Ignition coil failure

Good morning all. After a good blast yesterday on my 850T, I parked the bike up on the sidestand and soon noticed a slow drip of what appeared to be water from between the left cylinder fins. As this liquid reached the ground it solidified into what looked like candle wax. Trying to look between the cylinders I think the drips were coming from one of the coils. If this is the case I assume that coil is on its way out. Should I replace one or both? I have a vague idea they should be replaced in pairs. The bike, by the way, had mot missed a beat. Thanks in anticipation Dave.

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Tank off for a proper look, don’t guess. The metal can type coils I’m familiar with contain oil, but other types are solid plastic construction. But whatever it is has melted by the sound of it. :scream:

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I bought twin lead coils when I had my heads converted to twin plugs, someone gave me some bad advice about the rating of the coils, when I left the ignition turned on the outer plastic body melted like Salvador Dali’s watches.

I assume your coils have metal cases and that the goo is coming from the inside? I would think that you would only need to replace the one that is breaking down but personally I would replace both of them.

My bike started life as an 850T and I have an original coil in the shed from before I changed to twin spark. You are welcome to it free of charge. Not sure if it is any good! Pay me the postage or collect from the Three Horseshoes on Sunday 17th July.

Thank you both for your replies. You’re correct Mike. The first chance I get I’ll whip the tank off and have a proper look. The coils are the tin can type and look like they’ve been there since day one.
Chris, thanks for your kind offer but my coils and probably yours are heading for almost 50 years old so, assuming it is the coil at fault, I’d rather fit two new ones that should last longer than I do. I’m stuck for time at the moment but as soon as I get chance I’ll examine mine and if necessary but a couple of new ones.
Regards. Dave

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Hello again. I now have two new coils but after noting which wires go which wires go where I have somehow lost the plot. The old coils are marked with an R one one terminal. The new ones are marked + and - . Could someone explain in simple terms does the R equate to + or - please. Thank you in anticipation :angry: Dave

The terminal marked R on both coils goes to the contact breaker via a red wire on the RH coil and a green wire on the LH coil and so to earth. R therefore equals minus. The other terminal is positive and the RH coil is fed from the ignition switch via the handlebar kill switch via a white wire. The LH coil is fed from the RH coil via a blue/black wire.

Coil wires

This is an extract from my own wiring diagram, see here for more details

I hope this helps. Best wishes Chris

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Thanks Chris. That is exatly the information I need. I’ve had some guidance from a Greg Bender site stating that the R represents the Italian for contact breaker but you have clarified that information.
Incidentally, I have one of your excellent laminated circuit diagrams from several years ago when I had a Le Mans 2 but my present 850T was rewired by the PO who has done a good job and documented it in the paperwork I received with the bike but the colours of the wiring and some of the circuit layouts differ to the original.
Thanks again. Regards Dave

So did you find out where the dripping was coming from?

Hello Mike. The coil is a metal one and it is sealed around the top. However, there was a trace of a waxy deposit around one of the terminals so that appears to be the problem. I can’t see how it was escaping but there it was. The left coil is mounted about an inch above the left cylinder on what seems to be the original bracket whereas the right coil is mounted below the tank on the central frame tube placing it further away from the hot cylinder. I’ve replaced both coils although the right one which is not faulty is an original Magneti Marelli (please excuse any incorrect spelling) but the faulty one is a replacement similar to the two new ones I’ve fitted. Anyway, the bike started at the first push of the button and all appears to be well. Thanks again Mike and Chris. Regards. Dave

Thanks, I’m somewhat bemused by the ‘waxy deposit’; having taken one of these apart (albeit it was a Lucas jobbie from the late 1970’s), it definitely contained oil, which was definitely runny, not waxy. The top of the metal can is just crimped over the plastic ‘cap’ so not at all impossible to spring a leak there.

Seems to me was simply roasted by the cylinders and gave up, the coils are capable of getting very hot by themselves and don’t need extra excouragement! I know the T3’s etc. had them mounted under the tank between the cylinders but this seems wrong to me, indeed on the Spada and Le Mans that I had, the coils are located behind the left side panel. Cooling air flow is still not ideal but at least they are away from the engine.

I have to agree with you Mike. I’ve always thought that coils contained oil. However, when I first spotted what appeared to be water slowly dripping from the left cylinder fin I then noticed it congealing on the floor with thw appearance of candle wax. When I took the tank off there was nothing except the coil above the point where the drips had fallen and dripped around the hot fin to drop off at the lowest point. I can only go off what I saw but I admit it doesn’t make sense. The left hand coil especially which is close above the cylinder must get hot particularly when stationary at traffic lights etc. As previously stated, the right hand coil is the original Italian equipment whereas the faulty coil is an aftermarket replacement. The bike has done 56000 miles so perhaps that coil has boiled before. Who knows?
Regards. Dave

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