Jeep is running! Coil question -

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post #1 of 4 Old 07-13-2009, 10:37 PM Thread Starter
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1975 CJ5 
Join Date: Aug 2005
Location: Allendale MI
Posts: 276
Jeep is running! Coil question

Well I went to NAPA to see about a resistor wire for ignition and all they had was a resistor block to mount inline somewhere. The guy there suggested a coil with an internal resistor which I got since it sounded easier and cheaper and I hadn't actually replaced the coil yet anyway. I then ran a keyed hot wire to the coil positive terminal along with the existing hot wire from solenoid. The thing started right up! My questions: is the internal resistor coil a good way to go? The extra wire I ran to the coil is hot not only in the run position but in the start position as well, is this OK since it is already getting 12v in from the solenoid when starting? I'm a little fuzzy on the correct wiring as the ignition diagram I have printed out doesn't even show the ignition module in it which I believe my '79 304 is using the duraspark. I'm glad it starts and stays running but I just don't feel comfortable that I have things wired the way they should be so if anyone has a simple diagram...

'75 CJ5 w/304/T150/D20/D44's/3.07/33" Thornbirds/Glass Body...
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post #2 of 4 Old 09-14-2009, 05:13 PM
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1979 CJ7 
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I am having kind of the same situation with my 79 304. I have a .4 ohm external resistor installed with a 12v supply like you do and my new accel coil I just put in split and blew oil all over today. I had the parts place give me a MSD Blaster 2 coil but I am not sure if this one will live either.
The PO had redone the wiring and it does not match any schematic I have ever see. MSD claims for points ignition you must have a .8 ohm resistor but I dont have a points system I have an electronic ignition which they dont mention at all.

1979 CJ7 aka "Christine" Build Thread:

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bigredisb is offline  
post #3 of 4 Old 09-14-2009, 06:18 PM
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1973 CJ5 
Join Date: Aug 2007
Location: South West Indiana
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You two are compairing Apples to Oranges...

'77 had the PRESTOLITE ignition system in it.
It DOES NOT use an inline resistor or resisted coil!

If you have a plastic vacuum advance on your distributor, then you still have the stock Prestolite ignition system...
If so, you REALLY need to let me know so I can properly diagnose what you are needing to do!

You need to explain why you put in a different coil,
Why you think you need to rewire the ignition, ect.

If you have a factory Prestolite, this is the way it should be wired,
Notice the LACK OF RESISTOR in the system...


OK, for the HIJACK guy... Big Red ISB...

Your '79 runs a Jeep/Motorcraft ignition.
You have a Motorcraft distributor, DuraSpark ignition module, and should have a canister coil. (Like the Blaster II)

Your DuraSpark ignition module REQUIRES A 1.35 Ohm (or larger) ignition resistor to keep the module alive!

What you need to understand is the factory ignition has a 1.35 Ohm RESISTOR WIRE built into the harness, so you probably don't need an extra resistor.

The simple way to test is to use a test light,
Disconnect the EXTERNAL RESISTOR & COIL...

Connect the test light to the NEGATIVE battery terminal, and touch the POSITIVE battery post with the probe...
Both to test the light, and so you can see what a 'BRIGHT' light looks like...

Now, Turn the key switch to the 'RUN' position,

If you get a 'Bright' light, the resistor has been removed.

If you get a 'Dim' light, the resistor is in place.

Here is a wiring diagram of your '79 factory harness expressed in two ways.
(Sometimes it helps to have more than one interpretation of things)

Notice I've added a note on where to add a dedicated ground wire for the DuraSpark module, And I suggest you find the 'Black' wire from the distributor, and connect a dedicated ground to the HARNESS side of that wire...
(The distributor is supposed to be the 'Ground' for the system, but it doesn't do a very good job.
Adding a dedicated ground to the HARNESS side of things means your module will get a good ground even if the distributor isn't providing one anymore)

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post #4 of 4 Old 09-14-2009, 06:37 PM
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1979 CJ7 
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I am not trying to hijack the thread. I had a similar problem and in CJFreaks post he states that he has a 79 304 which was believed to have a motorcraft system in it.
I didn't want to start a new thread and get slammed for not searching so I thought because it was a 79 engine, a 304, and what I thought was the same ignition I would post what happened when it was incorrectly wired.
Thank you for the info as usual its top notch. I believe I found my issue.

1979 CJ7 aka "Christine" Build Thread:

2013 Rubicon Unlimited build thread:
bigredisb is offline  

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