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Unread 07-28-2010, 01:18 PM   #1
75CJLevi
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Ignition system problem

I have an issue with my ignition system. I recently encountered a problem where the engine runs rough and sputters and chugs on acceleration and misses at speed. While it was at the mechanics he was poking around with his voltage tester and found that when he poked the neg. terminal of the coil the engine reved up to normal speed and stopped sputtering. He had the timing light hooked up as well and the missing stopped and started running normally. As soon as he took it off it started running poorly again. I thought it might be a bad coil so I replaced it. But the new one did the same thing. I checked the ground wires and it didn't make a difference when i jumped a new ground.

Not sure what the deal is. Any help would be greatly appreciated.

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Unread 07-28-2010, 02:06 PM   #2
howelljeep
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What did the mechanic say?
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Unread 07-28-2010, 03:02 PM   #3
Shiftace
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Sounds like a resistor. I had an old Dodge that did similar things. Turned out to be a little white ceramic resistor with a coil of wire running up the middle of it mounted on the firewall.
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Unread 07-28-2010, 03:50 PM   #4
dslywalker
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If you follow the wiring print the ground wire goes back to the ignition module.Do you have one?It should be on the drivers side fender or follow the wire back to it.Possible that it could be bad
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Unread 07-28-2010, 05:20 PM   #5
Ken4444
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Try temporarily gounding the ignition module back to the battery. This diagram shows how poorly grounded the module is (to the distributor).
duraspark-wiring-1-.gif  
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Unread 07-28-2010, 05:30 PM   #6
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Here's a set of other diagrams, compliments of JeepHammer. The second diagram shows the suggested ground.

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Unread 07-28-2010, 10:38 PM   #7
75CJLevi
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He is just as stumped. He said that he has never seen any thing like this before.
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Unread 07-28-2010, 10:49 PM   #8
75CJLevi
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Thanks all!

I will take the diagrams to the mechanic and have him add the ground. I don't have a resistor on the jeep so it sounds like I need one.
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Unread 07-29-2010, 11:12 AM   #9
JeepHammer
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So when he probes the 'Green' coil terminal, the miss stops?

Sounds like you might need to work on that connection.
Terminals at the coil are a large problem, there isn't a summer goes by without me replacing two or three coil connectors because they give up.
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Unread 07-29-2010, 03:35 PM   #10
Ken4444
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Quote:
Originally Posted by JeepHammer View Post
So when he probes the 'Green' coil terminal, the miss stops?

Sounds like you might need to work on that connection.
Terminals at the coil are a large problem, there isn't a summer goes by without me replacing two or three coil connectors because they give up.
I even had a problem with a new connector; one of the metal terminals had popped out of place and I didn't realize it. The Jeep wouldn't start. I went back to check the basics and happened to notice that the new coil connector had crapped out. It was esay enough to fix, but unfortunate that a new part malfuncitoned.
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Unread 07-29-2010, 04:28 PM   #11
82JeepCJ7
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If you Jeep does not have the ballast resistor, Chances are it never had one. My 82 never had one in the system.
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Unread 07-30-2010, 09:56 AM   #12
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Quote:
Originally Posted by 82JeepCJ7 View Post
If you Jeep does not have the ballast resistor, Chances are it never had one. My 82 never had one in the system.
'78 to '86 Jeeps (and beyond) all had ignition resistors.
It is a wire in the harness instead of the ceramic block most people are used to seeing.
Motorcraft/Duraspark won't live without one.
1.35 Ohms or better...
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Unread 11-21-2013, 11:10 AM   #13
semihemi
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I wish someone had answer for this problem, I have the exact same symtom, 1975 cj w/258 prestolite ignition, I have replaced gas fouled plugs, replaced carb power valve, coil, module twice, distributor, added 3 grounds, isolated the alternator (rfi). When I touch test light to the neg side of coil the engine will start and run normal, without the test light it is hard starting, rough running, irratic ignition. All of the coils I have seen listed are USE WITH EXTERNAL RESISTOR. This ignition doesn't seem to use an external ballast or resistor wire, so im still at a loss as to why it shouldn't have a internal resistor coil installed.
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Unread 11-21-2013, 11:17 AM   #14
CSP
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The fix for '75-77 ignitions is just to get rid of the Prestolite ignition system and swap in something else, whether it's the '78 and up system, an HEI distributor, or an aftermarket system.

A good chunk of what was said in this thread (including the wiring diagrams above) isn't applicable to the '75-77 ignition system, which is just garbage.
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Unread 11-21-2013, 02:35 PM   #15
shadoow
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Quote:
Originally Posted by 75CJLevi View Post
While it was at the mechanics he was poking around with his voltage tester and found that when he poked the neg. terminal of the coil the engine reved up to normal speed and stopped sputtering. He had the timing light hooked up as well and the missing stopped and started running normally. As soon as he took it off it started running poorly again.
can the mechanic repeat his 'success' ?? then use that to narrow down what causes the improvement
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