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Tohatsu 2008 9.8HP coil infinite resistance in coil...

bushtor

New member
Hi,
I have just purchased the above used Tohatsu model MFS9.8A3. It ignites only on one cylinder (one black spark plug and the other completely 'new'). When I measure the primary exciter coil there is around 0.5 ohms to ground. However there is an open circuit (>200Mohms) between ground and any of the two secondary winding outputs that goes to the spark plug cables. The resistance between the coil's two spark plug utputs is around 8k4 ohms. When I pull the rope and have the spark plug hex nuts grounded I see a weak spark in both of them.

I have never encountered a coil's secondary winding as an open circuit. To justify that there must be a diode og capacitor involved. Or is the coil defective? There is no corrosion or signs of wear at the engine's interior. Everything looks as new...

Thanks a lot for comments on this. Especially from those knowing this particular scenario in detail :)

Tor
 
I know the 9.8A3 very well. Have worked on maybe a hundred of them. Own one. Best 4-stroke in the Tohatsu barn.

You have an exciter coil, a trigger coil, and the CD, then the ignition coil.

If you use a digital ohm meter, you will likely see infinite ohms secondary lead to ground. It is a siamese coil, so unless there is a bad spark plug wire (or boot) (possible), if one plug fires, they both do. Use a DVA setting (or DVA adapter) on an Analog meter to see what kind of pulse is going into the ignition coil from the CD.

The spark plug wire should have some resistance (between plugs), but 8K sounds a little steep.

You can reverse plug wires, as well as plugs, to help isolate. Many a bad spark plug has been found this way.
 
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