Re: Installed new/used stern drive now engine died when shifted into forward
There are a lot of wrong ideas on how and why the system works and why it's there to begin with. Maybe this will help explain how it actually works and why it's needed.
The ONLY purpose of the shift interrupt system in the Mercruiser Alpha style outdrives is to get the drive out of gear (forward or reverse) when the engine is running and the boat is in the water with a load on the prop.
The lower gears (Both Forward and reverse) have a dog type clutch. These dogs are reverse cut so they try to hold themselves into gear, and will stay in gear even under heavy load. Even at idle they are under a fair amount of load and without the shift interrupt system, it’s almost impossible to get out of gear. You can see the undercut and how the shift dog fits in the gears in the picture below.
What happens with the shift interrupt system when moving the shifter from Fwd, or Rev back to the Neutral position goes something like this.
When you start moving the shifter into neutral there is resistance from the engaged shift dogs that prevents them from coming out of gear. The outer sheath of the lower shift cable is attracted to the Y shaped spring loaded bracket that operates the shift interrupt switch. When this Y plate moves and activates the shift interrupt switch, the ignition coil is grounded and the engine starts to die due to lack of spark, and pressure on the shift dog is released. With no pressure on the shift dogs, the inner cable then pulls the drive easily into neutral. When this happens, the Y plate releases the shift interrupt switch and ignition is returned and the engine keeps running.
All this only takes a split second, but if you listen, you can hear the engine drop a few rpm before running normal again when coming out of gear.
With an old, corroded lower shift cable, the cable will not allow the Y plate to release and the shift interrupt switch remains engaged. Since the shift interrupt switch remains engaged for a long period of time which keeps the ignition system grounded out, the engine dies completely.
The system is NOT designed to work when putting the drive into gear, or when the boat is out of the water and on muffs. Without the load on the prop, there is not enough resistance on the shift dogs in the lower unit to cause resistance to the shift cable and engage the shift interrupt system.
As a trouble shooting note: If your shift interrupt switch is being activated going into either forward or reverse or coming out of gear when the boat is out of the water, then you have a problem with something binding from the lower shift cable to the drive itself.
The above describes a Mercruiser setup, but the OMC system with the ESA module works basically the same. The difference is the ESA module only stops spark to half the cylinders and will cause the engine to miss, which releases the pressure on the shift dogs.
