Problems with Mercruiser engine gear shift.

I started having trouble shifting in and out of gear with my Mercruiser 4.3 litre V6 engine and Alpha One stern drive. This was not a problem on the open water when uderway, but became a real nuisance when I was berthing the boat in a marina. Primarily because once in gear, I could not get the engine into neutral whilst the engine was running. There are a number of issues that can cause problems shifting gears and here are three potential causes of the problem.

1. The gear shift cable. With continued use and ageing, the shift cable will deteriorate and begin to break down. The inner cable begins to fragment and then cannot move freely. There can also be problems if the cables are not lubricated. They need to be greased by your marine mechanic and checked to ensure they move freely.
2. Gear shift bellows. These can leak and cause water intrusion which will also lead to problems shifting gears.
3. Shift Interuppter Switch. Sometimes referred to as a micro switch or shift change switch. This device cuts the engine in a split second to allow the gear change without damaging your engine. These can deteriorate or break, making it difficult to neautralise the boat's engine.

My engine would go back into neautral when the ignition was turned off. This eliminated the shift cable, as it would still be stiff when the engine was turned off and if it were the problem. The gear shift bellows had recently been replaced, which eliminated them as being the cause. In the end, my marine mechanic isolated the shift interrupter switch as the root cause of the problem. A small roller had broken free rendering the switch unable to neautralise the stern drive.