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Outboard Repairs - 1976 Merc 7.5 hp E/W Thunderbolt Ignition.
INDEX - Switch Box, Coils, Spark Plugs, Kill Switch, Test Run.

The idea of towing a disabled boat while under sail has always appealed to me.  But with the urgency of the situation I have always thought it prudent to tow with the control of the outboard and rudder.  I have towed a few power boats, much to their chagrin.  In 2024 the lake was a mill pond when I was on my trek to the far end of the lake for Panache's end of season haul out.  About half way there some guys waved me down.  There was zero wind and their outboard was dead.  They were actually sculling back to the marina that was still 5 miles (7 KMs) away.  Talk about optimistic.  So I did the honourable thing and offered them a tow.  When I look back on it I'm amazed at how well my 7.5 HP Merc pulled their boat, given the few strands left in the corroded wires of the Switch Box.  While the main drive of a sailboat are the sails, there are times when the engine is required in a marina, a narrow channel or for the foibles of Mother Nature.  I need my outboard working again.
 

What follows are related repairs.
 

IGNITION SWITCH BOX (2025) - The Mercury Switch Box (Part # 339-5287A1) generates the synchronized high voltage pulses for the spark plugs.  This is your basic Capacitive Discharge Ignition module (CDI), if you can recall your automotive electronics theory from the 1970s.  Merc calls this a "Switch Box", given that this is a twin cylinder and the "Box" zaps the correct spark plug, triggered from under the flywheel.  The CDI draws its DC power from the stator under the flywheel where the magnet in the flywheel passes by the pickup coils and the rectifier converts it to DC.

Much to my surprise the factory original Switch Box had badly corroded wires that connect to the ignition coil for the bottom cylinder.  The brittle insulation fell off as I rubbed the wires to remove a spark plug.  Judging by the extensive corrosion on the wires, the insulation was cracked for many years.  The local repair shop told me that early vintage Mercury outboards (3-25 HP) are noted for this problem.  My guess is the insulation finally broke down after years of exposure to oil and high temperature under the engine cowling.  Only very special insulation could withstand oil in the 1970s.  Just two strands were left which is too much resistance to create a healthy spark.  This might explain the fouled spark plug and erratic idle of the last few years.  At any rate this condition is not dependable. 

Now for the bad news.  The OEM Merc Switch Box is manufacturer discontinued but a local dealer found the last switch in Vancouver, BC at $600 Ca.  A tad rich for me!  When he found a slightly cheaper one for $500 I thought this expense for a 49 year old outboard made it time to replace it, almost!  The good news is that I found an after market module for $155 Ca via the good ole Internet.  Finally some common sense.

It is prudent to take a photo of the factory installed CDI module, noting where the wires are terminated and how they are routed, especially if you do the repair weeks after the replacement part arrives.  The wires on Panache's replacement module were not labelled.  The BN & WH wires are trigger wires for the top & bottom cylinders.  If you match colours each will trigger the correct cylinder.  Have faith!

Its quite a job to fit all the components of an outboard within the tight constraints of the case.  You have to give the engineers credit for their design.  The routing of the 8 wires of this module are no exception.  It took a few tries to achieve a layout that prevents chafe or overheating, all with minimal strain.  I twisted the wires slightly to lower the impedance and minimize vibration.

HIGH VOLTAGE COILS - The connection to the primary winding of a high voltage coil is accessible at the studs on the side of the red coil.  This is where the green or blue wires from the CDI module connect, as shown above.  The top stud is marked + and the bottom stud is marked -.

The secondary winding of the high voltage coil has two conductors; the obvious high voltage one connects to the spark plug and the hidden low voltage one connects to the engine block.  The latter is the return path from the spark plugs screwed in the block.  Without the return path the spark plugs cannot zap.  The hidden wire is located at the back of the coil (Fig 1 below), where it is pressed against the aluminum starter bracket.  It's worthwhile to expose this connection to clean any corrosion then apply a dab of conductive grease.  Conductive grease contains graphite that improves conductivity.  It also blocks oxygen, thereby preventing corrosion and is a good heat conductor to keep the coils cooler.  Tighten the 3 brass mounting screws equally to apply uniform pressure against the hidden wires.  It keeps them connected to the starter bracket.  The studs and ring lugs under the rubber caps are now coated with conductive grease.  The secondary winding should measure ~1100 Ohms (spark plug lead to block).  The primary winding should measure ~.2 Ohms (between + and - terminals).   Cont'd below 
 

Cont'd from above. - The coils are mounted on the aluminum starter bracket at the top of the outboard that also support the electric starter motor and the recoil starter.  The starter bracket is bolted on top of the power head (engine block) where the spark plugs are screwed in.  For this reason the mounting bolts are part of the electrical return path for the high voltage to the spark plugs.  The bolts experience vibration and loosen with time.  Once loose it creates intermittent spark.  Check them occasionally.  (A bit overly simplistic but not everyone understands electricity).

UPDATE IS THE PRIMARY COIL POLARITY SENSITIVE?

No, the ignition coil on an outboard is not polarity sensitive in a way that reversing the primary wires from the CDI can prevent it from generating a spark.  The basic physics of an induction coil means that any changing current passing through the primary winding (whether positive or negative) can induce a high voltage in the secondary winding.  BUT, for optimal engine performance and spark plug life, the polarity applied to the primary winding of a 1976 Mercury 7.5 HP outboard (or any other for that matter) must be such that the secondary winding produces negative voltage at the spark plug.  This spark is several thousand volts more negative than the case ground potential of the outboard.  That being the case, the electron flow is still negative to positive.

Key Points on Polarity

• Function: An ignition coil functions by magnetic induction.  When current in the primary winding is interrupted by the points opening or the CDI module firing, it collapses the magnetic field, inducing high voltage in the secondary winding.  This process works regardless of the direction of current (whether positive or negative) in the primary winding.
• Electronics Theory - Electron flow theory states that current flows from negative to positive.  The center electrode of a spark plug is insulated with ceramic to keep it hot, thereby taking advantage of the phenomenon that electrons like to leave a hot surface to go to a cold surface.  Applying  the negative voltage on the hot center electrode causes the gap to ionize, requiring less voltage to make the initial jump to ground.  (Ionization is required to pass a spark through the high resistance of the compressed gas in the cylinder).  In addition, the center electrode of a spark plug is made from hard metal to prevent metal transfer due to current flow, ensuring long life.  This is the basic physics for a negative voltage spark. 
• Optimal Performance: For the most efficient spark and to prolong the life of the spark plug, the center electrode of the spark plug must have a negative polarity relative to the ground electrode.  This requires the connection to the primary coil studs to be in the correct polarity.
• Consequences of Reversed Polarity: If the polarity at the spark plug is positive, the engine will still run, but the spark will be less efficient, requiring up to 20-40% more voltage to jump the spark plug gap.  This can lead to hard starting, misfires, or rough running, especially as the spark plug ages or is under heavy load. 
• Correct Polarity: If the polarity at the spark plug is negative, the engine will run efficiently, filling the spark plug gap.  This leads to easy starting, smooth running, even with the spark plug under heavy load.
• Summary - While reversing the primary wires will not damage the coil or stop the engine from running, connecting them correctly as per the manufacturer's specifications is best practice for reliable performance.  Ensure the coil's secondary winding (the wire to the spark plug) has a negative voltage for optimal results. 

Measure Spark Polarity - You can test for correct spark polarity by hooking up a voltmeter with the negative lead to the plug terminal (which should be of negative polarity) and the positive lead to the engine block (which should be of positive polarity).  Set the meter initially on the highest volt DC range to measure the negative spark as per the diagram below.  This measurement will be performed in Spring when I can immerse the outboard in the water.

A yellow or orange spark is weak.  A white or purple spark is strong.

Having the correct heat range of a spark plug can prevent the ceramic nose from carbon fouling.  The heat range is mostly determined by the the length of the ceramic nose and to some extent size of the gap between the ceramic insulator and the metal body of the plug.  The final operating temperature has a lot to do with how much heat the plug conducts to the cylinder head.
 

IGNITION KILL SWITCH (2025) - The kill switch on a Merc 75 is a momentary contact closure that short circuits the flywheel signal that triggers the ignition.

Upon closer examination in the dark areas under the block I discovered the insulation on the kill switch wires was also brittle and the wire corroded.  Another problem waiting to jeopardize my adventures.  Aaaarch.  This switch was also manufacture discontinued and equally expensive to replace.  But I was able to refurbish it, saving me ~$80 Ca in exchange for some sweat equity.

The kill switch was easy to remove by cutting both wires and spinning off the retaining nut and neoprene cap.  With the switch free of the outboard the remaining wires stubs were cut off immediately behind each hollow contact pin inside the switch.  This allowed me to use a narrow punch to drive each pin out of the housing.  It helps if you support the connector housing on a matching size socket clamped in a bench vice. 

Once extracted, the wire end of each pin was filed flat and a depression ground using a Dremel burring bit.  This facilitated drilling out the old wires in the hollow pin.  Since the metal surface of the inside of the pin was whistle clean I inserted a new wire and soldered it in place.  The excess solder was filed off both contact pins so they could fit back inside the connector housing. 

Next I slipped the neoprene cap over the connector housing and tested the switch for continuity.  This switch creates a momentary closure, being held open by the tension of the neoprene cap.  The switch assembly was installed in the outboard frame and the new wires directed to their respective terminal.  This allowed me to cut the wires to length, crimp and solder a new ring connector to each and tighten the rings under their respective terminal.

Success, the switch can kill the spark again.
 

FIRST TEST RUN (2025) -

I positioned my run barrel under the outboard, filled it deep enough to immerse the water pump, then hit the starter switch.  Voila, it started instantly and purred like a kitten.  It took only a couple of minutes to burn off the fogging oil, much quicker than previous years. 

Standing next to the outboard with it running at eye ball level is a strange sensation.  You notice little problems that you can't see looking down from the cockpit.  It sure is a lot easier to see a solution for each problem when you can see into the cavities around the engine.  For instance, the throttle in the full advanced position pushed the generator wire which would eventually break it.  All I had to do was direct the wire to the opposite side of the terminal and it was fixed.  Not the easiest to access but...  The wires for the kill switch should have been directed through a cavity deep behind the engine, instead of through the maze of wires the factory directed them through.  To "fix" both required surgical removal of two layers of hardware, much like accessing things on a sailboat.  Remove 10 things to tighten one screw.

THE DELIVERY - The delivery trip down the lake went absolutely perfect.  The engine started instantly with the electric starter and purred like a kitten at idle.  It gushed out a steady stream of cooling water.  The new spark plugs made only the slightest ignition noise on the VHF radio, sounding a lot like a needle on a dusty LP record; comforting and not at all interfering to speech.  As far as operating the kill switch goes, if I don't push and hold the switch till the engine is totally dead, it will just spring back to life again.  Ironic but annoyingly good.  Gawd I love it when everything works.
 

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