Coil Resistor Identification

Hey yall,
I went to get a couple replacement coil resistors for my 74 cj5 (strait 6 258) and my 63 cj5 (4 cyl 134) and I realized that there are a LARGE selection or resistors with different amounts of resistance.

And... I dont trust what was put on the jeep in the past, so I didnt know how I could find out what resistance I need for each coil...

Anyone have the answer? haha

Thanks!!

 

Yea, just went through that whole thing because I had a miss in my engine and No One could figure it out....except WaltCouch. Turns out, my combination of coil/resistor dropped the charge to 4v at the distributor. According to the shop manual, with the key on and the points closed, you should be getting NO LESS than 9v when you test between the closed points and ground. Don't know part #'s, but that's how you test it with a multimeter

 

now is that the same for both the 4 cyl and the 6 cyl engine?

 

My '60 never had one.

 

Yeah all of mine have one, but all of them are different, and my 63 has three spares on the firewall and they are different lol.
I just got a new coil for the 74 cj, and it said it required an external resistor

 

Yeah they are all 12v systems with external resistors. So basacally, with a fully charged battery, I have to check the voltage on the coil side of the distributor. It seems it would be 12 volts without the resistor, since that is what the battery is giving off, so I have to get a resistor that will bring it from 12 volt down to 9 volt.

So there was never any "set" resistor back then...? I would seem that the jeep/willys would have had one particular resistor they put on the jeep from factory.

 

My experience has been that resistors are meant to work with a specific coil resistance. So, if you go to the Standard Ignition catalog and look up a UC-12 (a universal coil, applicable to thousands of different vehicles) and then pick a ballast resistor from a vehicle that uses that coil (say, a 1970 Dodge), it will work fine.

Focusing on the voltage drop is (electrically) naive, IMO. This is not a DC circuit. The ballast resistor is there to limit current following the discharge cycle of the coil. The coil-resistor-condensor circuit is a classic RLC circuit, and you can pretty easily predict its behavior. Whenever you want to limit current, you add resistance... it's Ohm's law.

A '74 should have a resistive wire, IIRC. Check the circuit diagram in the FSM. If you are using the factory wiring harness, it should have a resistive wire in place already, and not need an external resistor. Measure the resistance from the coil wire back to the (+) battery post. Even if there is a resistor in place, it will only be a very few ohms of resistance. So compare the resistance of the coil lead to something that uses a pure 12V under the hood.

 

yeah, that makes sense. The 74 actually has been changed all around, has a new engine, new fiberglass body, and the electronic ignition changed out to the point ignition, and most of the wiring has been re done so the original resistor wire is no longer there.

 

You went from electronic ignition TO points?

 

This is from the shop manual under Primary Circuit testing labeled as step "a" in a list of 13 steps.

"Measure the voltage at the coil primary terminals while cranking the engine with the starter motor. If the voltage is less than 9 volts the trouble will be found in the primary circuit...."

Step b. is:

"To check the primary circuit, turn the ignition on, turn the engine until the points are closed, and then measure the voltage drop across each portion of the circuit with a voltmeter."

In my case, it turns out the coil was fine. The resistor was dropping the voltage to like 4 volts. The engine was missing because sometimes the pistons fired and sometimes they didn't. I took out the spark plugs one by one with the wires attached and the engine running. You could see just whispers of sparks, sporadically. All this was after I went through 2 sets of caps, rotors, wires, and sparkplugs untill I found the problem was voltage drop.

 

Yeah, after having the electronic ignition quite/break 4 times, and once it quits, there is NO way to get it running again. DONE, TOAST, STOPPED right there, until you have a new one or a friend to tow you out. When your are miles out in the middle of nowhere, with no cell service in the area, its not a good thing to have happen. The point system, there is always a way to get back. May not run very well, but its always a garrenttee that you will make it home. So thats why we did that. haha