Outboard Ignition Coils and Spark Plug Wires (HT Leads)
This video looks at outboard ignition coils and HT leads or spark plug wires. Ignition coils have a primary and secondary coil inside them that steps up to the voltage needed to make a spark. There are two main types of ignition: inductive and CDI.
Outboard Ignition Coils and Spark Plug Wires (HT Leads) – Video Transcript
Hey there, Dangar Stu here. Today's video is going to be on ignition coils and HT leads and is proudly sponsored by Marine Engine.com.
Before we look at that though I've got another viewer t-shirt photo. This one is Tapio Koosama, I hope I got that right, sorry if I didn't. Tapio has got a pretty sweet boat there so you're making me jealous.
Before we go and have a look at some real coils on the bench, we'll just go through a little bit of theory about how these coils work. There's two main types of ignition that you sort of see in modern motors and that's inductive ignition and CDI. CDI is much more common in outboards so that's kind of what we are focusing on. Inductive ignitions worth sort of understanding though because a lot of the principles are the same, but it also lets you understand how CDI compares, what's its strengths, what's its weaknesses. The general idea of an ignition coil though is the same in both situations and that's where you have a primary and a secondary coil inside one ignition coil so when you open one of those ignition coils up, which we'll look at one of cut in half later, you'll see there's two coils inside them. There's a primary which has fewer windings or fewer sort of spirals of wire than the secondary and the idea is it a step-up transformer so it's the ratio of the coils that increases the voltage in proportion to that, so in this case if we imagine this was a standard sort of inductive ignition system ignore the, you know no condenser, all this kind of stuff.
But the basic idea is inside your distributor you have a set of points so we're going right back to some old-school ignition. The points will close and when they do, low-voltage current can come in and charge this primary coil and the length of time that charges for is called the dwell and you'll see that's often expressed in degrees of the crankshaft rotation, but there's a certain amount of time that that primary coil has to charge in order to get a good spark. That used to be set by gapping your points, etc. but now with modern sort of electronic and CDI ignition that's kind of a bit of a thing of the past. Once that primary coil has been charged Up, the points will eventually open at the end of that dwell period and when it does the magnetic field generated around that primary coil will collapse and that will induce a very high voltage in the secondary coil. The high voltage induced in the secondary coil then comes down through your HT lead to your spark plug, you get your spark and then it dissipates into the engine block which is ground, so that's kind of a basic idea. The distributor provides current to charge the coil and it manages the timing of turning that current off and it's the ignition coil itself that steps the voltage up from a low voltage to a high voltage, which gives you the volts you need for the spark to jump that little gap in the spark plug itself.
So that's kind of very simplified version roughly of how an inductive ignition system works. With CDI a few things change so I'll just quickly change that diagram and give you once again a very simplified version of what a CDI circuit looks like. Here's the basic idea with CDI. CDI stands for capacitor discharge ignition and this component here, just here, is the capacitor. The idea of a capacitor is it's like a little battery, just stores current and it can be charged really quickly and it can discharge really quickly. I've drawn this having the battery come in to charge the capacitor but generally it's charged from a charge coil under the flywheel but really the idea is that low voltage current gets to it. Various CDI units can work differently, you can actually have just 12 volts DC coming from the battery into the CDI unit, have a little step-up transformer there that gives you a higher voltage you need to charge the capacitor, but quite often with outboards they get charged straight from a coil under the flywheel. The first thing that charging current gets to, is this point here with a little bit of a diode that means going it can flow one way and it comes in, and if your kill switch is out, that means that this connection is closed so this is the kill switch, that little lanyard on your outboard, if it's closed it to goes to ground, and this charging current just goes to ground and nothing happens, that's why the outboard switches off when you pull you lanyard and close that circuit. Presuming your kill switch is in, this switch is open, the current can flow through here and start charging the capacitor. That charge is slightly differently depending on whether it's a step-up transformer or an AC coil etc, because of the diode here if it's an AC coil it won't sort of charge/discharge it'll just keep charging every time it gets a pulse in the correct direction but at the end of the day this capacitor ends up holding current and holding it at a reasonably high voltage, certainly above your battery voltage in the sort of hundreds of volts.
Much like the points in the distributor, we then have a little unit here called a silicon controlled rectifier and it's essentially a switch. Instead of being a switch that you throw manually with a lever it's controlled by another current like a transistor does, so in this case we have a trigger coil that sends a little current in here and when it does this SCR closes so the switch turns off. When it turns on, it allows all the current that was stored in the capacitor so then flow through that primary coil. Now capacitors discharge very quickly, so as that current flows through the primary coil that really rapid voltage induces a voltage in the secondary coil which is then amplified again, it's stepped up to a high voltage high enough to fire the spark plug. So the main differences with CDI is that as the current starts flowing you get your spark whereas with the inductive you actually built a current and then when you shut it off you got your spark so these will fire straight away but as a result you also get a shorter spark.
A long spark is good in certain situations, certainly at idle when the motor is running quite slowly, so CDI's will sometimes even fire a couple of sparks at low rpm so they can fire them really, really quickly so they're great for high rpm, they've never sort of run out of steam, whereas the dwell time on a inductive can get so short that you actually can't produce a very good spark at high rpm. So for that reason it's bit of a trade-off, great high rpm performance, so pretty good for high revving two strokes, that kind of thing, but not as good at idle. Having said all that from the outside it does look pretty similar, coil has two leads in, one lead out, so let's look at some real coils. Somebody commented last week about they expected me to cut a spark plug open because you know who doesn't love cutting stuff open and seeing what's inside, and it was a good suggestion, so I've cut a coil open we will have a look what's inside it.
It's a standard Yamaha coil I think it was, two leads from the CDI unit, the HT lead, so with it cut open what you can see is here fractured, is the inner sort of metal iron core then you've got these thicker wires on the outside, they're actually the primary coil that goes on the outside and then inside here, just inside between the primary and the core here you can see there's the finer wires of the secondary windings. So the idea behind these coils is like any transformer, where you've got the thicker wires carrying high current but low voltage, and then they're inducing a high voltage but a low current in the secondary. The result there is the same amount of power, but what you're doing is trading current for voltage. You don't need a lot of current to jump the gap on a spark plug but you do need a high voltage. So these are the two wires, simply is a ground and then that signal from the CDI, so when that capacitor discharges it comes into this wire and then goes straight to the engine block on this one, and that's what you get the current flowing through that primary coil.
Then the induced current comes out your HT lead, through the spark plug, and then to ground through the engine block. That's all kind of nice in theory but probably not something you really need to know in order to install them, get your outboard working whatever, so it's going to a few of this sort of simple practicalities that you might need to know when it comes to working with these. As we touched on a little bit last week in the spark plug video, there's two main types of coils you'll see on an outboard, there's a single coil like this with your two input leads from the CDI and then a single HT lead out. Then you end up with a waste spark system like this which has two leads coming in the same, there's just a single coil inside here, single primary, single secondary, but it runs two spark plugs. The idea behind these wasted spark systems is that they still just have a single primary and a single secondary coil inside the ignition coil, same two wires, to fire that spark. The spark then comes down both leads, it comes in opposite directions as we talked about but it'll fire on both and both cylinders will be at top dead center, so for example if it's a four-cylinder engine it'll be sort of two and three and then one in four and they're coming up. One will be on the compression stroke so this will actually fire the start of the power stroke, the other will actually be top dead center on the exhaust stroke so although the spark plugs firing, there's nothing but burnt, you know, carbon dioxide in there so it's a wasted spark. So both these types of coils are really common to see on outboards.
I'll try not to repeat too much the stuff that's in the old CDI video that I did a while ago but there's a few resistance tests you can do on ignition coils that are worth knowing about. To do some resistance tests on a multimeter I've just got the positive lead into this section with a little ohm symbol then the common which is the ground then I've got the multimeter set to ohms which is the resistance test, and you can see here now it's basically saying infinite resistance because the leads aren't touching. There's two resistances we can check in a coil, so it's called the ignition coil but it actually has two coils of wire in it so as a unit it's an ignition coil then inside there's a primary winding or a primary coil and a secondary winding, a secondary coil. These two finer wires coming from the CDI are going to the primary coil so we can test this by simply putting the negative on to the negative here, positive to here. Once we got our multimeter on those two points we'll get a reading. Here you can see it's hovering around point two, point three ohms, even then two point one. The reason the resistance is so low is because your primary is these thick wires so thick wires generally low resistance. To test the resistance for the secondary coil I'm going to put the positive lead in where the spark plug goes and then I can put my negative lead on to either of the wires for the primary coil.
So with my positive in the spark plug lead and my negative just on the earth for the coil, I can see there I'm getting about eight kilo ohms. This diagrams for a different outboard but you get a basic idea here, they're saying primary should be 0.08ohms to 0.11 and here they say the secondary coil should be 3.5 to 4.7, if this was the manual for that coil then we'd see that that secondary coil is about double the resistance it should be, but it may well be right for that particular outboard. Once again it goes to show if you're going to do some diagnosis on an outboard you really do need the service manual so you've got the specs for your specific outboard because they really do vary. With a wasted spark system, the primary coil is tested in the same way but the secondary is tested by putting the positive and negative into the two spark plug holes, so you can see here I'm just putting positive negative into the two spark plug holes. I can't recall what outboard this coil comes off so it could be because of resistors built into the caps, all sorts of things but the manual for your outboard will say to test it with the caps on, you should get this total, whatever it is, so that's why you need that information.
Once the current exits the ignition coil it then goes into the HT leads so I can get to this but there's a few different types of HT leads around, there's sort of carbon ones, stainless steel copper, all sorts of things so I'll show what they look like. This particular one I don't know how well you gonna see but this is actually a carbon sort of wind, so this is a high resistance cable and then this type here is just a stainless steel wire, which I'd say is the most common type you'll see on an outboard motor. These spark plug boots that go onto the end of the HT lead, they come in various forms too. Some of them look just like this, which if you see comes through here and actually goes through and makes connection with the core on both sides. After that this lead is just designed to push on to a spark plug like that. They'll have some sort of rubber boot, this is just an empty rubber boot you can buy so this will push through so that it sits inside the boot like that, with the spark plug coming through but internally, that's all it is. Another common type you'll see is like this where, we take the spark plug out and then just pull this rubber boot off the end. Here you can see it looks like this so it's not the spring this time but it's a similar idea and it's crimped onto the wire here. So I'll pull that crimp off and show you what the core looks like. So all I've done here is just start to pull this crimp away and then if you pull this out you can see there it's just the core folded back, that core was then pressed against there and crimped over. So you can see it's a pretty simple connection because it's a really high voltage too, having it pressed up against the middle eye that's more than enough to have the current flow through.
When you buy those they come like this, sort of ready to crimp on but this one is actually an internal feeding spring, this ones is sort of designed to go at the distributor end, much the same idea. Fundamentally they're pretty robust, to be honest with you if you had an HT lead coming like this and you took this wire and wrapped it around the end of a spark plug like that, it would work, it's not going to work reliably you know with vibration or whatever but you get the idea, they're actually a pretty robust connection. And another type that's quite common, so compared to this type here where you fold the insulator back and then crimp a connector on, the screw in type, so this is a Yamaha one and if I pull the rubber boot off the end you can see in there it's just got a screw inside. With that type rather than having the core get extended, you simply cut them completely flush, so it looks just like that, and then just screw the connector down. The threaded section will go inside the insulation and then be pressed up hard against the inner core, so that's another very common type of HT lead terminal you'll see on outboards. Sometimes the HT lead is not actually removable, in this case this is a dual spark coil and you can see here I've just cut the end off and there's a spike here which is the terminal and then the core was up against that and then all of the outer casing inside here is glued in, so there was no way of pulling that out or winding it out.
Although I was saying before it's quite robust in the sense that as soon as you've got a bit of a connection of the core against that pin of the connector or the spark plug current is going to flow to the spark plug. They're not always very mechanically robust so make sure you pull on the plug itself rather than the HT lead, sometimes just giving the HT plug a little bit of a twist as you pull up makes it come out easier as well. These little crimp-on connectors that I was showing earlier, it just fits the middle, they've got no resistance in them but quite often these screw on ends have actually got resistors inside them. HT leads are designed to have a very specific total resistance, so the outboards electronics are designed to have those types of leads so it's important to get the right stuff. The total resistance is a product of the HT lead itself, some leads the carbon Leads, have a certain resistance per foot so longer the lead the higher the resistance and some plugs have these resistors built-in. If you don't have the right resistance you can get electrical interference but you'll also get a higher current flow which means you can burn out some of the electronics inside the outboard, so it's important to make sure that total resistance is correct.
To test the resistance of the plug itself, once again pretty straightforward, just multimeter lead in each end, like that. Once we've got the multimeter probes on you can see here it's a 4.98 kilos so it's obviously a five kilo ohm terminal end. Once again they vary the manual for this particular outboard, says the terminal ends should be 1 kilo ohm so if it is supposed to be a 1 and that's a 5 you know something's gone wrong inside it. When you're testing the resistance of the lead itself you obviously need to know the expected resistance so there'll be a particular, you know, how many ohms per foot with these carbon cables but I also recommend, once you got the leads on, just give the cable a bit of a reboot if it's got an intermittent break in it you might find that when you first put it on you're getting the right resistance then you wiggle a bit and suddenly you'll see the resistance go way up and that way you know there's a break inside the cable. Another thing that can happen is you have a failure of the installation so the actual core is okay but the insulations failed and it's allowing that spark to jump to ground, some on the engine block, or to another lead before it gets to the spark plug. Often you'll hear that if a motor is running you'll sort of hear that crackling noise and if you turn the lights off you'll see it, you'll see that flash of light. Another thing you can do is when you have the lights off and you're looking for that voltage leak, you can just spray some water around the HT leads, that way if there is any failure with the insulation it'll encourage that arcing across the ground, you'll see it straight away, you can swap the lead out.
Well that's about it for HT leads and coils, I hope it sort of gives you an idea of the basics of them, I thought it'd be a nice follow-up one just to wrap up from doing the spark plugs last week. Hopefully next week my back will be a little bit better and we'll pull the green machine back up to the workshop and I'm going to go and do a dual battery install as well as that hydrofoil installs, so we'll get on to those ones next. Alright well take care and I'll see you next week.