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Discussion Starter · #1 ·
So I have come to the conclusion that I don't know crap about the inter-relationship of all the components of an ignition system on my 84 CJ7. I have had intermittent starting problems that seemed to be solved by the installation of a new ignition module a few weeks ago. Well the problem is back, and no the module did not go bad, I tested yet another new one and same thing. So my problem is back and behaving erradically. I got a lot of good posts in response to my cry for help.

I want to better understand how to test the various components involved (limited strictly to electrical stuff for now) such that I don't blindly start replacing parts in the hope that I get lucky.

I either want to develop, steal, copy or otherwise get together something of a decision tree that might look something like;

Does Engine start? If not is battery dead, here's how to test. If battery is ok check selenoid, here is how and the various volt readings you should have. If Selenoid is ok, check coil, here's how. etc. etc.

You get the idea. I don't have this information such that when my intermittent, and damned frustrating starting problem arises, I can tackle it in a systematic fashion based on some facts (unique approach for me).

Sooooo, in terms of electrical for an 84 CJ7, TFI, no computer stuff, is this a definitive list of electrical items to consider.

1. Battery
2. Starter
3. Starter selenoid
4. Ignition switch
5. Switch relay (do I have one of these?)
6. Coil
7. Ignition module
8. Distributor (cap rotor)
9. Plugs and Wires
10. Basic wiring and connections

What am I missing?

Now given the vast accummulated intellect on this board. Can you guys tell me how to test each component listed. Perhaps you only know what the coil voltage should be in the RUN and CRANK state, great, let me know. I will put it all together and submit for everyones review, benefit, etc. If I am re-inventing the wheel, please point me in the right direction.

As mentioned, this is but a small part of starting issues (carb, fuel filter,fuel pump, blah, blah, blah). For sake of discussion and argument I would like to understand electrical for now. However, ultimately it would be great to include everything in one big decision matrix.

Please let me know your thoughts on this, and more importantly any info that I can use to debug an electrical system.


946 Posts
Well I'd start here:

This is TR er, JYG schematic with a few corrections. /ubbthreads/images/graemlins/smile.gif

I'll add pictures and schematics as I "Improve" them.
I think we should keep all the schematics in the same format and I find these JYG schematics quite readable.
It will take some work to improve my artwork. /ubbthreads/images/graemlins/smile.gif

My stuff is a little off center:

So I usually post a picture:
Image uploading. Refresh page to view

I have an 84 harness And will work on it.
I'll let JYG or RRich work on the explinations.

Oh yeah, you don't have a #5

10,815 Posts
That's a tall order - a flow chart covering everything.

But -- starting problems are not always ignition related.

Maybe a good explanation of the exact problem - and conditions it happens under would be more helpful.

When it won't start - cranks OK?
Spark at the plugs? Basics first.

Only hot, cold, only after sitting a year, month, week, hour, moon is full, etc?

Ignition mods already done?
Age of components?

All is helpful.

As we help you trace it down, we'll also include the "Why."

6,870 Posts
First off, please post what vehicle this is for, model, year, what you have done so far...
It's a PITA when we have to go reference your bio to find out what you are driving.

Don't assume the module is 'Good' because the parts store 'tested it'...
Modules act differently when 'hot' or under full load, or starved for current by inadequate (corroded connectors, carboned up switch terminals, misadjusted switches , ect.) wiring harness, and not just a test.

This may be something a complex as an intermittent module malfunction or as simple as a connector expanding with heat and breaking connection...

OK, you wanted a 'Flow Chart'...


1. Add the extra ground wire for the module wiring diagram posted before.
Harness side of the four wire connector, black wire. Use 12 AWG and ground directly to battery negative.

---If the distributor has an intermittent ground, it will disable the module. (some times intermittently)

Add an extra ground wire to the head(s) for the high voltage from the ignition coil to ground. Do NOT rely on rusty, corroded, painted or missing, bolts, brackets & ground straps to do the job.
Again, 12 AWG is large enough to do the job.

The added ground wires are insurance against the inevitable ground failure through the distributor housing, brackets and rusty bolts.

2. With the Key Switch in the 'Off' Position, do the following...
Check battery voltage and write it down for reference.
Check voltage at the Positive side of the ignition coil. It should be 0 (zero).
---If you have ANY voltage at the coil, the ignition switch is bad.

Check the voltage at the HARNESS side of the two wire module connector. Both sides should read 0 (zero).
---If you have ANY voltage at this connector, the ignition switch is bad.

3. Disconnect the starter side heavy terminal on the starter solenoid, (this keeps the engine from actually cranking during tests). Make sure you leave anything else hooked up to that terminal.
With the key switch in the 'Crank' position, check voltage at both of the small terminals on the starter solenoid. They should both be battery voltage.

---If you don't have battery voltage, the ignition switch or wiring harness is bad.

With the key switch in the 'Crank' position, check the voltage at the positive side of the ignition coil. It should be battery voltage (more or less).

---If you don't have battery voltage, the wiring harness between the 'I' terminal and ignition coil positive side is bad.

With the key switch in the 'Crank' position, check for 12 volts at the HARNESS side of the ignition module two wire connector. The White wire side should have battery voltage, and the Red wire side may or may not be 'Hot', depending on the ignition switch you have.

---If the white wire isn't 'Hot', the wiring harness is bad.

4. With the key in the 'Run' position...
Check for voltage at the positive side of the ignition coil. It should be reduced, around 9 volts. This means the resistor wire is intact.

---If you have battery voltage, your resistor wire has been eliminated. Add a resistor.

Check for voltage at the HARNESS side of the module two wire connector. The Red wire side should have battery voltage, the White wire side should be an OPEN CIRCUIT, 0 (zero) volts.

---If the Red wire feed is not active, or has less than battery voltage, module failure WILL RESULT. Ignition switch or harness wiring is bad.
---If this White wire feed is active in the 'Run' position, module failure WILL RESULT. Ignition switch or wiring harness wiring is bad.

Check the 'S' & 'I' terminals on the Ignition Solenoid. Reading should be 0 (zero), open circuit.

---If there is voltage at the 'S' or 'I' terminal, the ignition switch is bad or wiring harness is bad.

5. Unplug the distributor to harness connector (distributor magnetic trigger pickup coil). Test for resistance between the Orange and Violet wire terminals in the distributor connector.
Resistance should be 400 to 800 Ohms, depending on temperature.

---If the resistance is outside the specified range, replace the pickup coil.

Disconnect the ignition coil and test for resistance through the primary (low voltage) windings. The acceptable range is 1.1 to 1.5 depending on temperature.

---If the resistance is outside of the specified range, replace the ignition coil.

Test both the Violet and Orange wires against the Black wire for resistance. This reading should be infinity.

---If you have ANY reading other than an open circuit, the pickup is bad and needs to be replaced.
(you may consider replacing this pick up anyway, as a matter of elimination. I didn't read where you had replaced this component already.)

Disconnect the ignition coil and test for resistance through the secondary windings (high voltage). The acceptable range is 7,700 to 9,300 room temperature (around 75 degrees), and up to 12,000 Ohms at 200 degrees (according to the paperwork).

---If the resistance is outside of the specified range, replace the ignition coil.


Repeat #4 & #5 when the vehicle is acting up, and you will quickly determine if the problem is switch, harness or module related.


I know you probably don't want to hear this...
Your problem sounds like it's probably going to be one of two things,

1. The ignition switch is defective, improperly adjusted, or has linkage binding allowing for both the Start and Run circuit to be active at the same time.
This will quickly toast a module (serious over heating), usually with intermittent results.

Not only will this allow current to the Start circuit during Run conditions, toasting a module quickly, but this condition my also limit current to the module on the 'Run' circuit, also toasting the module, often with intermittent symptoms, so the local parts sore (I mean 'store' :p ) won't be able to detect a defect with the module.

You may consider cutting the 'Pigtails' off an old DuraSpark module, and connecting them to a GM HEI module for testing purposes.
The HEI module doesn't have a 'Start' circuit, and won't be affected by any switching problems.
John Strenk can post a very detailed (with correct pickup polarity) wiring diagram for this home built diagnostic tool.
Don't forget to mount it on something that will double as a heat sink!

2. You have a heat related problem. This may be under hood heat, or component heat from the currents being handled.
I'd be suspicious of the ignition coil and the magnetic trigger coil. It's not very common, but both of these can show intermittent heat problems like the one you describe.

The only 'Positive Testing' procedure for this type of intermittent problem would be to,
1. Heat the suspected parts in a 'Dutch oven' (controls heat better than a toaster oven).
You can also use the kitchen oven, but your wife will b!tch, and they are hard to control for temperature, thermostats are VERY inaccurate, so watch your thermometer!
When at the prescribed temperature, (around 200-250 degrees) do your tests.
(no garage is complete without a hot plate!)

2. Do the parts changer thing... Sometimes cheaper and always faster than doing real diagnostic work.

Here is something else that is a variable, UNPLUG YOUR FACTORY TACH.
See if the engine runs with the tach unplugged. If it doesn't, you may have to bypass the tach.
If it does run, just leave the tach unplugged and try for the problem.

Tachs are like ignition coils and mag trigger coils, they don't go bad very often, but they can cause intermittent problems when they do...

946 Posts
Note on last line in step #4.

If you leave the wire going to the "I" terminal connected the you will see the same voltage as what you measured on the (+) side of the coil. This is OK as the wire runs back to the coil.

If you disconnect the wire from the "I" terminal, then you sould not see any voltage on the "I" termnal.If you do than the solenoid is bad.

The "S" terminal should not have any voltage on it except when the key is in the START position.

I see you have an 84 CJ7. If you have an I6 and you do disconnect the FACTORY tach the engine WILL die. This is because the tach for the I6 is in series. The 4 cyliner tach is not and you can disconnect the 4 cyl tach and the engine will still run.

To correctly bypass the tach you will have to unplug the tach and reconnect the harness plugs together as shown in the picture above. If it runs this way and not with the tach connected then you have a problem with the tach.

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