1990 25 HP Mercury Ignition Troubleshooting

vibroluxe

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Aug 2, 2010
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I have a 1990 25 HP Mercury (0C26XXXX) that I need some help with. It is only firing on the top cylinder. Switched the wires on the coils as recommended here and the other coil fired the top cylinder as well. Coils are good.

Built a DVA to check voltages. Stator voltages and resistance appear to be fine. Trigger voltages did not register. Switched the trigger wires so that the brown/yellow wires went to brown/white wires on the switch box and the bottom cylinder got spark for the first time! Great ? I figured it must be the trigger. Had 2 triggers laying around and installed them on the engine. Neither worked. Spark only on the top cylinder ? ARRGH? :mad:

So I would like to hear what you think I should test for next. Or ? do you think I need to just replace the switch box?

Would appreciate any help ? would love to be on the water this year? Thanks! John
 

vibroluxe

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Re: 1990 25 HP Mercury Ignition Troubleshooting

Found this on the web:

Mercury Alternator Driven Ignition Systems
Two Cylinder Engines

•NOTE: These engines sometimes develop a problem where you only have one cylinder firing. This can be caused by the flywheel magnets coming loose and shifting around until they touch. If only one cylinder is firing, remove the flywheel and check the magnets to see if they are tightly bonded.

Talk about an obscure issue... Well, I took the flywheel off and sure enough - the magnets were no longer bonded to the flywheel. Now I have to figure out where these are supposed to be located
 

vibroluxe

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Re: 1990 25 HP Mercury Ignition Troubleshooting

Thought there were 3 magnets but it turns out there are only two. One had split in half and moved all the way over to the other side of the flywheel with the other magnet. Think I'm good, they look like they need to be on oposite sides

What an oddball problem. Thank goodness for Google! John
 

vibroluxe

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Re: 1990 25 HP Mercury Ignition Troubleshooting

Success!!! For the first time since I started working on this beast, both cylinders are firing! I know I am talking with myself here but am posting this to show anybody with a similar predicament that there is one more pice of the ignition that can sometimes be overlooked- the flywheel.

Splashin' my 1970 13' Boston Whaler this week with a now solid running Merc 25...
 

windy5849

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Aug 12, 2010
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Re: 1990 25 HP Mercury Ignition Troubleshooting

Great job trouble shooting.Thanks for posting the problem and solution.
 
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