Measuring Resistance in Spark Coil Assembly

Electrical and electronic topics for small boats
Soho
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Joined: Tue Nov 13, 2018 2:03 pm

Measuring Resistance in Spark Coil Assembly

Postby Soho » Sun May 21, 2023 11:10 pm

Q1: Is there a specific procedure or "trick" to replacing the plug cap onto the coil wire when installing a new ignition coil on a Yamaha 90-HP engine that is three years old?

Q2: once the ignition coil is assembled, how is a resistance test made to insure there is proper electrical connection to the little wire in the coil wire [to something unspecified]?

I have just [installed a spark plug cap assembly and an ignition coil assembly on a Yamaha 90-=HP engine. No spark is being generated on the cylinder associated with the ignition coil assembly.

The installation method looks intuitive: just screw the cap onto the wire.

Q3: is there a specific technique for installation of the cap onto the wire?

I am alone working on the boat. To simultaneously crank over the engine and also see the spark is difficult. Also I have no spark plug tester.

I just want to know if the spark plug cap assembly is properly installed onto the ignition coil assemby.

I want to test the resistance to verify

On Monday, May 22, 2023, I will be on the boat. I will check the bullet connector to be properly connected, I will check that the spark plug is clean.

To be able to perform a resistance test will be ideal.

jimh
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Re: Measuring Resistance in Spark Coil Assembly

Postby jimh » Mon May 22, 2023 9:38 am

Soho wrote:Q1: Is there a specific procedure or "trick" to replacing the plug cap onto the coil wire when installing a new ignition coil on a Yamaha 90-HP engine that is three years old?


Give the actual model designation of the engine and the date of manufacture. Requiring readers to intuit the model designator and the year of production from your description of the engine "age" is not particularly a precise method of identification of the engine model and year of production.


Soho wrote:Q2: once the ignition coil is assembled, how is a resistance test made to insure there is proper electrical connection to the little wire in the coil wire [to something unspecified]?
To give a general answer about resistance measurement, in any high-voltage developed with a spark coil, the high-voltage lead from an ignition spark coil will have continuity to the ground lead from the coil.

Measure the resistance at the spark coil lead that will attach to the spark plug electrode to ground. You should see the resistance of the spark plug coil secondary winding. This resistance should not be 0-Ohms, but will be the resistance of the wire wound onto the coil secondary winding. That could be in the range of 10-Ohms or higher.

You can deduce to proper resistance reading by measuring the resistance on the spark plug high-voltage lead to ground on another cylinder's ignition coil.

Soho
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Joined: Tue Nov 13, 2018 2:03 pm

Re: Measuring Resistance in Spark Coil Assembly

Postby Soho » Mon May 22, 2023 10:06 am

Thank you Jim.

Yamaha 90hp, Engine Model 90AETOL, Year of Manufacture 2019



Re Q1: Is there a specific procedure or "trick" to replacing the plug cap onto the coil wire when installing a new ignition coil on a Yamaha 90-HP engine

To answer this question, you need have the actual experience of doing this. The way Yamaha has this set up is the plug cap has a coarse screw on it that actually "screws" into the insulated wire from the coil - the screw goes down the center of the insulated wire, the insulation is thick and rubber, and makes contact with the wire in the center. These are 2 separate engine parts to purchase, the plug cap and ignition coil which ends in a wire that you attach your plug cap to.

Q2: once the ignition coil is assembled, how is a resistance test made to insure there is proper electrical connection to the little wire in the coil wire from the plug cap.

This has been answered and actually it was sort of a dumb question I admit. The goal was to confirm that a connection had been made between the newly screwed in plug cap and the coil, no continuity would imply no good connection and no spark will happen.

jimh
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Re: Measuring Resistance in Spark Coil Assembly

Postby jimh » Tue May 23, 2023 9:03 am

Soho wrote:Q2: once the ignition coil is assembled, how is a resistance test made to insure there is proper electrical connection to the little wire in the coil wire from the plug cap.

This has been answered and actually it was sort of a dumb question I admit. The goal was to confirm that a connection had been made between the newly screwed in plug cap and the coil, no continuity would imply no good connection and no spark will happen.


I don't understand your comments above.

Have you been able to properly measure the resistance of the connection between the "little wire in the wire coil from the plug cap"?

Also, some high-voltage spark plug wires are resistive wires, that is, they are not just pure copper wire is almost no resistance. The wire itself has intentionally been made to have resistance so that the delivery of the high-voltage to the spark gap in the spark plug does not create excessive radio-frequency interference.

Soho
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Joined: Tue Nov 13, 2018 2:03 pm

Re: Measuring Resistance in Spark Coil Assembly

Postby Soho » Tue May 23, 2023 10:56 am

To clarify - my objective is to confirm that the installation of the plug cap onto the new ignition coil wire has been made correctly and that the current from the coil can in fact reach the plug. I should probably have said "continuity check" not resistance, although one could argue that a resistance reading confirms continuity.

I have not been to the boat yet, it's raining here, to check that I have continuity on my new installed coil.

The root problem here is that I have intermittent problems with my #2 cylinder not firing, and the "fix" so far has been to stop the engine, reseat the spark plug cap, start the engine again and it has typically, well always, rectified the concern. This led me to want to check that my plug cap to coil connection was good, hence the desire to check continuity. This also was my reason for asking if there was a trick to installing the plug cap on the Yamaha, it's not one unit, it's 2 pieces that you have to install, hence the potential to get it wrong somehow. I bought a new coil "just in case" that was the concern.

What I can do now is test the continuity on the installed new coil, If I have continuity, check that #2 is working when running and go from there.

I did not want to solicit, yet, input on other potential problems for #2 not firing due to the nature of the fix to date - might just be a loose wire.

Hope this clarifies and again sorry for any confusion, I do appreciate the input being forwarded on.

jimh
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Re: Measuring Resistance in Spark Coil Assembly

Postby jimh » Tue May 23, 2023 1:56 pm

Even if there were no DC resistance continuity, as long as there was only a small air gap the high voltage would jump the gap. Be careful running the engine and seeing no spark at the plug. There could be voltage jumping to ground somewhere else, like inside the coil assembly. You should look closely at the coil assembly for any sign of damage due to stray arcing to ground.