Testing the ignition coil

[Vicm]

I have seen few posts lately with ignition trouble both here and on ccot discussion board, so I am posting this very common test for the ignition coil.
Regardless of what ignition system you have or what coil you have you can use this test .
You can use your truck battery to do this test or any good 9V batt.
Use a good spark plug and and make a good connection to the coil high tension wire, Then use a length of wire to ground the spark plug.
Make sure you take any wires connected to + and - terminals of the coil out before you do the conections for this test.
In the diagram you get the basic idea of how to make the connections. This test can save you a lot of time when you have staring problems that you think is not fuel related. If you don't get a spark from the spark plug recheck your connections and try again ,no spark , you need another coil. Remember it is a touch and go to the - coil terminal and not a solid connection that will make it fire the plug.
This post is likely useless to most of you people.

 

[IDave]

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This post is likely useless to most of you people.

 

[Pin_Head]

I tell people to do this all the time and they ignore it. Apparently it is the part about disconnecting the wires at the - side of the coil or turning the rotor until the points are open that skeers them off.

 

[Nomis]

Good info - can we add this to the Tech Links?

 

[gibgoaz8]

Sir: Might not be useful for most people, but it helped me tremendously..... Thanks. Now I know I have a good coil, I just have to figure out what else is wrong. Have a 78 fj40 with electronic ignition. Will have to start searching from the coil on down. Any suggestions on what to do next? Thanks.

 

[Vicm]

Sir: Might not be useful for most people, but it helped me tremendously..... Thanks. Now I know I have a good coil, I just have to figure out what else is wrong. Have a 78 fj40 with electronic ignition. Will have to start searching from the coil on down. Any suggestions on what to do next? Thanks.

With the electronic ignition , if your coil is good, about the only other thing you can do is swap another good igniter . That is abou the only disadvantage with this setup.
But I have traced problems sometimes to a bad connection to the igniter or bad ground to the igniter. A good test light will reveal a bad connection.
The 2 wire connector going to the igniter with the key switched to the (ON) position , both must have power to them.
And a good grounding wire from the -batt post to coil bracket screw will eeminate a bad ground.
Then the HT wire should also be checked for firing which I believe you have done already.
I have had one odd problem onetime , a loose connector in the 3 wire plug to the distributor.
Others will have better advise for you,but you should explain more about what kind of trouble you are having.
Truck doesn't start at all,
or truck runs for a while then dies.
Even better if you start a new thread and wellcome to mud.

 

[71cruiser71]

i dont wanna steal your thread or anything but this could actually help me alot. when im driving my tachometer works great untill i hit 2500. it starts jumping around. could this be a coil problem or a loose connection?

 

[bdsmitty88]

Hey. Thanks for the writeup for this test. I've been using it to troubleshoot my ignition coils, but old and new one. When you "touch and go" what is this process like and how consistent is the spark from the plug and how easy is it to get that consistent spark?

I hooked everything up to a fully charged 12V battery, disconnected from everything else in my FJ. I had everything hooked up correctly, but with both coils getting a proper spark from the plug was pretty sensitive to how I was making contact with the touch and go wires. It would only spark when I made a almost brush like connection with some forces, sweeping one wire (copper wires tightly wound into a point) perpendicular to the other and quickly dragging it across. Almost like a matchbox strike, but the two wires are perpendicular.

Both of my coils measured in spec per resistance specifications, both primary and secondary. One is an internal resistor model (3 ohms) and the other is not, with no ballast either (about .6 ohms measured for primary). I'd expect the lesser resistance one to spark stronger, and it did. They both were so very sensitive to the way I was striking the wires together. Is this normal?