Testing & fixing ignition coils - save some $$$ / €€€ / £££ / ¥¥¥

MaxPete

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Good morning all!

When I get into a new activity or return to one as I have returned to motorcycles after a 25 year absence, I always gather as much literature as possible on the new topic and read it. That is the best way to find out what is going on, who are the best suppliers of parts etc. and also, to make some new friends. With my return to motorcycles and particularly, old ones like my XS650s and even the 2007 Honda ST1300, I have subscribed to a couple of UK magazines: Practical Sportsbikes and Classic Motorcycle Mechanics. By the way, my local Chapters (like the US Barnes & Noble) stocks both of these so you can sample them without committing to a subscription. I also got the US-based Cafe Racer magazine which is also good.

These are all truly excellent publications and really informative because they actually show you how to DO something on your bike as opposed to simply how to BUY something for it which is what most of the "mainstream" bike magazines put-out these days - along with a lot of useless sh!t about riding the latest big-buck Harley / Indian / Victory / Ducati / BMW etc. etc. through "Wine Country". Sorry, journos, but not everyone is a California-based zillionaire whose business associates provide them with free bikes and hotel rooms and pays them to swan around writing about their adventures while wearing fancy biker gear and eating gluten-free, fair trade, free-range avocado-coriander-caviar-curry tacos.....

OK - I wish I was one of those people - but I'm not. Rant mode disengaged now. Anyhow....

....on the matter of ignition coils, I have read quite a bit on this Forum about how the old Yamaha OEM XS650 coils were weak and get worse over time and while the initially low kV rating of 1970s vintage coils did make sense, I did wonder why they deteriorated over time. It seemed to me that since an ignition coil is simply...a couple of coils of wire wrapped around an iron core and that the insulation would either be good or bad and that the wire would either be contiguous (not broken) or it would be broken. If the insulation is bad or if the wire is broken - you have a dead coil and can simply toss it because these things are sealed (potted) in epoxy and cannot be fixed. I suppose the insulation might be ok cold but begin to break down when it heats up - but whatever.

To rescue was the recent June 2017 issue of Practical Sportbikes magazine with an article on testing and refurbishing old ignition coils. The article essentially said that if the coil measures properly with an ohm-meter (ie. the primary side and the secondary side are within spec) - and this is a very simple yes/no measurement, then you can refurb the unit by simply cleaning the contacts really well with some steel wool or sandpaper and contact cleaner.

Due to copyright concerns, I can't reproduce the article here but the step-by-step gist of it follows:
  • CLEAN the terminals - each coil has a primary (low voltage) side and a secondary (high voltage) side.
The primary side is what connects to your bike's 12volt wiring harness and the two connections may both be wires with bullet connectors or one of the connections may be a part of the iron core of the coil sticking out the end of the unit. A bolt will go through that hole and act as a ground connection to the frame of the bike.

The secondary side of the coil is where the big voltage spark comes from. On our OEM XS650 coils I think it is around 15,000 volts but modern systems produce as much as 80-90,000 volts or 90 kV. There will only be one terminal for the secondary side as the spark plug screwed into the cylinder head provides the ground connection for this portion of the circuit. The secondary coil terminal has a large diameter plastic "nut" through which the spark plug wire goes. You can unscrew that nut and inspect the actual connector which is shrouded in a little plastic housing. Down in there is a thing that looks like a wood screw - THAT is the connector. To make the secondary connection, you simply screw the spark plug high tension (tension is another word for voltage) wire onto that "wood screw" thingy and slide the plastic nut back on and tighten it - with a little dab of silicone dielectric ignition system sealer grease to keep the water out.

You MUST have good connections on both the secondary and primary sides of the coil for it to work properly - and apparently, rust and crap on one or more of those three connections is the almost always the culprit when a coil is said to be "weak" or dead. The actual failure of a coil is not unknown, but it is rather rare, apparently. The magazine suggests a wire brush and a can of brake or contact cleaner to polish up the connections.

OK - so now you have cleaned the connections - let's test the coil!

TESTING an IGNITION COIL:
  • PRIMARY (low voltage coil - the 12volt part of the coil): this portion of the coil has about 300 turns of heavy gauge wire wrapped around an iron core. The resistance of this relatively short piece of large diameter wire should be quite low. Put your ohm-meter on the lowest scale and measure the resistance tween the two 12v terminals (one of which may be the mounting pad / bolt hole of the coil itself) - it should measure around 1-3 ohms (the value given in my Clymers manual is 2.25-2.75 ohms).
  • NOTE: measuring small resistance values is not easy, so when checking a primary side coil, what you are looking for is a small value. A large value - like 100s or 1000s of ohms is bad.
  • SECONDARY (high voltage - spark plug lead side): this side will have around 20,000 turns of small diameter wire and resistance will be quite high - likely around 10-15,000 ohms. Put your ohm-meter on the 20k ohm scale and measure between the "wood screw" and the ground terminal of the coil (which, again, may be a mounting bolt hole through the body of the unit) - or if the coil has two outputs, simply measure between the two "wood screws". You want to see a resistance of around 10-15,000 ohms here - if not, you either have a terminal which needs more cleaning - or a bad coil (which cannot be repaired).
Anyhow - I'll bet MikesXS/XS650 Direct has made a lot of money selling coils to folks who could still be using their old ones if they had simply cleaned and tested them. This is a very simple task - likely less than 30 minutes start to finish.

Don't forget to put a little dab of dielectric silicone grease on the secondary coil connection (the spark plug wire) to keep water away from the signal. If water gets in there - your spark plug may not fire consistently and the engine will "miss".

Once you've done all of that, wah-la - your coil should work like new again - or truly be dead - but at least you will know for sure - prior to spending a bunch of hard earned cash on a new one.

Pete
 
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Written well, good reading. Cycle magazine once speculated that the motorcycle manufacturers were only spending 50 cents on the OEM coils. Cycle experimented with replacing OEM coils with large, exterior mounted, automotive coils, resulting in easier starts and smoother idling. Low expense or not, they sure were long lasting.

Scott
 
Entertaining, insightful, and well-said. There is a slight problem with the secondary though, I think. It isn't either shorted, open, or neither at high voltages. To measure resistance of a high voltage winding you need a tool called a megger, which works like an ohmmeter, but it uses a high voltage to take the measurement instead of the low voltage of an ohmmeter and the scale is very high. Using a megger you will always read some degree of a short, and the thing is to know what is the lowest acceptable reading is for the particular thing under test.
 
Xjwmx - you are correct but not very many people have a megger and so I think that the method I quote will be adequate for a go-no go decision in a component.
 
^Very much so, especially if it reads open! But if you are interested in their degradation you need a megger.
 
Excellent Pete! Was wondering how to test those blighters. If mine come up smelling good, I'll put some more money in "Pete's Beer Jar" for when you finally visit Oz and look me up. End up owing you and Mailman a carton or two.
Cheers
 
nice write up Pete thank you . It just goes to show once again what a throwaway Society we have become. With manufacturers scimping on materials and quality all the time, to make a profit and stay viable its depressing to compare the quality of goods 30+ years ago with those of the present day.

I have always tried to salvage everything that I can throughout my life rather than buy new and it does give one a lot of satisfaction to return something to full use by ones own hands . It also gives one an insight and understanding into how things work plus it saves some money to spend on something else :)
 
Question:
On original dual output coils, you cannot unscrew the plug wire from the body to test the secondary.
How is this done properly?
I always have trouble due to the wires.
 
Hi Pat - great to hear from you! I just did the forks on my red '76 and once I have that all buttoned up (later this morning I hope) I will be getting at that cool Air-Tech seat I got from you for my '81 Special-to-Cafe bike!

I'm not sure what to do about your issue with the coils. So - you cannot replace the plug wires on the coil? Are they permanently moulded in there?

Hmmmmm. Picture?

Pete
 
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Stock coils from '80+
You may be able to remove the wire, but you will not be able to reuse the plug wires.

IMG_2079.JPG


IMG_2080.JPG
 
So, let's take this to the next level and talk about how the heck the coils work (and you electrical types out there, please correct me if I get something wrong).

In my post above we learned about the low voltage (LV) primary windings and the high voltage (HV) secondary windings. The basic idea is that the LV windings cause a current to flow in the HV windings through a process called inductance. This current sets up a magnetic field in the iron core around which the windings of the coil are wrapped. The strength of that electromagnetic field depends on the number of turns of wire in the coil (more turns = a stronger field and that is why the secondary coil has 20,000+ turns).

When the breaker points open or a pulse of current is sent to the primary side by a solid state ignition system like a PAMCO or a stock Yamaha TCI, the current in the circuit changes rapidly and thus causes the magnetic field in the primary circuit (and thus the secondary circuit) to also change rapidly which causes a massive spike in voltage in the secondary circuit as a result of an effect called the Law of Inductance. This voltage spike is what causes the spark to jump across the electrodes of a spark plug igniting the compressed fuel-air mixture in the cylinder of the engine. Clearly, a lot of stuff (electrical & mechanical) has to happen in the correct sequence for the engine to run properly and efficiently.

This inductance effect was discovered by British physicists Michael Faraday (1831) and Joseph Henry (1832). They described it in an equation which can be found at the following linky (NOTE: the following equation contains calculus which has been known to cause drowsiness among engineering students. Do not operate machinery while doing calculus).

https://wikimedia.org/api/rest_v1/media/math/render/svg/4d306618c5cd1c6fb8a6c5e3fe5894ed2055c0f1

Anyhow, it's all about the magnetism and the electricity - and that's what makes the wheels go round!

Cheers,

Pete
 
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Ah ha - responses:

LittleBill31: I think you're snookered as far as testing those coils. I'd bet that removing the wires will result in no way to replace them. The earlier units had the screw-on spike type connectors which, while being more prone to water ingress, make it easy to replace the HT leads.

David: OK, I'll replace whatever I consume and buy a bag of chips (crisps) for you and I to split when you return.

Cheers,

Pete
 
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LittleBill31: I think you're snookered as far as testing those coils. I'd bet that removing the wires will result in no way to replace them. The earlier units had the screw-on spike type connectors which, while being more prone to water ingress, make it easy to replace the HT leads.

Ya, you cannot remove them. It's difficult to get a good reading on stock dual output coils. Aftermarket coils can have the wires removed, but not originals.
 
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