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Merc 75 Outboard Repairs - Switch Box, Spark Plugs, Kill Switch, Test Start.

However, I am looking forward to the day when I can tow a boat while under sail but so far I have thought it prudent to tow with the control of the outboard and rudder, discretion being the better part of valour.  The lake was a mill pond when I was on my trek to the far end for Panache's end of 2024 season haul out when some guys waved me down.  Their outboard died and there was zero wind.  They were actually sculling back to the marina that was 5 miles (7 Kms) away so I offered them a tow.  When I look back on it I'm still amazed at how well my Merc 75 pulled their boat, given this corroded Switch Box.
 

IGNITION SWITCH BOX (2025) - The Mercury ignition Switch Box (Part # 339-5287A1) generates the timed ignition voltage for the spark plugs.  This is your basic capacitive ignition module (CDI), if you can recall your automotive electronic theory from the 1970s.  Merc calls this a "Switch Box", given that this is a twin cylinder and the "Box" directs the spark to the correct spark plug at TDC.  It draws its power from the generator like a magneto.

Much to my surprise this Switch Box had a badly corroded wire that connects to an ignition coil.  The cracked insulation fell off as I rubbed the wire to remove a spark plug.  Judging by the amount of corrosion, the copper was exposed for several years.  The local repair shop told me that older Mercury outboards (like this one) are noted for this problem.  My guess is the insulation finally broke down after years of exposure to age, moisture, high temperature and the harsh marine environment.  Only two strands were left which is too much resistance to create a healthy spark.  This explains the fouled spark plug and erratic idle of late.  At any rate this condition is not dependable. 

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

It is prudent to take a photo of the installed unit, noting how the wires are routed and where they are terminated, especially if you do the repair weeks after the replacement part arrives;
  2 GN - top cylinder primary coil.
  2 BL - bottom cylinder primary coil.
  1 BK - engine block ground.
  1 BN & 1 WH (each with insulating sleeve) - flywheel switch.
  1 OR - terminal under flywheel. 

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 logical layout that prevents vibration, chafe or overheating with minimal strain. 
 

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

Upon closer examination in the dark areas under the block I discovered the insulation on the kill switch wires was also cracked.  Another problem waiting to jeopardize my adventures.  Aaaarch.  This switch was also manufacture discontinued and equally expensive to replace.  But it was possible 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 contact inside the switch.  This allowed me to use a narrow punch to drive each contact out of the housing.  It helps if you support the connector housing on a matching size socket held in a bench vice. 

Once extracted, the wire end of each contact was filed flat and a depression ground using a Dremel burring bit.  This facilitated drilling out the old wires with a tiny drill bit.  Since the metal surface of the inside of the contact was whistle clean I inserted a new wire and soldered it in place.  The excess solder was filed off both contact barrels 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 is a momentary closure, being opened by the tension of the neoprene cap.  The switch assembly was installed in the outboard frame and the new wires directed to their respective termination.  This allowed me to cut the wires to length, crimp and solder a new ring connector to the ends and tighten each ring under its respective screw.

Success, the switch can kill the spark again.
 

TEST START (2025) -

I positioned my running barrel under the outboard, filled it to immerse the pump, then hit the starter switch.  Voila, it started instantly and purred like a kitten.  It took only a few 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 from the cockpit.  It sure is a lot easier to see a solution for each 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 would be 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.

And finally the delivery trip down the lake went absolutely perfect.  The engine started instantly, purred like a kitten at idle, gushed out a steady stream of cooling water and the new spark plugs made only the slightest ignition noise on the VHF radio.  The background noise sounded 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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