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Discussion Starter · #1 ·
Ok......I have a 1989 4.2 YJ (Carb)5 spd...Durning the past week or so i have been having a hell of a time trying to pin point a problem with the ignition. I have resently replaced Cap , rotor, plugs, battery, and ECU and also had the ignition module tested( and it tested good). The YJ turns over but dosnt Fire, It has Spark to the plugs,and Fuel. After i installed the ECU i was planning on test all the wires for a spark. And thats when i stumbled apon this !!! I had my ignition key turned to the "ON" postition( not the crank)and a spark plug grounded to the frame to test i attached my negative post on the battery and noticed a spark coming from the plug....Keep in mind that the jeep's ignition has not been cranked i disconnected the neg cable ...and the sparked stopped......touched it again...and low and behold there's the arc again.......I guess this is the reason why my YJ hasnt been able to start.........Please I need some advice. What would cause this????????....thanks for taking the time to help.


13,432 Posts
You may want to re-test. What you describe can happen if the correct set of circumstances are in occurance:

1. Ignition set to RUN mode and the engines stopped at about 8* BTDC of the compression stroke for the plug that's pulled.
2. Rotor pointing to the distributor tower of the pulled spark plug.
3. Hall effect swtich in position to pulse the ignition module.

So now you remove the ground wire for the battery and replace it.

4. The ignition circuit doesn't care where the battery cable is.
5. The ignition circuit sees that power has been restored. Just like turning the igniton to RUN mode.
6. The hall effect switch recieves power and sends a pulse...
7. To the ignition module which sets up a pulse to...
8. Pulse the coil. The coil being pulsed reaches saturation....
9. And builds up voltage which is sent via the distributor cap to the...
10. Spark plug and the voltage jumps the gap...
11. And you have ONE spark occur.

Now, move the engine by hand 10 degrees or 20 degrees CW and repeat the procedure...

You should not see the spark occur. If you do... somethings really messed up, likely in the distributor.


Discussion Starter · #5 ·
Got the module tested at Napa, and for good measure i also took it to AutoZone it tested good at both locations


Discussion Starter · #6 ·
Thanks Leve......The spark did not accur angain.......Damn and i thought i was on to ssomething....Guess iam just reaching for [censored]. This damn Jeep got me sracthing my heaD.

Discussion Starter · #7 ·
You can't test a module!
Module usally come and go, work and don't work.
Most of the time when they fail is do to HEAT. They can't test it for that.
I had a bad module in my 87. Every other day it would stall and when I started it (if it started) it would back fire. I had my Module test at AutoZone, they said it was good. My old man and his friends said "you can't test a module"
So I bought a new one for 60 dollars (because there wasn't anything left that could be wrong) and it fixed it. For 4 years so far!

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