HELP! Randomly no spark!

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kimmyc626

kimmyc626

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I have a 1972 Dodge Demon 340 with Mopar electronic ignition... starts up greats, runs beautifully, but while driving it will randomly shut off every once in a while and will no longer start due to no spark. This usually occurs once it has come up to temp BUT it doesn’t happen all the time. It is very random, with no indication of issues, seems like it just suddenly shuts down. It does not stumble or act as though it is flooding out or starving for fuel, it just seems like someone turns a switch and shuts it off. Sometimes it will start right back up, other times it has to sit and cool all the way back down. The engine has not ever run hot, once up to temp it maintains a steady 180 degrees. The carb is getting fuel, but it definitely is not getting spark when this issue occurs.
 
Ballast Resistor ? Anyways I have seen bad or loose wires at the connectors on the Ballast Resistor

Can you bring a test light with you or volt meter inside the car , so when it does shut down , first thing your going to do is check for cranking voltage on the positive side of the coil

If nothing , go to your ballast resistor with the test light - The one connector or plug with two wires on the Ballast and check for power with the key in the run position / cranking

If your getting power to the positive side of coil while cranking

Would bee looking at the coil itself

ECU ground

Pick up coil
 
Last edited:
Check your ecu ground! I was having trouble too here is how you can diagnose it.
 
Carry a alligator clip jumper, BIG, like maybe no 14 wire and "old school" full size clips. When it quits jumper from battery direct to coil and see if it will start. If it will not start using the key, try cranking it by jumpering across the starter relay. If that does not work, also carry a couple of jumpers made up so you can bypass the resistor, or maybe even just go ahead and buy one and try it.

Along the way, loosen and clean the mounting bolts on the ECU. The ECU MUST be grounded. To find out, turn key to "run" and measure voltage on both sides of coil. PLUS side should be somewhere around 6-8V not much higher. If it is "same as battery" IE around 12, the coil is not drawing current. Measure voltage on coil NEG. If it is 12V the coil is not drawing current through the box. If it is 1-2V or so "normal." If it is zero the coil is open assuming you have power on the PLUS side
 
What color is your control box. factory 72 340's are blue and externally grounded with the green/red wire. The later orange control boxes are internally grounded and that pin is not used. Those old units would fail when hot and start when cooled down. They came on 72 and 73 340 cars. If you have a blue box that may be your culprit. Been there more than once. Its a ground you can't check. Put a 4 pin box on if you have a 5 pin.

Any color will work for a test.


I just thought of this. I must be really old. Who the hell would have a original blue box after 50 years
 
Is the vac adv hooked up? If so, the pick up wires break [ no spark ] from flexing after a lot miles. Test with an ohm meter while wiggling the wires.
 
Post 3 is good. Also an old trick is carry a glass of ice water. When it quits, dump the ice water on the ECU and see if it will restart. If it does, put an ECU on it. It's breaking down when it gets hot. We used to diagnose intermittent Ford Duraspark boxes back in the 70s and 80s like that. Worked every time.
 
Bad ground from brain to body?

Broken wire in distributor pickup plate?

That second one has got me more than once.
 
Prob one of two things Pick up or the ECU typical symptoms. Hate to say this you have to troubleshoot it when it happens. Change out the ecu when it happens and if it persists prob the pickup, not much you can do without a Mopar ignition tester otherwise.
 
kimmyc626 said:
I have a 1972 Dodge Demon 340 with Mopar electronic ignition... starts up greats, runs beautifully, but while driving it will randomly shut off every once in a while and will no longer start due to no spark. This usually occurs once it has come up to temp BUT it doesn’t happen all the time. It is very random, with no indication of issues, seems like it just suddenly shuts down. It does not stumble or act as though it is flooding out or starving for fuel, it just seems like someone turns a switch and shuts it off. Sometimes it will start right back up, other times it has to sit and cool all the way back down. The engine has not ever run hot, once up to temp it maintains a steady 180 degrees. The carb is getting fuel, but it definitely is not getting spark when this issue occurs.
Click to expand...

Twice now I have had the distributor connector cause this exact problem.
Heating and cooling was causing the plug to loose the connection.
Pinched the plug a little and problem solved.
 
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