Ignition Coil Testing | GTAMotorcycle.com

Ignition Coil Testing

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Gkaros

Member
Hi all, new to the site. Have an old but nice Ascot VT500FT Vtwin.
Will not start.
Ive suspected the coil for a while because it use to play the classic I wont restart if I'm hot game.
Ive removed the coils to bench test them.

2 questions..
1. The 2 outputs on each coil. Do they fire at the same time? Does that mean that there are 2 spark plugs in each cylinder?
2. Testing the resistance in the coil. Im getting about 3 ohms on the primary sides of each. A bit more than the 2.2 ish that the manual states.
For the secondary side: Manual states to test across the 2 secondary boots but everything I read states that the secondary test involves testing across 1 of the primaries and the secondary... Which is correct? Is there a physical connection between the primary and the secondary?

Hope my question makes sense.
thanks alot.
 
1. How do you not know how many spark plugs are in each cylinder? I would start with new plugs as a first cheap step because obviously they haven't been out in a while
2. Did you zero out the meter before measuring the resistance (or do mental math if your meter doesn't do relative)? 0.8 ohms could easily be lead resistance in the multimeter.
What reading does it tell you is correct for the second measurement? Between primary and secondary should have lots of resistance, my guess is the secondaries are in parallel and resistance between them should be lowish.

You have two coils with two outputs each? Is that stock or has someone put 2 half working coils in? The wiring diagram seems to show a single coil. I found a VT500C with dual coils, but they are single output coils and that bike uses points not a CDI.
 
Hey GreyGhost. Thanks for the reply.
This is the VT500FT which is a twin 500cc. Yes the stock coils are 2 outputs per.
Let me clarify re the 2sparks per cylinder. Yes I do know that each cylinder has 2 plugs. My question was more on whether they were both used in the compression chamber to fire cylinder. I ask because they are on opposite sides of each cylinder head and not beside each other. Is one on the exhaust side?

I retried the resistance on the primaries. I'm getting 2.6 and 2.7 Manual states 2.0

I'm still confused about secondary testing. Watching stuff on youtube and everyone says that if there is no continuity between one primary lead and the secondary, then the coil is shot. My manual states to test only between the 2 outputs on the secondary and not between prim and sec.
a bit confused...

thanks
 
Gkaros said:
Hey GreyGhost. Thanks for the reply.
This is the VT500FT which is a twin 500cc. Yes the stock coils are 2 outputs per.
Let me clarify re the 2sparks per cylinder. Yes I do know that each cylinder has 2 plugs. My question was more on whether they were both used in the compression chamber to fire cylinder. I ask because they are on opposite sides of each cylinder head and not beside each other. Is one on the exhaust side?
Click to expand...

When they are using two plugs/cylinder they normally spread them out so there are two flame fronts. This speeds up the burn. Other than the ridiculous aftermarket flame throwers, I haven't seen any vehicle put plugs outside of the combustion chamber.

What do you get when you touch the leads of your multimeter together (just the leads, no coil in the circuit)?
 
There are ignition coil testers you can buy that go inline between the wires and caps that allow you to see the spark 'jump'.
Its a primitive way to find out if the coils fail under hot conditions.
One more thing to check - spark plug cap resistance. You can only check that cold, but if its not within spec its best to replace.
 
bitzz said:
1) If the coil has two secondary outputs, two plug wires, it is a wasted spark ignition and it does fire both at the same time.
2) Here is everything you need about ignition coils http://beru.federalmogul.com/sites/default/files/ti_07_ignition_coils_gb_2013_lowres.pdf

An ignition coil is a very simple device that seldom goes wrong.
Click to expand...

coils do fail, and the older it is the more likely it will fail.
a coil on its way out will often work cold - and not when it is hot - re: hot no start.
not saying it is or isn't a bad coil, just food for thought. I usually try replacing coils with a known good one and see if there is a difference. check that the coils are well grounded w/o corrosion to frame - bike wrecker nearby? coil is probably ~ $20 each
 
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Had a coil fail before too. Swapped with a working one and that worked. Might have also been the 2 cables coming out of it, but since it would have been major surgery to replace the cables and not knowing if it was that or the coil itself not worth bothering, a replacement worked.
 
Ascot500.com has a lot of useful information, including issues with coils and coil testing. Some owners have had coils test within spec and have issues starting when hot.
You can find info on coil testing and solutions for your specific model.


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Keep in mind buying from a wrecker and the coil could test OK when cold...and still fail when restarting the bike when hot.
If battery, wiring and electrical components all test okay, I would bite the bullet and buy new coils. One less thing to worry about.


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Bought my first bike in '85 - a '84 VT500 Ascot. Dual plugs - fires both at once.

Never had a coil problem. I wouldn't think the primary and secondary are connected...

One time I did have an issue where it would bog-down when trying to accelerate. Took the Cycle World West tech a while to fix it but turned out to be a bad ground - under the seat, there's a ground wire that screws into the frame - he clipped and replace the lug. Not sure where this ground goes, but if to the ignition circuit then perhaps may be related.

I think that the VT500 Ascot and VT500 Shadow coils are the same - as the Shadows were more numerous, might be easier to search for that.
 
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+1 something to keep in mind with any older bike is all the grounds. A rainy no ride sunday is great for going over them. but it can get worse as copper wire will rot. looking closely, if the wire is starting to turn green it needs to be replaced. In some cases it is easier to run a new auxiliary ground.
 
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I just re-read your first post and am wondering is the bike not starting from cold now? If that's the case make sure to begin troubleshooting with a fully-charged battery in good condition.
Before removing the coils to bench test, did you try removing one spark plug at a time and testing for spark (ie reconnect the wire to the plug and lay the plug against the cylinder head metal to metal...then try starting the bike and look for a strong spark from the plug)? Let us know as we don't want to assume you have done all of the necessary tests.


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My former VT500. OP - is your bike red? We should chat. chance that it's my old one, LoL.

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Hey. I put the coils back on and I do get a spark in all 4 plugs so coils are at least "working"
I did notice a shock though as I held one of the wires, so I'm wondering if there is a leak in the plug wire itself. Ive had that happen in my car once.
Maybe time for new wires..?
G
 
Hey guys. One more question. Whats the purpose of the resistor and the spring in the spark plug boot. It appears the only 1 of the 2 plugs in each cylinder have the resistor setup and the other doesnt...??? Do I need to maintain this if I get new plugs?
thanks
 
That's good you made progress. New wires might be in order. You'll get a much healthier spark too.

Next you need to check if fuel is flowing into the carbs. Unscrew the float bowl drain screw on each carb one at a time and see if fuel flows out. If you have fuel coming from both carbs, then tighten it all up. Now again with the fuel tap on, do you get any fuel leaking out of the carb overflow tubes? If so, you may have a stuck float.
Also, check the airbox and air filter for blockage.
If that all checks out, pull a spark plug from each cylinder and see if you smell gas down in the cylinder. If not, pull the carbs. You may need to disassemble and clean them.


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