Need help understanding 4.2 ignition system - JeepForum.com
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post #1 of 17 Old 02-10-2015, 09:27 AM Thread Starter
1Coyote
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Need help understanding 4.2 ignition system

88 yj 4.2. I seem to be electrically challenged about the ignition system. I am only getting 1.5 volts to the coil in key on position or when cranking. I know it needs 6-7v and 12v cranking but don't understand where that voltage is actually coming from or why it's low.

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post #2 of 17 Old 02-10-2015, 09:31 AM
mike134
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Just to be sure, you have above 12.4 volts at the battery right?
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post #3 of 17 Old 02-10-2015, 09:42 AM
Mike Romain
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There is a crimp connection common to the two circuits about a foot away from the coil in the loom that can fail from vibration. I think I would look there first. It will have factory high heat duct tape on it.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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post #4 of 17 Old 02-10-2015, 05:09 PM Thread Starter
1Coyote
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Battery is up, cranks good but not enough juice to coil. Will check the crimp connection. I just cannot understand what provides the voltage to the coil. Does it come from the ballast, starter relay, ECU?? I know the coil, pick up coil and ICM are good, all new..
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post #5 of 17 Old 02-10-2015, 06:32 PM
dinger240
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http://www.jeepforum.com/forum/f12/c...iring-1365305/ Look at post 7 Looks like yellow wire goes to starter relay.You should have 12 v there
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post #6 of 17 Old 02-10-2015, 08:17 PM
Mike Romain
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Quote:
Originally Posted by 1Coyote View Post
Battery is up, cranks good but not enough juice to coil. Will check the crimp connection. I just cannot understand what provides the voltage to the coil. Does it come from the ballast, starter relay, ECU?? I know the coil, pick up coil and ICM are good, all new..
The battery supplies the voltage to the coil. In run, the voltage goes through a ballast resistor wire to drop it a few volts so the ignition module doesn't overheat. In start, the coil gets it's power direct from the starter relay.
These two power circuits meet at that crimp connector before they go to the coil.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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post #7 of 17 Old 02-10-2015, 11:02 PM Thread Starter
1Coyote
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Thanks, found the crimp connector, it is good. However, when I turned the ignition on, to take a voltage reading (1.7 volts), the crimp and wire above it were very hot. Seems like I have a short. As you stated, the crimp has three wires, the yellow from the coil, a black going down to a small loom and a large redish orange going into the main loom. I am guessing the black goes to the ballast.
Getting tired so I'll put this on hold until morning.
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post #8 of 17 Old 02-11-2015, 06:11 AM
Mike Romain
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Quote:
Originally Posted by 1Coyote View Post
Thanks, found the crimp connector, it is good. However, when I turned the ignition on, to take a voltage reading (1.7 volts), the crimp and wire above it were very hot. Seems like I have a short. As you stated, the crimp has three wires, the yellow from the coil, a black going down to a small loom and a large redish orange going into the main loom. I am guessing the black goes to the ballast.
Getting tired so I'll put this on hold until morning.
The fat wire is the ballast wire and it gets hot which is why they used duct tape, black tape glue will melt away. The other wire goes to the starter relay to provide power when the key is in start.
Where are you taking your voltage readings from? That crimp connector should show 9 volts with the meter on it and on the battery negative with key in run. If not, try unplugging the coil and see if the volts come up. Same deal testing at the coil, meter on positive side and on battery negative. Then unplug it and see if the volts come up. Thinking maybe you have a dead coil. This can happen from leaving the key in run with the engine off.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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post #9 of 17 Old 02-11-2015, 04:20 PM Thread Starter
1Coyote
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Mike, thank you for sticking with me on this. I really appreciate the help.
With key in run, I get 3.1 volts at the coil. When I disconnect from the coil I get 12 volts. As I understand it, I'm guessing still, the coil is toast and the 12 volt reading means the ballast wire or the starter relay is bad. By the way, I have the TFI upgrade. Am I going the right direction?
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post #10 of 17 Old 02-12-2015, 08:20 PM Thread Starter
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I checked the starter relay and it seems fine. I disconnected the wire from the relay but still have 12 volts to the coil. I can only assume that the ballast wire has gone bad. So, having read a few posts, I believe I need to replace the ballast wire with a ballast resistor. Question is, what size resistor? There seems to be some differing opinions.
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post #11 of 17 Old 02-12-2015, 08:44 PM
Mike Romain
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I am thinking the other way. With no coil plugged in you have 12 volts unloaded. With the coil plugged in, it changes down radically. I think the coil is shorted out.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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post #12 of 17 Old 02-13-2015, 12:07 AM Thread Starter
1Coyote
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I won't disagree about the coil. However, as I understand it, There should only be 7 or 8 volts through the resistor wire, not 12. Does it make sense that the wire and coil are both bad? This is the second Ford type coil. Should I change to a performance coil?
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post #13 of 17 Old 02-13-2015, 04:10 AM
Mike Romain
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Quote:
Originally Posted by 1Coyote View Post
I won't disagree about the coil. However, as I understand it, There should only be 7 or 8 volts through the resistor wire, not 12. Does it make sense that the wire and coil are both bad? This is the second Ford type coil. Should I change to a performance coil?
The volts show 8 or so when there is a load on the circuit, that means plugged in to the coil. When the wire is in the air, it can read 12 volts.

Lots of, well most new parts these days are made in China junk. I think I would scrounge at the wreckers to get an OEM ford coil, same for an ignition module, far too many of those come just plain dead in the box.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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post #14 of 17 Old 02-13-2015, 05:15 AM
sainmatt
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I'm not the kind of person to throw parts at a problem, but the HEI eliminated all of my electrical issues. My icm tested well at autozone even though it was bad. Now, a single 12v line controls all of my ignition.

I know you already went tfi, but you could swap over and be done with it and have a much simpler system. I'm sure you could stick it out and resolve your issue. My biggest thing is that I wanted that icm out of my jeep.

89 YJ 4.2L Laredo, Howell TBI, CRT HEI
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post #15 of 17 Old 02-13-2015, 10:48 AM Thread Starter
1Coyote
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I was thinking about the voltage change, you mention, last night. I'll see if my neighbor has a coil that I can use to test the voltage drop. Maybe, I'm starting to understand something.
Would love to get rid of everything but California will not let you change much.
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