Ignition coil testing

How to test a coil

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old man emu
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Ignition coil testing

#1

Post by old man emu »

My WLA's engine stopped working last July and for a variety of reasons (Winter, Spring Rain, other activities) it has been sitting idle until this weekend.Before that short run in July it had only done about 200 miles since 2020. So I decided it was time to get it going. Which is a brilliant idea as Winter is approaching.

I did all the correct things - fresh oil and fuel, charged the battery and cleaned the wire connections and plugs. Tried to start, but no go. No spark at the plugs.

Here's the results of examinations:
1. Power to the Primary coil inlet terminal.
2. Power leaving the coil outlet to the circuit breaker. Does this mean that the Primary coil is intact?
3. Power at the contact points.
4. Battery just about 12 volts.
5. Generator light illuminated.
6. Unusual finding: No power at the oil pressure switch.

The coil is relatively new, and I've only done about 500 miles since it was fitted, I think - definitely more than 1000 miles due to COVID and really crappy weather since 2019. I can't remember who I bought the coil from, but my usual USA supplier is Flatland in Arizona. At the time I also replaced the H/T leads and the bike was running well.

It's been a while since I played with the ignition system and Old Timer's has made me forget how to test the coil, especially the Secondary coil.

Based on the examination results, can I please have some advice and guidance on possible causes for the lack of spark and how to test the coil?

I can't go to an auto electrician as none working nowadays knows anything about simple ignition coil systems.

Thanks heaps.
Andygears
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Re: Ignition coil testing

#2

Post by Andygears »

The simplest way I know of to check an ignition system and thus the coil is look at the points, see if they’re opening when you turn over the motor. Pull the plugs, plug them into the wires, let the points stop at a closed spot then with a screwdriver, open them and play with that, the plugs should fire. Of course the ignition should be “On”.

Andygears
Excalibur
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Re: Ignition coil testing

#3

Post by Excalibur »

What Andy said.
If still no go, ensure plugs have some sort of common ground. You can't have one plug dangling mid-air on the end of its' HT lead and the other grounded because both plugs will not fire in sympathy with each other.

Wet weather/sitting for long periods/etc can oxidize points so give these a good clean. Shiny-metal-clean.

Test secondary circuit of coil/plug leads/caps simply as illustrated in pic, ohmmeter clipped to plug HT caps. A reading here indicates there's continuity. No reading indicates open circuit. Also it verifies HT leads are in proper contact with plug caps and coil output terminals.
coilHT.jpg
coilHT.jpg (14.47 KiB) Viewed 943 times
Primary winding should register about 4 - 5 ohms
6. Unusual finding: No power at the oil pressure switch.
Normal because the OP switch terminal is itself grounded when there's no oil pressure.

HTH. Good luck
wannchev
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Re: Ignition coil testing

#4

Post by wannchev »

Secondary coil resistance should measure about 7000ohms without the resistor type spark plug caps. These coils are very reliable.
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Re: Ignition coil testing

#5

Post by wannchev »

Measure resistance between the 2 sparkplug leads. Not an automotive coil.
Mongrel505558
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Re: Ignition coil testing

#6

Post by Mongrel505558 »

6. Unusual finding: No power at the oil pressure switch.
Normal because the OP switch terminal is itself grounded when there's no oil pressure.

HTH. Good luck
[/quote]

Assuming the bulb is in place and the filament intact, in which case it would be lit because current has to pass through the bulb to ground. Is the bulb lit? Anyway, if there's voltage at the points and coil primary then you can figure out the oil pressure light later.
old man emu
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Re: Ignition coil testing

#7

Post by old man emu »

Excalibur wrote: Sun Apr 09, 2023 4:52 am Test secondary circuit of coil/plug leads/caps simply as illustrated in pic
Please upload a bigger file size of picture. I can't make out detail.

ohmmeter clipped to plug HT caps. A reading here indicates there's continuity.
Do I see one probe connected to, say the rear cylinder HT and the other to the front cylinder HT?
Wasted spark ignition.jpg
Wasted spark ignition.jpg (7.95 KiB) Viewed 876 times
Is that simply testing the continuity of the Secondary coil? Do you have the probes connected the same way to test the resistance (7000 ohms)?

Oil Pressure light - Normally ON if there is no oil pressure. Oil pressure opens the switch contacts and breaks the circuit to the light, turning it off. My problem is that there is no power to the switch. But that circuit is not related to the coil, so I'll leave it for Ron. (Later Ron)

When I tried for a spark, I had the rear plug resting on the cylinder head and the front plug fitted. Both plugs were cleaned and gapped before going in.

What is annoying is that a new coil from the USA is about $US100, but shipping to Australia would not be much less. It appears that all the NOS in Australia was sold off back to the UA and Europe a couple of years ago.

Thanks again for your help.
Excalibur
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Re: Ignition coil testing

#8

Post by Excalibur »

old man emu wrote: Mon Apr 10, 2023 5:14 am
Do I see one probe connected to, say the rear cylinder HT and the other to the front cylinder HT? Yes
Is that simply testing the continuity of the Secondary coil? Yes, plus leads and plug caps simultaneouslyDo you have the probes connected the same way to test the resistance (7000 ohms)?If you want the most accurate coil ohms figure go directly in to HT outputs. Look for about 16k - 20k ohms. I don't get too hung up on exact values here. The last couple failed coils I seen happened to be open circuit on the secondary

Oil Pressure light - Normally ON if there is no oil pressure. Oil pressure opens the switch contacts and breaks the circuit to the light, turning it off. My problem is that there is no power to the switch. But that circuit is not related to the coil, so I'll leave it for Ron. (Later Ron)Could be simply a blown bulb filament or poor bulb holder connections. Power has to come through bulb before it gets to OP switch.

When I tried for a spark, I had the rear plug resting on the cylinder head and the front plug fitted. Both plugs were cleaned and gapped before going in.
Sounds ok
Prudently you should also charge battery, rest it overnight then test it under load. Sometimes a dying battery can test ok for voltage but pull a load on it and volts fall off a cliff.

Good luck, hope this helps and gets a man back on the road.
old man emu
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Re: Ignition coil testing

#9

Post by old man emu »

I'll check the oil light socket. It might be dirty.

When I get a free moment I'll do the HT resistance test. If it fails, I won't be a happy little Vegemite because the coil is newish. Been on the bike a year or two, but the bike hasn't been used.
old man emu
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Re: Ignition coil testing

#10

Post by old man emu »

I had the resistance of the Secondary coil tested today. The result was in the Mega Ohms range. Also the test for continuity of the HT leads shows that they there is none.

Last year we had incredibly large amounts of rain and the bike wasn't kept too dry. The auto-electrician who tested the coil suggested that water has gotten into it and has destroyed it.

I have the old coil, but can't remember why I changed it. I'll get it tested later this week. If it fails, then I'll have to import one from the USA. I can accept the fact that one Australian dollar equals sixty-nine US cents, so a $US93 coil costs me $140 of my dollars. ON top of that, the cost of postage will be paid in US dollars.

Oh the joys of preserving history!

It looks like the oil pressure light is suffering from rust in the socket, so I'll pull the plate off and refurbish it.
old man emu
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Re: Ignition coil testing

#11

Post by old man emu »

On the 25th May I had the old coil tested. The secondary coil had a resistance of 17,000 Ohms, which I thought a bit low, but worth giving it a chance. The HT leads were not showing continuity. I managed to source a length of 7-strand copper HT lead and set about pulling the old leads apart to use the terminals and boots.

I found that the bad leads from the newest coil were relying simply on a spike through the cover to the wire inside. There was no better connection with the terminal. Little wonder that there was no continuity. So with the leads I made, I exposed the end of the copper lead and soldered it to terminal. Those leads have a resistance of 2 Ohms. Close enough for Government work.

I can only hope that the coil has the oomph to create a hot enough spark to start the burn.

it was too cold for an Aussie to go to the workshop today. It only got to 13 C (55F) and was cloudy with 18 mph winds. Hopefully the wind will drop tomorrow and the clouds go away so I can work outside.
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Re: Ignition coil testing

#12

Post by panhead_kicker »

Weather and corrosion at wire connections can be devilish. I've been working on a Mule 4 wheeler that's lived a hard life in the outdoors. Got it running then parked it last week for a couple days. Next day turned the key to move the vehicle, and it just cranked... fuel but no fire at either plug. Bad electronics? Bad Coils?
I started pulling all the main wire harness connectors apart and re-seating them with a little squirt of wd40. Exercised the last connector, and the old Mule fired right up. Just a couple nights of dew was enough to finish fouling up a connection; corrosion that was years in the making I suppose. Looking at the wiring schematic and starting with the ignition wire connectors first, would have been most logical :roll:
old man emu
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Re: Ignition coil testing

#13

Post by old man emu »

I had a bit of luck and found a new 12V coil in my Sydney. my State's Capitol. The price was only ten Aussie dollars more than the advertised price from the USA, but the shipping was only twenty Aussie dollars as opposed to around one thirty dollars from the USA, and that was shipping it by the slowest means.

I will give the old coil a chance to see if it works, and then I'll throw it in the Spares box as a backup.
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Re: Ignition coil testing

#14

Post by harleych »

the new coil should be for points ignition. replace one double spark coil possible to use two single spark coils. low voltage circuit easy to check by bulb with wires. for cleaning points dads used small coin. a large resistance may be due to the disconnection of the winding and wire inside the coil body. you can try to push the wire into the coil
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