Index

Merc 75 Outboard Repair - Switch Box, Kill Switch.

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 rudder and outboard, 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 season haul out when some guys waved me down.  Their outboard died and there was no 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 with 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 CDI ignition module, if you can recall your automotive theory back in the 1970s.  Merc calls this a "Switch", given that this is a twin cylinder and the Box directs the spark to the correct plug at TDC.

Much to my surprise the Switch Box in Panache's outboard had a badly corroded wire connected 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 on this wire, the insulation was leaking for several years.  The local repair shop told me that older Mercury outboards are noted for this problem.  My guess is the insulation finally broke down after years of exposure to oil.  Only a few strands were left in the wire which explains the occasional erratic idle due a fouled spark plug not receiving enough juice.  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.  Well that expense for this 50 year old outboard makes it almost time to replace it!  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 part arrives;
  2 GN - top cylinder coil.
  2 BL - bottom cylinder coil.
  1 BK - engine block ground.
  1 BN & 1 WH (with insulating sleeves) - 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. 

I operated this outboard for years with Bosch automotive spark plugs as they were the ones it was equipped with when I bought Panache.  I cleaned them with each annual service but now they were looking a bit shoddy.  Not having the manual it was difficult to know which spark plug this outboard wanted.  When I discovered the correct plugs to be NGK BUHW, I thought it high time to install some.  It was difficult to source these plugs locally but as luck would have it, I had a set in my parts box.  There is an expression for this, but...  Success, a healthy spark at both plugs.  And all I wanted to do was test the new Ignition Switch Box.
 

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 in exchange for some sweat equity.

I removed the kill switch by cutting both wires going to the engine connections 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 contactor inside the switch so I could use a narrow punch to drive each connector out of the housing.  It helps if you support the connector housing on a socket held in a bench vice. 

Once extracted, the wire end of each contactor 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 contactor was whistle clean I inserted a new wire and soldered it in place.  The excess solder was filed off both contactor 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 connector 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.
 

Return to Tech Tip Index. . . . . . . . . . . . . . . Have a Question?