Help me understand the ignition system...

[SecondMonkey]

I have a 1972 150hp Mercury outboard that I have completely rebuilt. I would like to upgrade the entire ignition system with modern parts, but I'm a little confused as to how it works.

Basically, this motor has a very early version of electronic ignition - no points. I would like to replace the "switchbox" which I believe does the job points would have.

There are three wires coming from the distributor's trigger to the switchbox. Then there is a wire from the coil to the switchbox, and wires for power and tach.

Can somebody help me understand how this works? Is this something I could replace with an electronic ignition upgrade for old cars and a new coil? Are the three wires from the distributor similar to that found in older cars?

 

[dwendt44]

Mercury's Thunderbolt ignition was pretty good then, still fairly decent today.
If there are a lot of hours on the engine, I'd just change the coils and check the wires(they were solid core back then IIRC) and be done with it. Maybe clean all connections.
You may want to get a spare switchbox, JIC.
They are expensive new, but like a lot of manufacturers, they will stop making replacements soon if they haven't already.
Don't know if there are outboard junk yards for a source for spares.

 

[SecondMonkey]

Yeah, mine still works ok, but I think it could be better. The switchboxes tend to wear out and cause hard starting, which I have. They no longer make them, though there is an aftermarket version...but it's absurdly expensive because it says "marine" on the package.

I'm mostly just trying to understand exactly what the switchbox does. Is this ignition system like what you would find on a car...or is it some crazy weird thing that mercury came up with?

 

[andyd]

google on capacitor discharge ignition. IIRC, a module that used to replace the points and condenser in old school distributors

 

[tom slick]

your ignition probably operates using a hall effect.
simplified explantion:
a reluctor moves past magnetic pickup causing a disturbance in the magnetic field. the magic ignition box senses that disturbance and triggers the ignition to fire. it doesn't matter what coil you use as long as it is a non-resistor type.

Using GM HEI modules is very common to replace other manufactures' modules.

This might work for you if you are crafty.
http://www.tercel4wd.com/forums/viewtopic.php?p=27113&sid=4e36ec8e76e52a235eb813c46580539f

I've also heard of people replacing their module with an MSD 6A but I don't know if it is possible or how it is wired.
http://www.msdignition.com/ignition_4_6200.htm

 

[dwendt44]

I'd stick with the Mercury coil if it's still available.
It's a high out put 30Kv, IIRC, and these coils have been very
reliable.
Some snowmobile racers used Mercury coils for upgrading their
ignition systems back in the 70's.

 

[Donald]

You should be careful with using anything non-marine in your marine
engine. The material could corrode more easily or it could be dangerous if it produced a spark without a screen (such as an automotive starter or alternator). I have no problem with aftermarket parts, but they should always be marine.

 

[tom slick]

good point about marine applications. MSD does make a marine version of 6A, its called a 6M-2.

http://www.msdignition.com/mari_1.htm