1983 CJ-7 No Spark, Low Coil Voltage on crank - JeepForum.com
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post #1 of 8 Old 07-15-2020, 01:21 AM Thread Starter
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1983 CJ-7 No Spark, Low Coil Voltage on crank

Hey All,

New to me Jeep...I have a 1983 CJ-7 4.2L i6, just bought it a few weeks ago, it passed smog less than a month ago. Ran great for a couple weeks, then just died on me while running and now it wont start. Cranks, but I've got no spark.

I've scoured most of the ignition issue threads and I've done most of the troubleshooting (printed out the Medics instructionals!), but I'm still up against a wall.

At this point, we have new spark plugs, wires, distributor cap & rotor, coil and ICM.

When the key is in the on position, I've got about 7v at the coil (tells me the resistance wire is good, since the voltage at the beginning of the resistance wire is 12v), but when it cranks to start it's only going up to 8v at the coil. Battery has sufficient voltage.

No nutter bypass, I do have the ECU under the dash and I'm suspecting this is the issue at this point...but I haven't found any good info on the operation of the ECU to help me make sense of the damn thing.

Also having trouble finding diagrams for my engine. I have the Haynes manual with some diagrams but I'd like to print something bigger to reference while I'm diving in.

Any thoughts at the lack of voltage while cranking?

Thanks in advance for your time!

"Patience and fortitude conquer all things" ~ Ralph Waldo Emerson
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post #2 of 8 Old 07-15-2020, 03:33 AM
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there should be a wire from the I terminal of the solenoid to the + terminal of the coil (red with tracer in diagram below). In Start this is energised to bypass the resistor and provide battery power. If you have the +ve wire disconnected you should see 10 -11.5V in Start on that wire.

By itself this does not explain your lack of spark!!!! The ICM works on battery power (nominal 12V) supplied direct from the ignition switch and not via the fuse box.

I would start with checking the IGN LPS fuse in the fuse box (Fuse 8). This feeds the MCU computer under the dash, not the ignition module or coil.

The MCU has eight inputs including from the distributor i.e. the orange and purple wires from the distributor which are the sensor wires. It only uses this input to know the engine speed.

Based on the eight inputs and the programming within the MCU it issues fake sensor readings to the ICM. This then drives the ignition. If the MCU is not powered up, or there are multiple problems with sensors, it will not pass the signal to the ICM.

On the other side of the MCU it is issuing instructions to the carburettor and Solevac throttle positioner. If you look down the throat of the carb you will see a stepper motor, this should be moving pins in and out if the MCU is working. When you first turn the key to Run you should see the Solevac operate for 60 seconds.

What did your investigations find? Have you got power and ground at the ICM? i check the light green wire for intermittent ground in time with the engine turning over.

A few more diagrams which may help.
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post #3 of 8 Old 07-15-2020, 07:57 AM Thread Starter
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Thank you Bagus, this is great info and I appreciate the diagrams.

I will spend some time with the jeep tonight and report back what I find!

"Patience and fortitude conquer all things" ~ Ralph Waldo Emerson
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post #4 of 8 Old 07-15-2020, 12:18 PM
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Do you still have the OEM 'Horseshoe' connector on the coil?
They are notorious for the issue you are now having.

Have'n you along, is like loose'n 2 good men
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post #5 of 8 Old 07-15-2020, 12:58 PM Thread Starter
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Hey Lumpy, I do not have the horseshoe connector, new coil is wired direct to terminals. Good to know, thank you!

"Patience and fortitude conquer all things" ~ Ralph Waldo Emerson
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post #6 of 8 Old 07-15-2020, 02:16 PM
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Below is a document I copied from one of jeephammer's posts.
I agree check the I terminal is spliced with the red green tracer ballast wire, that provides the full 12V at crank.

1. Get yourself a test light with a bulb.
Looks like an ice pick with a bulb in the handle and a wire coming out of the handle.
Around $6 at the parts stores.

2. Hook Test light WIRE to your battery NEGATIVE terminal.
Probe the Positive terminal to see if the test light is working.

3. DISCONNECT your coil connector from the ignition coil.

4. Probe the 'Red' wire terminal in the coil connector (Disconnected from the coil).

Turn the key switch to the 'Run' position, and see if you get a 'Light' at the tester.

This will tell you if the Ignition fuse, Ignition Switch, Factory Tach and all those little connectors under the dash are working,
And if that power is getting out to the ignition coil.

You do NOT get a light at the coil connector 'Red' terminal, then you have an issue INSIDE the Jeep, or with the fire wall connector, or resistor wire.



6. Have someone CRANK the engine (Key Switch to 'Start' position) and probe that same 'Red' wire terminal while cranking the engine.
You should get a 'Light' at the tester.

*IF*... You do NOT get that light when cranking,
Then you have an issue with the 'Red' wire on the small terminal of the starter relay marked 'I'.
That wire connected to the 'I' terminal on the starter relay goes over to the ignition coil 'Red' wire terminal to supply increased voltage to the coil while cranking.

Now you have Tested, and hopefully VERIFIED the power circuits to the ignition coil.


7. Move the tester wire to the POSITIVE (+) battery terminal and clip it on there.
Touch the Negative battery terminal to verify the tester is working.

8. PROBE the COIL CONNECTOR 'Green' wire terminal.
You should get a 'Light' showing the electrical 'Ground' path to the module, and... ->Through->... the module is working.

The coil electrically 'Grounds' by passing through a switch in the module.
If the 'Green' wire circuit is broken, corrosion, broken wire, whatever,
The coil can't 'Ground', and the module can't switch the coil 'On' and 'Off' to make it fire the plugs.

You SHOULD get a bright 'LIGHT' when you probe the 'Green' wire terminal.

*IF* you do NOT get a 'Light' then either the 'Green' wire circuit between coil connector and module is broken,
Or the 'Black' wire from module to distributor housing is broken,
Or the distributor is no longer making ELECTRICAL contact with the engine block.
(Mechanical Contact and Electrical Contact are two entirely different things)

9. Pull in anything you don't want mangled by the engine when it cranks,
And crank the engine while probing the 'Green' wire at the coil connector.
If you get a 'Flashing' light, then it's probably the coil, coil wire, distributor cap, rotor that isn't working correctly since a flashing light means the ignition module is trying to fire the ignition coil like 'Normal'...

If you DO NOT get a flashing light at the 'Green' wire terminal while cranking.
That means the ignition MODULE is not firing.
You need to find out why...


The next tests work with the module, mounted low on the back side of the DRIVES fender, under he washer tank on '78 to '86 Jeeps.
It's usually easier to work on the harness if you unbolt the module and raise it up as far as you can.

10. Connect the tester to battery NEGATIVE terminal, touch the Positive post to confirm the tester is working.

11. Unplug the TWO WIRE connector going to the module, The module side of the wiring will be 'Red' and 'White'.
You need to test the HARNESS SIDE, which will be a large (10 Ga.) 'Red' and a smaller 'Blue' wire.

Start with the smaller 'Blue' wire on the harness side of the connector,
See if you get a 'Light' when cranking the engine.

Don't worry about power to this one with the key switch in the 'Run' position, it shouldn't be 'Hot' when the key switch is in the 'Run' position, Just when cranking the engine.

If you get a 'Light' when cranking the engine, this power feed to the module is working.

12. Now test the larger 'Red' wire.
This terminal should have power when the key switch is in the 'Run' position, and will sometimes show 'Hot' when the engine is cranking, but that doesn't matter,
Just the key switch in the 'RUN' position is all we are looking for at this time.

*IF*... You got power at the coil in the 'Run' position,
And you DO NOT have power at this terminal in the run position,
Then you have a problem with the 'Splice' between the fire wall connector and where the wire splits into 3 wires (Called a 'Splice') in the harness,

If you found power when and were you needed it,
Then RECONNECT the two wire connector and move on.

13. UNPLUG the 4 wire connector at the module.
Connect your test light to the battery POSITIVE, we are testing the 'Black' wire, HARNESS SIDE.
Probe the black wire, Harness Side, and see if you get a strong, bright 'Light'.
This will confirm your module is getting electrical 'Ground' through the distributor.
That 'Black' wire goes to the distributor and 'Grounds' through the distributor housing.

14. Make up a 'Jumper' wire, just strip the ends of a wire,
Connect one end to the battery NEGATIVE, the other end to the COIL CONNECTOR 'GREEN' terminal.

Then probe the 'Green' wire in the HARNESS side of the 4 wire connector.

You should get a strong 'Bright' light. If you do NOT get a 'Light' then the 'Green' wire between coil and connector at the module is broken somewhere.


From this point forward, you will need a 'Multi-Meter' that is capable of testing for resistance, that means it has an 'Ohms' reading.

IF YOU HAVE A COMPUTER CONTROLLED IGNITION ('83 to '86), SKIP #15 since there won't be a 'Violet' wire in the harness side of the module 4 wire plug.
Move directly to #16 and test the distributor it's self if you don't have a Violet wire in the HARNESS side of the 4 wire module plug.

15. Set your Multi-Meter for 1,000 Ohms, and probe the 'Orange' and 'Violet' wires at the 4 wire connector, harness side.
You should see a reading of 400 to 800 Ohms, with about 625 Ohms being 'Normal' or 'Optimal'.

If you DO NOT get a reading of 400 to 800 Ohms, then you will have to move to the next test.

16. Unplug the distributor from the harness,
Test the DISTRIBUTOR SIDE of the connector, male plugs, you are testing the two running PARALLEL TO EACH OTHER ('Orange' & 'Violet' or 'Black' with 'Orange' stripe and 'Black' with 'Violet' Stripe on some distributors).
Again, you are looking for a reading of 400 to 800 Ohms, with around 625 being 'Nominal'...

*IF*... You DO NOT get the specified reading, the 'Stator' or 'Trigger' winding in the distributor is shot.

*IF*... You DO get the 'Nominal' reading at the distributor, but DID NOT at the module 4 wire plug,
Then you have a wiring problem between the distributor and the module connectors.


Testing the module...

Since there isn't any practical way to test the module other than replacing it,
I suggest you take the module to one of the parts stores and have them test it on the bench.
Since many of the stores DO NOT have properly trained personnel, I would suggest you take it to 3 or 4 of the stores, get a consensus on it's function.

A KNOWN working module is always a good thing to have, but you have to test that module while the vehicle is working, instead of waiting until the vehicle has stopped working since you won't know if your 'Test' module,
Or even a 'New' discount store module is working when you put it in...
Many of these 'Import' or 'China' modules don't work right out of the box, so you can't really rely on a 'New' module to give you a definitive answer.


This SHOULD give you a quick, easy way to find out at least WHERE the issue lays...
The Coil Connector Power test will let you know if the issue is on the firewall side of the coil,
Or in the engine harness it's self.

The module power test will confirm if you are getting power to the module while cranking, and if the 'Start' circuit is powering up like it's supposed to.

The coil ground test will let you know very quickly if the 'Ground Path' is the issue or not.

The Ohms test at the distributor and 4 wire plug let you know if the trigger is working in the distributor,

DO NOT forget to plug in the distributor, module and coil before you go cranking with a new module!
(Ask me how I know that one! )
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post #7 of 8 Old 07-15-2020, 02:31 PM
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Just a note:
Before you go crazy looking for a fuse, there is not one for the ignition. Meaning Tach, ignition coil and ICM.
So once the key is ON you should be getting power to the coil and ICM.

There is a fuse for the Computer strangely enough. (IGN LPS)

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post #8 of 8 Old 07-16-2020, 01:43 AM Thread Starter
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Alright...Thanks again everyone, Bagus, Lumpy, Basod and John. I appreciate your time and feedback.

Spent the evening going through all these troubleshooting points, and we're running now!

So last night, I did not have spark from the coil to the distributor...and low voltage to the coil on crank.

Tonight, I started with the troubleshooting points from Bagus' post. And I was mindful to clean all terminals and plug connections while going through all of these steps. If a ring terminal looked sketchy, I replaced it. Any connection looked oxidized or dirty, I cleaned it.

Solenoid checked out, had good voltage on 'I' when cranking. Fuse was good. Only thing I couldn't figure out was checking the pins from the stepper motor in the carb. I gave that a few tries but couldn't see anything so I couldn't verify that step - but I was more focused on the ignition electrical so I didn't spend too much time trying to figure that out.

From there I moved on to the troubleshooting points from Basod's post. I went step by step and everything checked out...everything, just as it was supposed to as listed. When I got to the very end, I put it all back together and checked for spark...and I had great spark from the coil. Then I hooked it back up to the distributor and checked spark at plug wire 1 coming off the distributor and had great spark there as well. This was the best progress I had so far.

Turned over but still would not fire, so then we started adjusting the distributor and that's when it wanted to start to run. So we moved on to timing and adjustments, and got her running.

Best I can figure is that cleaning all the connections and ensuring we had good grounds all over etc. as we went through all the troubleshooting was what got us there.

Learned a lot through this process, so thanks again to all that responded!

Take care.
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