Ignition coil question?

Does the 74 CJ5 require a resistor or resistor wire for the coil? or did they come with an internal resistor coil? How do I tell if I have a resistor wire?

 

Two easy options - 1) look at the wiring diagram in the FSM to see if there is a resistor or resistive wire between the coil and the ignition switch, or 2) look at the coil spec'd for that year in the Standard Ignition catalog
http://www.standardbrand.com/web_app/catalog/smp_bgbulk.aspx?strSearch=UC-12X
Now, this doesn't say explicitly whether it uses a resistor or not, but then if you look in the marine catalog on the same site, a UC-12 or UC12X is a "12 Volt coil for use with external resistor or where the resistor wire is an integral part of the wiring harness."

 

I've got a follow up to this question.... I picked up a 72 CJ5 with a 304. It runs really crisp at part throttle, but when you get into it past half throttle it starts missing. It's not an operational problem for driving it - I'm not trying to get myself killed - And I had thought it might need the carb gone through.

The previous owner, who pretty much left this Jeep parked out behind his barn and did little with it, stated the owner before him upgraded to electronic ignition. Sure enough, the box is there on the firewall. Today I was listening to the radio in the shop (my jeeps radio) with the key on. (and the engine off) for a couple hours as I went through fixing lights, etc and generally rehabbing my Jeep. I was tracing some wires and for one reason or another ended up touching the coil and it just about took my fingerprints off it was so hot. You could have cooked with it. So I think I may have found my missing problem.

I broke out the WD40 and a rag and wiped the crud off of it and sure enough it says "12v do not use without external resistor". I don't see any little resistor blocks on the firewall anywhere, and I REALLY don't want to dissect my wiring harness looking for a resistor wire if I don't have to.

Will I be hurting anything if I go out and pick up a coil with an internal resistor and replacing the one I have with it?

 

What happens when you leave the key on depends on whether the points are closed at the engine's stopping position or not. When the engine is running, current flows through the coil intermittently, roughly only half the time. If the points are closed and the key is on, current will flow through the coil continuously. This is not how the coil is meant to operate, and is most likely why the coil was hot.

The usual side effect of leaving the ignition on with points ignition is 'burning' the points - the points get hot and their surface is damaged. Points ignition is not meant to be left on all the time. Use the 'accessory' position on the switch, or disconnect power to the coil if you must leave the ignition switch on and the motor not running.

What color is your spark? You are jumping to a (likely incorrect) conclusion here by condemning the coil. Having a spare coil isn't such a bad thing though. If you want to replace the coil, I'd suggest you replace it with another external resistance coil like the Standard UC-12 or UC-12X. Unless your wiring harness has been replaced, I'd expect there is a ballast resistor there somewhere. Adding a second ballast resistor (internal to the coil) will cut the current through the coil in half, and could replace one (supposed) problem with a different (real) one.

If you are worried about this, there is a factory service manual (FSM) for 1972 online at www.trailforge.com - the diagrams there should show you where to look for the resistance wire or ballast resistor. <edit> It's right there on P. 22-3.

 

I reread your post, and I see you have some kind of electronic ignition. Can you tell us what the name or label on the control box is? There are a few different types of pointless ignition that could be installed in the engine, and there's also 'capacitive discharge' ignition that works with the original distributor to boost the spark intensity.

The point about leaving the key on is still correct though.

I'd suggest you look for the ballast resistor. The stock location is shown on P. 22-3 of the manual. Which coil you need depends on the type of ignition you have. Some electronic ignitions work without a ballast resistor, and some need one. You don't want to add a second resistor though, if you already have one.

 

All gm hei's require a full twelve volts to the unit with a #12 wire or larger or You will get the loss of power you discriber. I had this problem comverting to hei on early 350 chev. replace wiring to the full 12 volt no wire and walla

 

thats should read 12 volt no 12 wire

 

Thanks for the info guys.

- Nope, it's not an HEI. I didn't even know they made them for AMC engines

- Went all through the harness, found something that looked like it could have been a resistor, but it was sorta half bypassed. I don't think the previous owners were so good at wiring. In any case before I found it, I swapped in a 12V internal resistor coil and it completely solved my high rpm missing problem, so I left it in. If it ain't broke...

- The electronic ignition box on the firewall has the word "Standard" stamped in it and nothing else. If it's not obvious yet, I don't know all that much about working on the ignition system. (although I think I grasp wiring pretty well in general)

-Thanks for the tip about not leaving the key on. Someday when I have lots of time and nothing better to do I'll probably re wire the whole jeep (it seems like it could use it) but for the time being I'll just not use the jeep for a crummy portable stereo.

 

"Standard" is Standard Ignition, an aftermarket supplier of good quality ignition parts.

You probably have Prestolite or Motorcraft Duraspark ignition from a later Jeep. Look at the distributor - Prestolite has a plastic vacuum advance can, Duraspark has steel.

Use the "accessory" position (turn key 1 notch CCW) for stereo - if it works, you're fine leaving that on.