How to diagnose a coil or electronic ignition system fault

[ThePinkPanther]

On my OMC Cobra inboard V8 engine, I have zero spark(s) coming out of the plug leads.

The starter motor turns fine and there is a a good 12+V out of both batteries.

When I disconnect and "lay" a plug lead across the block and turn the engine, there is not the slightest sign or sound of a spark ............. likewise I can attach a spare spark plug into the coil lead and once again there is no spark from it when the engine is turned over.

I have also tried installing a spare coil but still no luck. That gets me to thinking of the electronic ignition module in the Dizzy and to my knowledge there is just no way to test those mongrel things for output other than buy another one ($600) and see if it fixes the problem .......... Expensive bit of blind troubleshooting!

Any of you guys know what could could be causing this lack of spark on all the plug leads and associated failure to start?

My Option B is to install a new Dizzy with what I believe has it's own internal coil plus an ignition module all being 3 in 1, if such a thing exists though I have been told it does!

Anybody know if I am on the right track here and if so, is there agood sparky on the Bayside of Brisbane to help with installation, timing etc?

Thanks all!

[Fed]

Have you checked the shift interrupt system?

[ThePinkPanther]

Have you checked the shift interrupt system?

Gulp! Hmmmm! Ahhhhhhhhhh! Duh! Yeh, well, geeeee, thanks Fed, like what the hell is that?

Never heard of it in my life. Is it somthing to do with the system that stops me from starting the boat when in gear?

If so, that it was disconnected ago as it gave heaps of trouble and eventually the dealer simply chopped it off during a service.

If this isn't it, hen you have got me beat!

I have done a bit of homework since the post and am pretty convinced it is the ignition module that is shot as it has all the symptoms of a (failed) unit.

Appreciate your comments.

Cheers

[Fed]

In a nutshell it's a small switch connected between the coil negative and the engine ground that activates (closes) when there is strain on the shift cables imposed by trying to into neutral against the reverse cut dog clutches. This causes the engine to miss a beat and lets you seamlessly get out of gear. If the strain mechanism sticks your ignition remains dead.

It should be very easy to find & test, rather than write a tree about it I pinched a description from iboats for you.

HOW TO: How the Shift Interrupt system works

Usually in the spring and summer we get a lot of posts regarding the shift system for the MC-1 (falsely referred to as Pre-alpha drives) to the Alpha 1 drives. 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.

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.

Don't let the tree above scare you it's really a very simple system and you could test it by simply disconnecting one wire.