Tohatsu 2008 9.8HP exciter coil issue....

[bushtor]

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

 

[pvanv]

I replied in the other forum. You should get an Analog meter with DVA, take some readings, and much will become clear. It is apparent that you do have spark to both plugs, but you could have a bad plug. It does happen, and it can fool any mechanic (I know!).

 

[km1125]

If I understood Paul's response in the other forum, you have a "waste-spark" system. When the coil fires, the current goes out through one high-voltage port of the coil, to that spark plug, to ground, to the other spark plug then back to the coil to the high-voltage port. It only goes to "ground" to get from one plug to the other plug. You might think of one plug operating in 'forward', where the spark jumps from the center electrode to the tab electrode, then in the other plug it runs in 'reverse', by jumping from the tab electrode to the center electrode to get back to the wire going back to the coil. In essence, it doesn't really matter which way the spark jumps, as long as it jumps.

 

[pvanv]

Um. No. Close, but not quite:

Yes, it is a wasted spark system, but it is CD/Magneto, not a 12v system at all. There is an exciter coil to charge the CD, then a trigger coil to discharge the CD. The CD discharges into a siamese coil, which has 2 plug wires, and both fire in the same direction at the same time, so each cylinder fires around TDC-compression (actual timing controlled by the internal programming of the CD), and also fires on TDC-exhaust, which is the wasted spark.

So, if only one plug is not firing, there is either a bad plug, a bad plug wire (or plug cap), or a bad connection where the plug wire attaches to the coil.

Troubleshooting would include swapping plugs, and swapping plug wires between cylinders. It is often a defective plug.

 

[km1125]

Well, I wasn't really referring to the primary side at all. The graphic I grabbed off google does show a 12v and edis, but that could easily be the magneto and CD circuit. I believe the secondary is the same. In some terms, when you refer to a "siamese" coil, that could just be two standard coils in the same housing, with a common ground. In a waste-spark system, there is no ground in the secondary side of the coil. In the waste-spark system, the plugs MUST fire in "opposite" directions, as otherwise there would be no return circuit to get back to the coil. IF the coil had a ground on the secondary, THEN you'd have an appropriate return circuit.

BUT, the troubleshooting is exactly as you describe... plus or wires... one has to be shorting to ground somewhere.

 

[pvanv]

KM, think of it as a 1-cylinder setup, with both plug wires both connected to a single coil. It is not two separate coils in a single housing. The definition of a "wasted spark" system is that the coil fires both on compression and exhaust. Hence the wasted spark on exhaust. They also use the wasted-spark system on the 2.5/3.5 and 4/5/6 1-cylinder motors, as well as the 8/9.8 2-cylinder systems, And yes, as you noted, if there is any spark at all, the troubleshooting comes down to plugs, wires, and plug caps. Of course, if the coil itself is very weak (highly unusual), you could get some spark, but not enough. In that case, a weak plug, weak wire, etc., or a leaky cylinder, might not fire, while the other one, being in better condition might still fire. Highly unusual.

 

[km1125]

Paul, all I was trying to resolve was his concern that he could not find a path to ground from the secondary outputs. In a "normal" coil you'd have that, but in these two-cylinder coils you don't.

This also affects how you have to troubleshoot, as you can't just focus on one cylinder or the other, as they are both tied together, unless you understand how they interact. Just disconnect plug 1 and plug 2 will not fire, no matter how good it is, or disconnect plug 2 and plug 1 will not fire, no matter how good it is.

You can isolate them by grounding one of the secondary outputs from the coil and working on the other cylinder. This effectively bypasses that cylinders spark plug. This will cause an increase in voltage across the other plug though, so it may still fire with the higher voltage, even though it is defective. A grounded plug or wire would still show as bad.

 

[pvanv]

Interesting thought, but incorrect. Again, think of it as a 1-cylinder coil, with 2 wires attached. The reason he sees such high resistance from the coil output side to ground is that the resistance is in fact very very very high. Also... If you unplug cyl1, cyl2 will continue to fire. If you ground either of the coil outputs, the coil will go dead, and nothing will fire.
Wasted-spark systems are in fact even used on older OMC 2-cylinder, 2-coil systems. It's just that they fire on exhaust as well as compression. That is all.

 

[km1125]

No, it's not incorrect.

Forget the 'wasted spark' thing.... what you said before was correct and I agree with that. Firing on the compression but then on the exhaust is "wasted spark". It is used because they trigger on the crank instead of the cam, so they can't tell when any cylinder is at compression... using waste-spark is cheaper to trigger and really doesn't have much downside. It can be used regardless of the coil configuration.

When you used the term "siamese coil", that's when I said there are really two types of coils that could be referred to as "siamese coil". One, is where there are two conventional coils in the same housing. These have been used in automotive a lot to save space. Each coil is internally grounded on one side of the secondary and there are two high voltage terminals - one for each plug. When you measure resistance, from each high voltage coil to ground you'll get about 10K resistance. Between the two high voltage terminals, you'll read about 20K. The other type of "siamese" coil is where there are two high voltage terminals (one on each end of the coil winding) and no ground.

If I understand this particular coil correctly, then the OP stated he has virtually infinite resistance to ground on either high voltage terminal, and about 8K ohms from one high voltage terminal to the other. That means it is the type in the diagram that I provided above, where the spark plugs are in series on the high voltage side and fire simultaneously. In order for this to work correctly, both plugs must be connected to complete the series circuit.

The reason you don't have them in parallel as you suggest, is that it would be possible for one plug to always fire and the other to never fire. Electricity always takes the "easiest path to ground" (or to complete the circuit). In a waste-spark system, the spark plug NOT in compression would fire before the spark plug that is under compression (btw, the real way to test a spark plug is to put it in a cylinder with >150PSI of air, because it's harder to fire in compressed air)

 

[km1125]

To the OP: Have you had any success in resolving the issue?