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Confirm bad coil?

Mike 71X

Mike 71X

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So had a nice drive today, came home and parked for an hour or so. Went to put car in the garage and won't start. Set the coil wire up to the exhaust manifold and no spark. Found some other forums on here with info on the coil. One said if there is voltage on the negative side with the key in "run" that is not a good thing. I have almost 10v on the negative side so one strike against the coil? Another linked to a page on mymopar on how to test the coil. Says the resistance between the side terminals should be 0.75 and 0.81. I measure 1.0. Also says the resistance between either side terminal and the center should be between 10,000 and 11,000. Mine measures about 9300. This is out of the car with no wires to it. So I guess I'm buying a new coil tomorrow. Sound right?
 
NO spark while cranking? The ohm values aren't too far off, but I'd suspect it as bad. I had one that would be bad when it was hot only. Does it start ok cold? The negative side gets switched by your ignition. Is it points or electronic? Closed points should read 0 volts (the points are conducting to ground and burning ). But you'll read voltage normally if the switch is open, so not necessarily bad.
 
Won't start cold, cranks over but won't fire. Tried both pulling a plug and the coil wire up to the exhaust manifold and no spark visible when cranking. It was near dark out by then so thinking if there was spark it would be easy to see? Electronic ignition. Had a spare ballast so swapped that out but no change.
 
As Demonic said... negative coil post get triggered by the ignition module to fire the coil. If that sucker went bad, and they do, or something feeding the module is missing; you will get exactly what you have. Might see if you can borrow a coil for a quick test. Usually parts stores won't take returns on electronic parts.
 
Help us help you.

What brand of coil?
@@@**What ignition system do you have**@@@
Do you know how old it is?

China parts generally aren’t to good/reliable.
 
Last edited:
For electronic ignition, install the coil, take the coil/cap wire loose on the cap side & place the end VERY close (not touching) to a good ground. Disconnect the twin-bullet plug at the distributor. With the key "on", ground the exposed terminal from the wiring harness on that twin-bullet plug. Each time you ground the wire, a spark will jump from the coil wire to ground. If no spark, then the problem is either the coil, the ballast resistor, or the wiring itself.
 
Jacob's Electronics coil. When trying to find info on it sounds like they are out of business, maybe for good reason. It was on the car when I bought it along with the ECU. ECU is orange box. The stated resistances I found on the coil are quite different from what I measured. Wiring harness was all new last year. Distributor was someone else's extra but was new in box and was installed last year also. Don't know brand off hand. Going to buy a new coil tonight.
 
R413,
What do you mean by what ignition system? Points or electronic? I had electronic in my first post. Saw you emphasized the question so I don't think I'm understanding the question. Sorry, I'm a rookie on engine electrical.

R413 said:
Help us help you.

What brand of coil?
@@@**What ignition system do you have**@@@
Do you know how old it is?

China parts generally aren’t to good/reliable.
Click to expand...
 
New coil didn't change anything. Someone suggested trying a test light on the coil negative terminal to see if it would pulse when the coil fired. Tried that and it stays on all the time while cranking the starter, does not flash.
 
I've read on several forums that the "Orange Box" was more prone to failure than most. Haven't a clue why but it sounds like you are on the way to solving that problem.

I recently converted my '74 to HEI ignition; totally eliminating the OEM electronic ignition box and ballast resistor. Now have a HOT park that will easily jump a 3/8 inch gap!
 
I noticed the harness plug to the orange box has 5 wires but there are only 4 terminals in the box. I see online there are both 4 and 5 terminal boxes available. What is the 5th wire for? Apparently it's not needed?
 
My '74 Charger SE had the four wire ignition box when I bought it. The green/with red tracer wire was still there it goes to the ballast resistor. The opposite side where the dark blue run power connects. Don't know what it's purpose was but obviously the newer four pin units do not use it. Full run power is fed to the four pin box on the light blue wire so maybe internally they run it through a dropping resistor then to the circuit the green/red wire once fed.
 
Tried grounding the distributor wire as suggested above, no fire. On the new coil I can hear a little static from it but no spark. My understanding that means a coil, ECU, ballast, or wiring. Coil is new, ballast checks out, Napa store tested the ECU and said it was good, have power to the coil +, power to the ECU and the wires to the distributor from the ECU check out. Still always have power to the coil -. Disconnected just the coil + to see if I had power leaking into the coil - wire but it dropped out so I believe that wire is OK. Tried a ground wire on the ECU just to make sure, no change.

Should I trust the NAPA test on the ECU?

I found a diagram that says one of the distributor wires from the ECU is for start and one is for run. Should I be reading voltage on the run wire when the key is in "run"?
 
Last edited:
I believe this might help. Full Run power is only there on the light blue wire when the ignition is in the RUN position.

'74 ignition.jpg
 
The Lord shined on me a little extra today. Found a post online where someone had an orange box "test" good at the parts store but still prove to be bad. With all things pointing to the box and no other ideas I bought one, put it in and said a little prayer. Fired up so fast it scared me! Thank you to all who provided guidance. Without the advice I'd still be out there leaning over the hood with a dumb look on my face. Started out Sunday knowing nothing about electronic ignition, boy did that change!
 
Good to hear.
I once had an orange box that I had been running for about 10 years.
One summer the car would start to break up above 3500-4000 rpm, otherwise it ran fine.
On and off I would try and diagnose the problem with no luck, not much spare time back then.
Finally around September on a whim I stuck in an old aftermarket replacement from about 1980 and the car ripped down the road.
Problem solved boy was I happy.
 
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