Ignition Troubleshooting - 1978 700

[Berdink]

I have a 1978 Mercury 700.
130+/- compression.
I have spark on 1 and 3.
I have no spark on cylinder 2.
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I'm sure there are several methods of process of elimination.
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Here's what I'm thinking.
Let me know if this sounds right

Test 1
If there is Spark on 1 and No Spark on 2, swap the two trigger wires at the SwitchBox.
If spark returns on 2, and now no spark on 1, it’s the trigger.
If still no spark on 2, and there's still spark on 1, it’s not the trigger.

Test 2
If still No Spark on 2, swap the green wires going to coils 1 and 2.
If spark returns on coil 2, it’s the SwitchBox.
If still no spark on 2, it’s the coil.
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[Chris1956]

Sounds about right. Those older Mercs had issues with insulation falling off the wiring harnesses. You might also look for that.

CDI Electronics should make replacement components.

 

[Nordin]

Most of those Mercury and Mariner the switchbox fails.
From my experience the switchbox and stator fails, not so common with the trigger.

 

[Berdink]

Yeah, much looked good before I started touching things and the insulation started crumbling off the green corroded wires.
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I'll definitely be going through all the wiring and making repairs before I attempt to start it.
Same with the whole fuel system.
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[Berdink]

Okay, switching green wires around from a known good circuit/switch on the switchbox, I get spark with all three coils.
And alternating the Violet, White, and Brown Trigger Wires, connecting them to a known good circuit/switch on the SwitchBox, I get spark with any of those three Trigger wires..

Conclusion, the Stator is putting out voltage.
The Trigger is sending the signal to the SwitchBox with all three wires correctly. (Violet, White, and Brown)

The issue is with the SwitchBox.
The SwitchBox is not sending current to #2 coil/sparkplug wire.

I'll order a new SwitchBox and post back the results.
Thanks Again

 

[Berdink]

Most of those Mercury and Mariner the switchbox fails.
From my experience the switchbox and stator fails, not so common with the trigger.

Would I have better chances of success with Sierra Marine? I'm thinking they at least have a lifetime warranty.

 

[Mc Tool]

can you get into the switch box ? is there a circuit board you can see ? . Im just thinking that by the way you describe your diagnosis it sounds like the switch box is 3 separate switches /circuits and thusly you have two good ones to compare against the bad one .... may be a single component that could be replaced . I dont like chucking out stuff like that when theres nothing to lose by having a fiddle .

 

[Chris1956]

The switchboxes are potted with epoxy. It is very difficult to get to the innards.
You might look for a new engine harness. Sierra may make them.

 

[Berdink]

can you get into the switch box ? is there a circuit board you can see ? . Im just thinking that by the way you describe your diagnosis it sounds like the switch box is 3 separate switches /circuits and thusly you have two good ones to compare against the bad one .... may be a single component that could be replaced . I dont like chucking out stuff like that when theres nothing to lose by having a fiddle .

Likely not...The circuits are emersed in a pool of epoxy

 

[Mc Tool]

Mmmm bugger ! Looks like there are 3 either bulges or discolorations down the left side adjacent to the three nuts on the right side ....I wonder if thats where the power transistors are . Probly wouldnt be hard to make one

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[Berdink]

View attachment 419944

My shopping for a new one I'm noticing they all have BLK/YEL where mine has ORN for Orange.
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Mine might likely be a pretty old one.
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Mercury switched from the solid orange (or salmon) kill/stop wire to the standard industry black/yellow stripe wire around 1978–1979, coinciding with the adoption of the universal Boating Industry Association (BIA) wiring color codes.
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