Bad ECU? How do I check my brainbox... - Page 2 -
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post #16 of 19 Old 02-16-2020, 09:42 AM Thread Starter
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1993 YJ Wrangler 
Join Date: Jan 2015
Posts: 46
Yea, it's not starting. It's not getting spark. I checked the fuel pressure, it's good. I pulled a spark plug wire and checked for spark. I got none. The coil and plugs are new, maybe 2 k miles on them however I can check them with a meter. I checked the crank position sensor and it's good I can take a meter to the coil and check it out too but I'm like 80% sure it's not a mechanical problem. I was going to pull all the plugs today and check them. I did get a pretty loud backfire right before she died, so I hope everything in the engine is okay, although it was running while it was still in gear and sounded okay. I would like to check for compression if it's not super hard but it sounds good when i crank it. The computer I have can only check codes I cant read anything else with it sadly. I know she's not getting spark, the crank position sensor is good, shes getting fuel, and she cranks.

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post #17 of 19 Old 02-16-2020, 12:25 PM
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1987 YJ Wrangler 
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You did have the end of the plug grounded when you checked for spark? Right? I am not trying to insult your intelligence but have to question how you do things based on this thread and your posts. If you missed the most simple of things it could sidetrack your diagnostics and results. Your lack of common terminology or acronym knowledge leads me to believe that your knowledge is basic but yet some of your other observations are very encouraging that we can all get on the same page and accurately diagnose this problem.

Remove the coil to distributor high tension wire at the distributor end and check to see of spark will jump at least a half inch if not close to an inch to ground. You can do this with a spark tester or by installing a Philips screwdriver in the end of the coil wire and holding the metal blade of the screwdriver at least a half an inch from a good ground surface. It will probably require a buddy. A reasonably insulated handle on the screwdriver is wanted and it will probably require a helper.

If have spark going to the distributor cap but not coming out to the plugs it can indicate a few things. Water or moisture in the cap, a burned through rotor, a distributor that does not turn, a carbon tracked cap, a messed up rotor to cap connection, lots of bad plug wires or at least where you tested. etc.

If you do not have spark from the coil wire you need to test for power to the coil, and a pulsing negative coil circuit, preferably with a test light.

It really does not sound like your ECM is dead in the way that most of them go dead. You were able to do things and bring your cluster back to life after the fact. You were able to flash codes after the fact. The ECM does control ignition timing and pulses in this engine but other basics have to be covered. The majority of ECM failures that are documented involve no Check engine light and not fuel pressure... They come down to bad capacitors and the ECM never really powers up for a few minutes.

I believe that we have crossed that basic bridge.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #18 of 19 Old 02-17-2020, 10:43 AM
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1989 YJ Wrangler 
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The random “backfire” you mentioned

Classic symptom of water in the fuel....if it’s been sittin a while with ethanol-laced fuel that is a possibility

But follow the other steps first. No spark won’t fire either water OR fuel.

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #19 of 19 Old 02-22-2020, 02:37 PM Thread Starter
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Originally Posted by Boojo35 View Post
I believe that we have crossed that basic bridge.
Hey thanks for the advice. I am a novice when it comes to the mechanical world It looks like it might be the coil. I tested it like you said and watched some videos on how to test it. Judging from the videos mine isn't pulsing. Test light on the grounded(-) side of the coil should pulse and the (+) side should be steady. Mine was steady on both sides. which I think means it's bad because it requires the difference to generate the spark, like the spark is created when the test light goes dark? if the light never goes dark the spark is trapped?

I made videos...

Here is checking for spark from the coil. I unplugged the distributor cap from the ignition coil and wedged a screwdriver in the plug and checked it against a good ground, i tested the ground before hand, and it's my main ground, and I cleaned it.

Here is me using the test light on the positive and negative sides of the coil plugged in with little copper back probes i made from wire. Ignore the annoying of my voice...

The random “backfire” you mentioned

Classic symptom of water in the fuel....if it’s been sittin a while with ethanol-laced fuel that is a possibility
I'm not too worried about that. She had been sitting for a month, but I ran through a tank, and I've got a water separator on my fuel filter. She sat over the summer but had an empty tank and I ran through 4 tanks before I had issue. If it was water how do I fix that?

Thanks for all the advice though!

I checked under the distributor cap and the rotor looks clean and I didn't see anything that jumped out at me there. I can take pictures and post them as well.
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ecm , ecu , starting issue

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