermarket parts for outboards, inboards, and inboard / outboards to Dolphin Marine Service.
This page was last updated on: November 3, 2008

The purpose of the switch is to kill the engine for a fraction of a second to release the gears.  Normally adjustment of the shift assist switich will be required if the engine kills when shifted into gear.  The mere fact that it is killing the engine proves that the switch is good, it is working at the wrong time(going in gear).

If the boat has no neutral, when you try to shift to neutral it fights you and passes from fwd to rev or rev to fwd, then you have a  problem of a sticking shift cable or a sticking shift arm and roller on the upper shift shaft.  This adjustment is addressed in part two of this page below.

The gears in the outdrive are undercut so once together they stick together if there is a prop load(in the water).  When the forward cable tries to move the lower shift cable it fights the action and is felt as a resistance on the shift handle.  Pressure on the lower shift cable shifts the arm that trips the easy shift switch, thus the engine starts to die.  One second later during the lapse in h.p. the gears let go, then the switch resets and the engine regains power.  The whole process takes only a second and the engine recovers with only a hesitation, which you hear (whooomp).

If this is what you think is happening, then follow the procedure below.

1)Test if the switch will kill the engine; Press the arm of the switch and roller inward, engine dies if switch is good.

2)To test the switch; jump across the wires going to the switch; if engine dies the circuit is good.

3)Test when the switch kills the engine; Press the shift actuating arm to either side, roller rises up engine dies.

4)It is important where the switch starts.  It should kill when the roller is 1/8" out of the valley of the arm.

5)If it trips lower it will trip at the wrong time , If it goes too high it won't work.

6)To adjust the position of the killing switch slightly bend the roller arm.  Bending it in makes it kill earlier.  Bending it out makes it later.  Retest.

7)It is important that the cables do not restrict the shift process, be sure the barrell under the cotter pin is free.

8)Be sure the three nuts that hold the cables down are not tight, just past the lock on the nuts,

9)Be sure the spring loaded plate that trips the switch is free and center springloaded.

10)If the plate is free to move and spring back to the center and the switch trips at 1/8" idle under 1000rpm; It should shift.

If you continue to have problems you may need to adjust your lower shift cable.


1)Be sure the pre set 6" length from tip hole to barrell (center to center) has beeen set and do not change.

2)Remove the forward shift cable from the shift arm assy.

3)Put shift box into forward gear.

4)Test feel the muvement of the springloaded metal piece that trips the easy shift switch.

4a)It should move freely from stop to stop with no resistance except the return spring.

4b)It should stay in the center with the switch roller in the middle of the valey on the end of the metal arm.

4c)When pushed either way it should feel only light spring pressure to the stop.

5)Move the shift arm with the lower shift cable attached to collapse the cable to its shortest position (fwd gear)

6)While moving the cable to its shortest position the prop must be rotated counter clock wise to the stop.

7)When the prop is jamed against the stop counter clock wise, and the cable compressed, this if full forward position.

8)With the arm in the above position adjust the forward cable barrell to just fit between the studs.

9)All slack should be eliminated but excess tightness will unseat the roller on the shift assist switch (this is not good).

10)Ideally the roller should stay in the center, but if you move the metal bar that operates the easy switch you should feel pressure.

10a) By pressure I mean you are forcing the cable before you hit the stop, it feels different than step 4c.

11)This is the forward adjustment.

12)To adjust reverse; Remove the tip of the lower cable from the stud on the tip.

13)Shift control into reverse gear.

14)Pull on the tip of the lower cable while turning the prop cw to the stop.

15)This is full reverse position.

16)Move the stud that the cable mounts to up or down the slot to fit the cable, do not shorten the cable to fit the stud.


1)You have gone too far; if you put it as tight as it goes and it feels almost tight enough, you can back off of forward to balance the adj.

2)Slack in the shift chain of linkage (bad cable, loose connection between parts, or old lower shift shaft, loose slide)

To check for the above problems there are three procedures:

Procedure #1 (Lower Unit parts wear)

1)Put the lower cable in full forward, stand on the prop blade to lock the shifter.

2)In this condition the shift cable should move very little; if you can slide it any distance, something is giving or shifting.

3)Determine what two parts are making the slack and replace them.

Procedure #2 (Not enough travel in shift arm)  MOST COMMON FIX!

If the cable adjustment in 16 is not far off, drill a new hole in the arm to accept the stud.   Or you can elongate the existing hole.

Procedure #3 (Problems in the shift box up front.)

Many shift boxes have adjustments to make them universal.  Sometimes people use the wrong adjustment shortening the travel of the box.  The common problem with this issue is lack of cable travel. 

1)Check if the arm behind the shifter that holds the shift cable is in two pieces. 

2)If it is in two pieces be sure it is fully extended. 

3)The farther out the cable is mounted the farther it travels. 

4)Some arms are multiple drilled, make sure the cable is in the outermost hole, or is there room to drill another further out? 

If the tests above do not indicate a problem then the other adjustments in the box and arm should correct shifting.

Remember if you have a problem, help is only an e-mail away.
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