‘72 and ‘73 Imperial intermittent no spark.

[Clover]

Okay, this one has me pretty well stumped. I’ve owned my ‘72 Imp for 26 years and it is a late build, so it has electronic ignition. Once in awhile, I get a no start condition. Crank, crank, crank and nothing, then it fires as the key is let off “crank”, to “run”. It’s pretty intermittent, so it’s hard to track down. Over the years, I’ve replaced the ECU, ballast resistor, cap and rotor, wires, plugs, ignition wires and ignition switch. It’ll be okay for awhile and then the problem will recur.
Now, I’m working on a friend’s ‘73 Imp and it has the same problem but it occurs more often on this one. I’ve now replaced the ECU and added an auxiliary ground, replaced the ballast resistor, distributor, spark plugs, battery and coil and it still will have the problem recur. It’ll be fine for a week, then the problem crops up.
I don’t usually just throw parts at a car- I try to diagnose the problem, using logic. This car has a strong +12v to the positive side of the coil but gets no spark out of the coil, until you’re shutting the key off- intermittently of course. Sometimes, it’s just fine. Has anybody ever figured this out? @DocMcNeedy does this happen to any of your Imperials? Help!

 

[Big_John]

Unplug the plug(s) at the firewall and clean the contacts.

Since you've replaced the parts that could cause this, like the ballast resistor, it really has to be in the wiring, or another possibility is the magnetic pickup in the distributor.

 

[Clover]

Unplug the plug(s) at the firewall and clean the contacts.

Since you've replaced the parts that could cause this, like the ballast resistor, it really has to be in the wiring, or another possibility is the magnetic pickup in the distributor.

Thanks for the reply John. It has a new pickup in the new distributor that I installed. I agree that It does seem like it has to be a wiring issue but the intermittent nature is really baffling to me. Cleaning the bulkhead connector is a good idea. I’m gonna try that next.

 

['66 Fury I]

All good suggestions above. Could be a failing ignition switch. Had a similar problem on my '66 Fury. Lindsay

 

[Clover]

All good suggestions above. Could be a failing ignition switch. Had a similar problem on my '66 Fury. Lindsay

I appreciate your input. I tried a new switch on my ‘72 and it didn’t correct the problem. Also, both cars show +12v at the positive side of the coil, when cranking, whether they start or not.

 

[Ross Wooldridge]

Check that the 12V feed wire to the coil is not chafed and intermittently grounding out against the engine block.

 

[Clover]

Check that the 12V feed wire to the coil is not chafed and intermittently grounding out against the engine block.

Thanks for the suggestion, Ross. I have checked the wire and it looks fine. This is a 45,000 mile car and is very original. I usually put the test light onto the coil positive, so that I can watch the light while cranking and it always has a consistent, bright light, whether or not it sparks. The fact that it fires up as the key is released, is very perplexing. It runs great after it fires up. This is very odd, especially since both cars do the same thing. I can’t believe that I have the only two that do this.

 

[detmatt]

Thanks for the suggestion, Ross. I have checked the wire and it looks fine. This is a 45,000 mile car and is very original. I usually put the test light onto the coil positive, so that I can watch the light while cranking and it always has a consistent, bright light, whether or not it sparks. The fact that it fires up as the key is released, is very perplexing. It runs great after it fires up. This is very odd, especially since both cars do the same thing. I can’t believe that I have the only two that do this.

I just can’t help but think it’s the ignition switch(s).

 

[Ross Wooldridge]

I agree - or something is connected incorrectly off the ignition switch the same way on both cars. If for some reason, the 12V feed to the coil is interrupted while cranking, or the switches BOTH are not turning enough to engage the contacts that permit 12V to the coil but does engage the starter for cranking, this would have the result the OP is relating to us.

I would still be tracing wires and looking for chafing it if was only one car, but the fact that he says BOTH are displaying the same problem makes me think something got connected incorrectly somewhere along the way... it's too much of a coincidence. Was anyone rummaging around behind the ignition switches at any time in the past?

Worthwhile seeing if another ignition switch might prove to solve the issue, but at the same time, even if it's only got 45,000 miles on it, things can get loose. Incumbent upon the troubleshooting thing to ensure that all connections are tight behind the ignition switch in the connector, and that the connection terminal block is firmly fastened to the back of the ignition switch.

Just grasping a bit at straws...

 

[Clover]

I just can’t help but think it’s the ignition switch(s).

Thanks for your input, Matt. I replaced the ignition switch on my ‘72 with an NOS one and the symptoms didn’t change. Also, the +12v at the coil is consistent on both cars.

 

[Ross Wooldridge]

Bad condensers? Still running points?

 

[Clover]

I agree - or something is connected incorrectly off the ignition switch the same way on both cars. If for some reason, the 12V feed to the coil is interrupted while cranking, or the switches BOTH are not turning enough to engage the contacts that permit 12V to the coil but does engage the starter for cranking, this would have the result the OP is relating to us.

I would still be tracing wires and looking for chafing it if was only one car, but the fact that he says BOTH are displaying the same problem makes me think something got connected incorrectly somewhere along the way... it's too much of a coincidence. Was anyone rummaging around behind the ignition switches at any time in the past?

Worthwhile seeing if another ignition switch might prove to solve the issue, but at the same time, even if it's only got 45,000 miles on it, things can get loose. Incumbent upon the troubleshooting thing to ensure that all connections are tight behind the ignition switch in the connector, and that the connection terminal block is firmly fastened to the back of the ignition switch.

Just grasping a bit at straws...

Thanks, Ross. I replaced the ignition switch with a genuine NOS one, and carefully adjusted it on my ‘72, with no charge in symptoms. The coil always has +12v while cranking, on both cars. It never loses it. This is a very strange one!

 

[Clover]

Bad condensers? Still running points?

They’re both electronic. Both have had the ECU and ballast resistor replaced. Between the two cars, I’ve tried new ECU, aux ground to ECU and distributor, new distributor, including the pickup, new ignition switch, new ballast resistor, new plugs, cap, rotor and wires, and new coil. What are the odds that I have the only two ‘72-‘73 Imperials that have this problem? I wish @DocMcNeedy would weigh in, on if he’s experienced this with any of his…

 

[Big_John]

They’re both electronic. Both have had the ECU and ballast resistor replaced. Between the two cars, I’ve tried new ECU, aux ground to ECU and distributor, new distributor, including the pickup, new ignition switch, new ballast resistor, new plugs, cap, rotor and wires, and new coil. What are the odds that I have the only two ‘72-‘73 Imperials that have this problem? I wish @DocMcNeedy would weigh in, on if he’s experienced this with any of his…

Thinking about this, you are getting current at the coil when cranking, so that would tell me that the ballast and ignition switch are good. The other thing that needs current is the ECU and that gets power through the ballast resistor if it's an older 5 pin ECU.

So, if you took the plug off the ECU, is there 4 pins or 5? The 4 pins ECU is plug and play and will also eliminate the need for the dual ballast resistor.

If it is a 5 pin, I'm wondering if there is an issue with the wiring/plug on the ECU. The thing to do is look at the wiring off the ballast first.

Also: 1972 Imperial & Chrysler Ignition System Service Book (Session 292) Probably nothing there is going to help, but it's still good to read.

 

[Ross Wooldridge]

Agreed - I had to swap out my ECU (orange box).

I'd forgotten all about that, as it was years ago, and that car is currently in pieces undergoing a protracted restoration (read - I haven't had time to continue with it).

I remember that I was having intermittent starting and run issues much like what you describe, and then total kaput on any running. However, all electrical tests showed OK. Swapped out the ECU and put in an aftermarket elcheapo one, and the car fired right up.

Perhaps swap out the ECU and report?

 

[Clover]

Thinking about this, you are getting current at the coil when cranking, so that would tell me that the ballast and ignition switch are good. The other thing that needs current is the ECU and that gets power through the ballast resistor if it's an older 5 pin ECU.

So, if you took the plug off the ECU, is there 4 pins or 5? The 4 pins ECU is plug and play and will also eliminate the need for the dual ballast resistor.

If it is a 5 pin, I'm wondering if there is an issue with the wiring/plug on the ECU. The thing to do is look at the wiring off the ballast first.

Also: 1972 Imperial & Chrysler Ignition System Service Book (Session 292) Probably nothing there is going to help, but it's still good to read.

The ECU is a brand new 4-pin. It does seem like it’s gotta be a voltage interruption to the ECU, somehow. I’m studying the FSM wiring diagrams right now. This is so weird. I just tried starting it, seven times in a row, one right after the other. The first five, it started as I was turning the key back to “run” the sixth time, it fired up in “crank” and the seventh time, it again didn’t fire until I was turning the key back to “run”. Had +12v to the coil, every time.
*edit* after studying the FSM, I think the likely culprit is high resistance in the bulkhead connector, alright. Specifically, pin 12, which eventually feeds the ECU, in “crank”. I also see where it will backfeed power to the ECU in “run”, from the coil, through the ballast resistor.
“Edit 2* thinking about it, it can be that it’s backfeeding through the coil, or it would fire up every time. The backfeed must be elsewhere. But I’m gonna do some more testing later, to see if full-time power to the ECU makes a difference, and go from there.

 

[Fireater1968]

Okay, this one has me pretty well stumped. I’ve owned my ‘72 Imp for 26 years and it is a late build, so it has electronic ignition. Once in awhile, I get a no start condition. Crank, crank, crank and nothing, then it fires as the key is let off “crank”, to “run”. It’s pretty intermittent, so it’s hard to track down. Over the years, I’ve replaced the ECU, ballast resistor, cap and rotor, wires, plugs, ignition wires and ignition switch. It’ll be okay for awhile and then the problem will recur.
Now, I’m working on a friend’s ‘73 Imp and it has the same problem but it occurs more often on this one. I’ve now replaced the ECU and added an auxiliary ground, replaced the ballast resistor, distributor, spark plugs, battery and coil and it still will have the problem recur. It’ll be fine for a week, then the problem crops up.
I don’t usually just throw parts at a car- I try to diagnose the problem, using logic. This car has a strong +12v to the positive side of the coil but gets no spark out of the coil, until you’re shutting the key off- intermittently of course. Sometimes, it’s just fine. Has anybody ever figured this out? @DocMcNeedy does this happen to any of your Imperials? Help!

i would hook a temp light bulb up to the positive (ignition circuit) side of the coil. just to make sure the power stays on while cranking. Also check the numbers on the coil. some of those old coils (Ford) have an internal ballast resistor inside the coil and wont fire very well with the external ballast resistor the mopars have. Also check the gap on the reluctor inside the distributor and make sure is stock distrbutor@.008,with a brass feeler gauge in the middle of the vacuum advance stroke. reluctors have also been known to become intermittent .(broken wires inside distributor or heat issues when old)
One last thing the dual ballast resistors had a dual ignition hookup. one wire that is hot in both on and start postions (ign 2 on the switch ) and the run postion only ( ign1 on the switch) this was done to supply coil with full 12 volts while cranking and then it would cut back the voltage a bit once key was released.

 

[Clover]

i would hook a temp light bulb up to the positive (ignition circuit) side of the coil. just to make sure the power stays on while cranking. Also check the numbers on the coil. some of those old coils (Ford) have an internal ballast resistor inside the coil and wont fire very well with the external ballast resistor the mopars have.

I appreciate your input. I’m definitely monitoring the positive side of the coil, with a test lamp and it always has +12v in “crank” mode.

 

[Boydsdodge]

Check the wires going from ignition switch through firewall. I had a B body in with an intermittent shut down while driving, after much searching I found that the blue power wire was loose enough to lose connection

 

[Clover]

Check the wires going from ignition switch through firewall. I had a B body in with an intermittent shut down while driving, after much searching I found that the blue power wire was loose enough to lose connection

Thanks for your input. I think maybe I’ve fixed it (shouldn’t have said that, lol) @Big_John suggested bulkhead connections and after studying the wiring diagram, I targeted pin 12 as the likely suspect. Sure enough, it had green corrosion on it (the only one). I cleaned it and so far, so good!