Coil voltage, what is the correct coil voltage on startup?

Hello, finding wiring diagrams for this vehicle is difficult due to the age, but what color wire are you reading the voltage on? With the key in just the run/on position you should technically read 12volts on both sides of the ignition coil until you crank the engine, once cranking the ignition module will pulse the ground side of the coil and this induces high voltage into the secondary windings inside the coil, and therefore a spark out the spark plug. So, I'm looking at a 1988 Jeep wiring diagram for a J10 which was all that was listed, but if the power wire for the coil is a specific color, I would try to follow that wire which is going to go to a fuse. Low voltage issues are usually corroded wires or connectors if it's not a battery issue.
The wiring diagrams I'm looking at have a yellow wire that powers the coil coming from a fused ignition switch location. So, you can see if you have that low voltage at any of the fuses in the vehicle, especially any fuse boxes in the engine compartment where everything is exposed to the elements and moisture.
I will see if I can locate any wiring diagrams but stay with the basics on this one. Try to trace that power feed wire. This will be a very basic setup for tracking down a corroded area. Any connectors under the fuse panels should be checked as well if you find low voltage going to a fuse box.

Using a test light can be very productive because it puts a small electrical load on the circuit.

Another test you can do with a multimeter is voltage drop testing. This is where you would have your meter set on DC volts, and as an example, to test an Alternator charging wire to the Battery, you would put one meter lead on the Battery Positive and the other lead on the Alternator main post which goes to Battery Positive. And whatever voltage you are reading on that side of the positive circuit it the amount of voltage being lost on that wire from B+ to Alternator +. You can do the same test from Battery Negative to the Alternator housing which is the Alternator grounding to the engine block. You need to do these tests with the key on because there needs to be current flowing to read any voltage drop on a circuit.

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Okay, I do see a couple of resistance wires coming from the Alternator, so Mi not sure exactly how accurate these diagrams are. What voltage are you reading on the other coil post? Are you reading the same voltage on both ignition coil posts(connections) with the Key On engine off (not cranking)? I assume you are not getting any spark at all from the spark plug wire coming off the coil. First check if you are reading the same voltage on both sides of the coil with just key On.
If you are the next test requires a test light. If you hook a test light to Battery Positive and touch the test light to ignition coil negative, you should have a dim flashing while cranking. This is the ignition control module controlling the ground side of the coil.

If you are not reading any voltage on the other side of the coil, switch your meter to check the resistance of the coil. Set the meter on Ohms setting and with the key off check the resistance across the coil. It's possible that with the new components the ignition coil failed. If the Primary circuit inside the coil has no resistance, the meter reads OL (out of limits) that means it's an open circuit inside.
If the coil does read a resistance across the positive and negative of the coil, it should be around 2 Ohms or less.
Checking the Secondary circuit of the coil should be from either post on the coil to the Spark plug wire output. Check from both coil posts because some coils are wired from the negative and some from the positive depending on the design. It should read around 7 to 10k Ohm (7000 to 10,000 Ohms). Here is a quick video on checking the coil resistance.
If either the Primary or Secondary ignition coil circuit is open (OL) that is why there is no spark.

Here are a couple of guides to help as well:

https://www.youtube.com/watch?v=LbL_P_2LvmU

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

https://www.2carpros.com/articles/how-to-test-an-ignition-system

Let us know what you find.

Okay, if you have an automotive test light, I would check the module operation first. Do you still have the old coil? I suspect the issue is either the coil, the low voltage, or the control module. If you use a test light and see the dim flash on the negative side of the coil, then you will at least know the module is okay. The vehicle ran before you changed any parts, correct? If so, try the old coil and see if you get spark back, if not try the old control module.

Okay, sorry for the limited-service info for a vehicle this age, but we should be able to get you going here. Try swapping out the coil first.

None of the coils you have is giving you any spark at all. There is a current limiting resistor in the feed circuit on the yellow wire. But did the vehicle run before you started the tune up?
There is also a capacitor on the feed circuit that comes off the same power connection to the coil and it goes to ground. It's to suppress any transient voltage that might interfere with other sensors on the vehicle. It should also be on the same yellow wire and just bolted near the coil to the engine block. If this diagram is correct, but if the vehicle ran before the tune up, then there's something else going on. Possibly a bad control module, but if you get a test light you can check for coil control, and it will limit the possibilities here.
This is the capacitor, but I really, we need to know if you are getting any spark at all, if you are then we know the coil is being controlled at least. These older ignition modules went bad all the time, especially the aftermarket parts they are making now. We see a lot of bad brand-new parts. So, the module is suspect too. If it's not able to completely ground the coil or for long enough,
I would stick with the coil that has at least 10-volts for now but see if the module is controlling it. I think these in series resistors are a very low value,
Does the Tachometer jump while cranking if you have one?

Okay, I was just looking through some old posts, and it looks like there won't be a full 12-volts at the coil on these. I'm going to ask if I can post this info for you.

I looked over some older posts on these Jeeps and around 8volts is normal with an exterior resistor in the circuit. There was a picture with a tech reading 7.5volts with the Key on at the coil positive feed. You should be able to read the resistors ohms from the Alternator to the coil positive post.
Next step you should check for any voltage drop on the ground side of the vehicle, With the key On, measure from battery negative to the engine block and to the body. With the meter on DC volts and see what you get for a reading. It's not a loaded test really because the vehicle is not running, but you could turn on some loads like the headlights, you shouldn't have much over 0.5v reedling on the ground side. I also marked out some of the fusible links, they should have battery voltage on both sides. They tend to corrode after so many years. Check any grounding locations from battery negative to be sure they are good. A test light hooked to battery positive would be best for checking grounds. Then there is at least some load. Connections can look okay, but not be able to carry current and that's why using a test light is good. Even if you have some old headlight bulbs that are 12volt rated, that will work, Headlamps can carry a few amps, you could rig something up and use one to check your grounds. I don't think power to the coil is the issue at this point. If you have a Tachometer on the dash, it should jump while cranking since the Tach signal looks like it comes right off the coil negative/control side.

You can also try running some temporary grounds from the battery to the engine block and body, but a cheapo test light from an auto parts store can go a long way. I wouldn't use a head lamp for checking the ignition coil control, but a standard test light will show if there is control on the negative side of the coil. It's a quick test that gives a lot of direction when it comes to diagnosing many different circuits.

I'm more in tune with Jeeps 1986 on back.

If yours has a 258, it may kinda be set up like a CJ

Here's a diagram that explains how it works and why "I" terminal

works while cranking.

This happens to be a partial shot from a 1979 wire diagram that I labeled

'79 did not have a computer, as later CJs did.

I'm sure your wiring from the ignition module to the distributor runs

through the computer.

Being an '88, has it been "nuttered"? Any other modifications you know of?

Original carburetor?

Over the years, folks tend to modify things just because their buddy says

it's better. This may or may not be your case. Send some pics of your

engine with the breather removed, maybe we can spot something!

---The Medic

I've never used one,

I've heard of a "out of the box HEI" have sometimes chewed up the cam

because the HEI distributor gear is hardened.

But,

I've read that can be overcome by swapping the gear off of the OEM

distributor to the HEI and all is fine!

I have a '77 and a '46, I upgraded the Prestolite system on my '77 to

Duraspark (like came out on a '79) My system looks exactly like

the diagram I gave you----with one exception, my '77 (prestolite system)

never had a resistance wire, I had to "Counter" that by installing

a Ballast resistor.

In place of the resistance wire. Diagram below.

I've seen folks install a ballast resistor (on the resistance wire) again,

an ignorant party suggested it cause they heard of 'em.

We don't need 2 resistances!

----The Medic