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Old 11-18-2012, 11:47 PM   #1
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'98 Jeep not firing, not starting with A/C on.

My dads '98 2.5 Jeep wrangler starting cutting out about a month ago. It would cut out and die every time you hit a small bump. Checked all connections, couldn't find anything, no codes. Finally it got where it wouldn't start at all without starting fluid, after which it would rev and drive fine until you hit a bump. So we starting replacing things that probably needed replacing anyway at 110'000miles. Plugs, plug wires, air filter, eta. Nothing helped. So we replace the fuel pump (the in-tank filter was clogged pretty bad) that helped some, it would now crank fine without any starter fluid, but it would still cut out.

A friend suggested it could be a clogged cat. Didn't think that would cause this problem, but it needed the old leaky exhaust replaced anyway. The cat DID seem to be clogged pretty bad with the water test. We ran a straight pipe back to a autozone muffler.

Low and behold, that fixed the issue. The jeep had more power then it ever did, could hit any bump and not miss a beat. But at that same time another weird problem developed..... it would only start with the A/C or heat OFF, with ether of them on, it would just turn over and not start. Once started, could turn A/C on no problem.

Everything has been fine up until today when it just died in the middle of the road and would not start back up no matter what. After a tow home, we went over every thing we can think off. It's not getting spark (won't start with starting fluid). We went over every single connector, none are loose. Trowing a code for 02 sensor bank 1 sensor 2, which is after the cat (or lack of one) and shouldn't make a difference.

My dad is about to buy a new coil, but I'm not so sure its the issue. Something with it not starting with the A/C on sounds really weird to me...

Could anyone point me in the right direction?

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Old 11-19-2012, 12:25 PM   #2
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Old 11-19-2012, 12:27 PM   #3
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not sure on the jeep, but some vehicles don't like to run with the cat removed
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Old 11-19-2012, 06:07 PM   #4
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Got an update.

Bought a new crank sensor. The old one had the insulation melted off the wires from the hot exhaust. No-go.. Tested the voltage from the wires going TO the coil. With the key "on" it had 0.095v for a couple seconds before going to 0. With it starting it went back to 0.080v. Thats less the 1 volt! What could be causing the coil not to get enough juice?
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Old 11-19-2012, 06:15 PM   #5
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One side of the coil will have +12v on it whenever the key is on, the other side has a pulsating ground connection on it provided by the engine computer which is what causes the coil to generate its spark voltage.

It's almost sounding like the +12v side of the coil has a bad connection to its +12v source which is a "dumb" source of +12v. The negative side of the coil that gets grounded to generate the spark voltage is connected to the computer and is thus the "smart" side. I'm guessing the problem is on the dumb side of the coil.
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Old 11-20-2012, 12:05 AM   #6
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One side of the coil will have +12v on it whenever the key is on, the other side has a pulsating ground connection on it provided by the engine computer which is what causes the coil to generate its spark voltage.

It's almost sounding like the +12v side of the coil has a bad connection to its +12v source which is a "dumb" source of +12v. The negative side of the coil that gets grounded to generate the spark voltage is connected to the computer and is thus the "smart" side. I'm guessing the problem is on the dumb side of the coil.
So, if I'm reading this right, if I were to put another 12v source to the coil (I'm thinking simply run a wire from the + battery terminal) while leaving the negative side connected, it should start? Obviously this would just be to test it, I would still have to find the bad connection for a permanent fix.
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Old 11-20-2012, 12:21 AM   #7
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That'd be a good way to test it for sure.
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Old 11-22-2012, 11:50 AM   #8
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Update

Tried the test, still reading the same ~.080v. I then tested the negative side of the coil wire (one lead on the wire, one on the + battery terminal), and find the + side is getting the correct 12v all the time, but its the negative side that is weak.

I'm going to get a manual and see if I can trace where the negative wire goes to find the short/problem. Fun fun.....not, lol.
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Old 11-22-2012, 03:14 PM   #9
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Your measurements confused me. What is showing .08v, the negative side of the coil? Why do you feel that is unusual? The negative side of the coil goes directly to the PCM's ignition coil driver circuit which provides a pulsating ground connection to the coil which is how it generates its spark. The ignition coil gets its +12v from the ASD relay circuit.
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Old 11-22-2012, 07:07 PM   #10
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Your measurements confused me. What is showing .08v, the negative side of the coil? Why do you feel that is unusual? The negative side of the coil goes directly to the PCM's ignition coil driver circuit which provides a pulsating ground connection to the coil which is how it generates its spark. The ignition coil gets its +12v from the ASD relay circuit.
If the positive side is getting a full 12v, (tested by the leads on the positive side of the coil connector and a chassis ground) then when I put one lead on the pos side, and one on the neg side of the coil, when starting the engine, shouldn't I get a full 12v everytime a signal is sent to the pulsating ground and not less then 1v which wouldn't fire the coil at all?
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Old 11-24-2012, 07:52 PM   #11
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Ok, it's official, I'm in over my head...

Went to the yard and grabbed a ECU. No difference. I keep checking with a test light and all of a sudden, BOTH the positive and ground (orange and green wires) on the coil connector start lighting up when one lead is on the wire, and the other on the positive battery terminal. Don't know if it means anything, but when the far right ECU connector (the one closes to the drivers side) is unplugged, the green wire (which I've been presuming is ground) will stop lighting the test light, but the orange wire will light it (12v) ALL the time, key on or not, ECU plugged in or not.

I'm at a total loss. My dad is only experienced with older carb and point settups. Even had a mechanic friend take a look at it and even he is just as confused as I.

My dad is actually considering selling the Jeep, which would suck as I have many fond memories in it. I would be extremely grateful to whomever can help.
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Old 11-25-2012, 04:41 PM   #12
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Old 11-26-2012, 06:37 PM   #13
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More update.

From what I read, orange wire is positive, green wire is pulsing ground. With old ECU, orange wire gets 12v all the time (one lead on the wire, and one one the pos. battery terminal). The green wire only test .080v when starting, and DOES pulse. New ECU, orange wire acts the same, but the green wire is acting JUST like the orange one, with a consistent 12v (again, testing one lead on the wire, and one one the pos. battery terminal). There is a difference between the ECU's, but nether is working. Could it be BOTH are bad? Seems very unlikely...
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Old 12-05-2012, 01:16 AM   #14
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I just noticed that I'm testing on the wrong batt. terminal (the pos + side), but I tried testing on the neg = side with no reading from ether wire. I ONLY get a reading with the pos + battery terminal. It's acting like the positive coil wire, is grounded. Where as the pulsating ground, ether isn't pulsating with one ECU, and doesn't show much of a reading at all with the other ECU.

My last resort before giving up is to pull the dash and look for a short or something (remember, it wasn't starting when the AC or heater was "on"...).

Thanks for the help.

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