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Unread 09-17-2012, 06:37 AM   #1
levergun
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1979 CJ5 
 
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88 YJ with 93 zj 4.0 blows coils

I have a 88 YJ I swapped a 93 ZJ 4.0 in over 120k ago. In the last few months it started having an issue with blowing coils. I vaguely remember it doing it right after I swapped it in. It currently blows one every month. I am at my witts end on this problem and have exhausted every thing I could think of and am looking for new ideas.
What I've done-
New cs
New cap, rotor, wires, plugs
Replaced capacitors in PCM, part of a different problem

I also read the tsb on it that mentions the resistor wire to keep it from happening? The part is no longer available so I wired in series to the + side of the coil a .8 ohm ballast resistor. I'm thinking maybe I just need to go up more? I'm also running the spade type coil.

My battery is older but I don't think it makes a difference.

The coil opens when it's hot and if you let it cool down for a while it will crank back up and start. It also occasionally explodes them. Also sometimes the coil is so hot it will instantly burn you. As in way hotter than the engine. Any ideas would be greatly appreciated.

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Unread 09-17-2012, 07:00 AM   #2
Jeepsr4me
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Check the voltage going to the coil with the engine running.
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Unread 09-17-2012, 07:27 AM   #3
Old4X
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You can upgrade to a TFI coil. They run at full system voltage.
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Unread 09-17-2012, 07:28 AM   #4
1991welder
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Have you checked to see if its making good ground it could be shorting its self out or theres always the more logical explanation the 4.0 just has to much horsepower for the coil lol jk
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Unread 09-17-2012, 07:40 AM   #5
Old4X
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Engine should have a solid ground to frame and battery.

Check the resistance on all your plug wires and coil to dizzy wire. High resistance can burn up coil.

Check that cap and rotor aren't damaged or worn.

Check wiring between coil and ICM, should show no resistance on ground wire (tach connector). Add a dedicated ground to ICM.
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Unread 09-17-2012, 11:40 AM   #6
Que89YJ
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The resistor should work in bringing down the voltage and effectively lowering the current. The ecu is grounding the coil. Check the ecu grounds to make sure they are all good. No ICM on yours. I am wondering if you are getting the wrong coils. The ealier Jeeps did have a resistance wire but the later models were designed to run on 12 vdc. I would x2 Old4X original post of looking at a different coil then the one you are using.
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Unread 09-17-2012, 01:59 PM   #7
Old4X
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88..93..4.0, What exactly is the setup here? The engine can have 4 or 5 different ignition setups depending on the install and components chosen.
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Unread 09-17-2012, 02:44 PM   #8
levergun
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Koeo. 7.2dcv
Koer. 12.3dcv

I have the tach wired to the - side of the coil and I'm wondering of that might be a issue as when I disconnect the readings change. Also iirc I vaguely remember there being a tach out on the PCM. It's been way to long since I wired though so I've forgotten a lot.
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Unread 09-17-2012, 02:51 PM   #9
Old4X
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Engine running (Koer ?) should be 7 to 8 V. Key in start position should be 12+ V, and should drop to 7+ V in run.

But, this is for the Duraspark ignition system and coil.

Tach is wired to neg side of coil.
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Unread 09-17-2012, 02:54 PM   #10
levergun
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Jeep is a 88. All engine wiring from the bulkhead fwd is 93 4.0 with wires spliced at the bulkhead. I originally got it with the spade type and have been using those. I've heard the round type have a different resistance value in them. Thinking maybe I should try the round type. All ECM ground test good. Less than .2 dcv drop from ECM to batt.
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Unread 09-17-2012, 08:22 PM   #11
Que89YJ
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Different coil for sure.
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Unread 09-18-2012, 11:50 AM   #12
levergun
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So, I have both kinds of coils in front of me. The wierd part is both the round and the flat terminal ohm out to 862 +- 5 ohms. I thought thy had different resistance values?

Also after looking at a PCM connector schematic it's saying pin 43 (gy/bl) is the tach out. Could having my tach on the coil vs the PCM be causing this?

Cap and rotor, plugs and wires are brand new. CS is 2 months old.
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Unread 09-19-2012, 08:44 AM   #13
Que89YJ
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The tack should just be a minimal draw. You can try disconnecting it and see if it has an impact. You can see why there is confusion on the coils. They do appear the same but they are definitely different. I would recommend getting a master blaster coil.
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