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Ignition problems

Tiny
RABBITT2003
  • MEMBER
  • 1975 JEEP CJ5
  • 4.2L
  • 100,000 MILES
I'm a mechanic that has experience in jeeps, but unfortunately not much with the Presto-light system. Replaced everything. It's in time, has spark, plenty of fuel, even ran a compression test on all cylinders, but for some reason no fire. The costumer loves this Jeep and has completely restored it to show room quality and would like to keep it stock. Was trying to push an hei, but he wants to try all options to keep it stock. Any ideas?
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Friday, November 15th, 2019 AT 8:02 AM

16 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good afternoon,

Use a test light on the negative side of the coil while cranking.

See if it flashes while cranking.

If it does, you need a coil. If it does not then it could be a module or wiring.

Did you check the cap and rotor as well?

Roy
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Friday, November 15th, 2019 AT 12:48 PM
Tiny
RABBITT2003
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  • 10 POSTS
It came in the shop when the owner took the coil out and replaced it with a MSD Blaster coil. Unfortunately in this process he removed the restrictor. This in turn shorted out the control model and fried the holo. It now has new distributor, coil, resistor, and control model. Getting spark from the coil as expected. So the control model and holo is working like it should, but no spark at the plugs replaced the cap and rotor and still have this problem. I checked the measurements of the rotor and cap and they are right. Have had 3 other guys walk over a couple of idea's. Aka test voltage, checking the control model, making sure the holo is telling the model to fire. All check out. Still no spark to the plugs. It would be a simple fix seeing as I tracked it down to anything past the cap and rotor, but everything has been replaced. Incredibly stumped.
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Friday, November 15th, 2019 AT 1:51 PM
Tiny
RABBITT2003
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  • 10 POSTS
Also no tachometer, so that's a non issue.
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Friday, November 15th, 2019 AT 1:52 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Okay, if you have spark out of the coil, then it leaves only the wires, especially the coil wire, ignition wires to the plugs, cracked cap or the rotor may not be turning.

Roy
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Friday, November 15th, 2019 AT 2:00 PM
Tiny
RABBITT2003
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  • 10 POSTS
Rotor is turning. Checked the timing while doing the compression to every cylinder making sure it was in time with every one I was checking. That was after setting the timing at 3 degrees past top dead. Even replaced the new cap and rotor that came with the new distributor. Also put 2 sets of wires.
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Friday, November 15th, 2019 AT 2:28 PM
Tiny
ASEMASTER6371
  • MECHANIC
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If you have spark coming out of the coil then it can only be from there forward.

Did you ground the coil wire and confirm spark?

Roy
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Friday, November 15th, 2019 AT 2:31 PM
Tiny
RABBITT2003
  • MEMBER
  • 10 POSTS
Yes. Should there be a rubber gasket between the cap and distributor. There is something stupid wrong and I needed to step back and get another pro's opinion. I know jeeps in and out, but this is my first for this bad setup they had from 75 to 78. I have my own shop so ordering testing and sending back parts is not a problem. Been doing it all week with this thing. Lol
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Friday, November 15th, 2019 AT 3:02 PM
Tiny
RABBITT2003
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If this was a points system. It would be out of my life 4 days ago. Lol
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Friday, November 15th, 2019 AT 3:04 PM
Tiny
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I would try another coil.

Roy
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Friday, November 15th, 2019 AT 3:08 PM
Tiny
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LMAO, did that. Twice.
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Friday, November 15th, 2019 AT 3:17 PM
Tiny
RABBITT2003
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  • 10 POSTS
Getting a spark out of coil any mechanic would be proud of.
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Friday, November 15th, 2019 AT 3:18 PM
Tiny
ASEMASTER6371
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Okay, what about the voltage to the coil? Did you make sure you have battery voltage when it is cold and a little lower when it warms up?

Roy
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Friday, November 15th, 2019 AT 3:19 PM
Tiny
RABBITT2003
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What difference would that be if the thing never fires in the first place?
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Friday, November 15th, 2019 AT 3:38 PM
Tiny
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Low voltage equals low output. Basic electric.

Roy
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Friday, November 15th, 2019 AT 3:44 PM
Tiny
RABBITT2003
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  • 10 POSTS
Actually went over to an old head with a Clinton book and found the two missing pieces of the puzzle. A felt pad between holo wheel and shaft and a dust cover between the rotor and distributor. Both of which made the rotor to far from the cap. It's running. Thanks for the help though.
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Friday, November 15th, 2019 AT 4:18 PM
Tiny
ASEMASTER6371
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You are welcome.

Always glad to help.

Roy
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Friday, November 15th, 2019 AT 4:36 PM

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