Coil Voltage question

[XS22J8R]

I've had a very frustrating intermittent starting issue with my 360 powered '78 Magnum ever since I bought it nearly 2 years ago. It will crank with not even a hiccup some times, then after several to as many as 5 or 6 attempts at cranking, decide to fire up. I went through a period last month where it was usually starting on the first turn of the key, and I thought maybe the problem was fixed. Then at a cruise when I went to leave, it wouldn't start and after a half hour of me and some people there at the cruise helping out, it finally decided to fire up. It gets fuel, it just seems like it isn't getting spark when in the cranking position of the ignition.
Otherwise runs perfectly, I just need it to start!
All the lean burn stuff was removed prior to my ownership and has a Mopar electronic ignition, orange ECU box etc.
It doesn't help I'm not very good with electrical issues, so I've been chasing this issue since last year, replacing the ECU, ballast resistor, cleaning up the ground on the ECU etc.
The other day my friend brought over his multimeter and did some checks. One thing that caught his attention was he was only reading 5.5 v going into the coil. I checked with another guy I know who is knowledgeable on electrical and he said it should be 12 v.
Is that correct, and if so, what would cause the voltage to be low? The guy who is good with electrical troubleshooting said a weak battery or bad voltage regulator can cause that problem. Does the voltage regulator even tie in with the voltage to the coil?

 

[RemCharger]

The voltage is reduced to the coil - mopars, ballast resistor-- GM Ford through a resistance wire.
But should be around 8-9 running and 11-12 while cranking

 

[XS22J8R]

We also measured the ballast resistor pins, upper left was 5.5v in both run and crank, the other 3 were 11.1 v in run and crank. Coil wire removed so run was just in run position of ignition, not actually running.
With the newer type ECU boxes, my understanding is that the top half of the 4 pin ballast resistor is taken out of the equation and replaced by a internal ballast resistor in the newer ECU?

 

[CarolinaCharger]

Was the 5.5 V present before starting, while cranking or while running?

 

[XS22J8R]

I believe we were getting 5.5V to the coil when cranking (with the coil wire removed to keep the car from firing).
Plugging and unplugging the coil wire back in afterward, my friend noted that it hadn't been plugged in completely. He wondered if that had contributed to the issue. I can't say if it had previously been not completely pushed in, as when the car wouldn't start at the cruise a few weeks ago, one of the guys there unplugged it and had me crank to look for spark. No way to know if it hadn't been completely plugged in before he unplugged it. My friend said it was a bit tricky to tell when it is fully plugged in.
After we had been messing around with the car the other day and the coil wire plugged back in, I tried starting and it fired right up, and fired up first try when I tried starting it several more times. The problem has been intermittent, so I still have no confidence anything has changed and I can count on it starting when I am ready to head home from a show or cruise.

 

[RJRENTON]

I've had a very frustrating intermittent starting issue with my 360 powered '78 Magnum ever since I bought it nearly 2 years ago. It will crank with not even a hiccup some times, then after several to as many as 5 or 6 attempts at cranking, decide to fire up. I went through a period last month where it was usually starting on the first turn of the key, and I thought maybe the problem was fixed. Then at a cruise when I went to leave, it wouldn't start and after a half hour of me and some people there at the cruise helping out, it finally decided to fire up. It gets fuel, it just seems like it isn't getting spark when in the cranking position of the ignition.
Otherwise runs perfectly, I just need it to start!
All the lean burn stuff was removed prior to my ownership and has a Mopar electronic ignition, orange ECU box etc.
It doesn't help I'm not very good with electrical issues, so I've been chasing this issue since last year, replacing the ECU, ballast resistor, cleaning up the ground on the ECU etc.
The other day my friend brought over his multimeter and did some checks. One thing that caught his attention was he was only reading 5.5 v going into the coil. I checked with another guy I know who is knowledgeable on electrical and he said it should be 12 v.
Is that correct, and if so, what would cause the voltage to be low? The guy who is good with electrical troubleshooting said a weak battery or bad voltage regulator can cause that problem. Does the voltage regulator even tie in with the voltage to the coil?

Bear in mind that the Mopar ECU control box does the switching of the coil. The ECU keeps the coil ON, with 12 volts during cranking and approximately 8-9 volts when the key is in the run position (actual voltage is dependent on the ballast resistor ohms)......but.....the ECU turns OFF the coil to produce the spark. Your problem may be attributable to in order:
1. ECU ground...run seperate wire from ECU mounting screw to engine block.
2. Faulty ECU control box (transistor)...test by substitution
3. Faulty coil ....test by substitution
4. Intermittent ballast resistor....test by substitution
5. Very unlikely that the pick up coil assembly in the distributor is bad
6. Loose or corroded bulk head terminal feeding ignition system
IMO....the battery or voltage regulator would not cause these problems. The alternator's voltage regulator is a separate independent circuit.
BOB RENTON

 

[XS22J8R]

In throwing parts at the problem, I’ve previously replaced the ECU, and the ballast resistor several times. I’ll work on further grounding the ECU. I’ve got spare coils on the shelf so I can try swapping it also.
And I’ll look at the bulkhead connection.
Thanks!

 

[LowBikeMike]

When it acts up run a jumper wire from battery positive to the positive side of the coil.

 

[RJRENTON]

When it acts up run a jumper wire from battery positive to the positive side of the coil.

And watch the coil go up in smoke......
BOB RENTON

 

[LowBikeMike]

I'm not advocating you leave the jumper there, and you know that. Its a diagnostic tool, if the vehicle starts he can generally figure the ECU portion of the system is just fine and not worry about the mysteries inside the box. My way of testing this system would be using a solder iron to trigger the ECU with the key on, simulating the engine running. Sort of a load test for the ECU and other components.

 

[bearman]

Bulkhead connection found bad connections on the engine side. Have cleaned and made sure making contact. Problem gone but always make sure grounds are grounded no paint grease etc. I have had coil get hot and zero start when cooled down it would fire up so check temp of coil after it's been running should be able to touch and not be to hot. Just my 2cents.

 

[TopBanana72]

The OP problem description sounds exactly like vapor lock to me.

 

[Geoff 2]

The voltage [5.5v ] measured at the coil will depend on the coil being used. If the primary resistance of the coil is lower than it should be [ wrong coil ? ], then you could get 5.5v instead of 7-8v. [ Assuming correct 0.5 ohm bal res ]
If you change coils, make sure the new coil is designed to work with a 0.5 ohm BR, which is the what Mopar uses. Do not mix coils/BRs, as you can get the problem you have now....

 

[MoparLeo]

 

[XS22J8R]

The voltage [5.5v ] measured at the coil will depend on the coil being used. If the primary resistance of the coil is lower than it should be [ wrong coil ? ], then you could get 5.5v instead of 7-8v. [ Assuming correct 0.5 ohm bal res ]
If you change coils, make sure the new coil is designed to work with a 0.5 ohm BR, which is the what Mopar uses. Do not mix coils/BRs, as you can get the problem you have now....

Are all vintage style Mopar coils correct to work with .5 ohm? I didn't look at the coil closely but I think its an aftermarket one.
My spare ones are like this one Ehrenberg is selling
https://www.ebay.com/itm/225616776192?

The OP problem description sounds exactly like vapor lock to me.

I don't doubt I've had some fuel issues this year, but I've had the intermittent starting issue since I got the car, and it has occurred even after the car is cold, having been parked in my garage for days, and with temps as low as 50°. I had it down to 1/4 tank in fall and took it out Thanksgiving weekend to put gas in it along with Stabil to get it ready for hibernation. I'm sure the stations had changed to crappy winter blend by then. I was out memorial day weekend, and shortly after getting to an event, went to move the car and it was running rough and stalling a lot before I finally was able to get it to drive. I attributed that to the crappy gas. Since then I put in 5 gallons when it hit 1/2 tank, and put in another 5 gallons on my way home from that cruise I had all the trouble at a few weeks ago (with the engine running so I wouldn't end up stranded at the gas station!), so I think that winter blend gas is out of the picture now. I know better than to put winter blend gas in my vintage rides, it's just that I was in a bind due to having the car in a indoor show in November where fire codes required 1/4 tank or less in the show car tanks, and I store my cars with full tanks to prevent condensation while they sit for months.

 

[RJRENTON]

I'm not advocating you leave the jumper there, and you know that. Its a diagnostic tool, if the vehicle starts he can generally figure the ECU portion of the system is just fine and not worry about the mysteries inside the box. My way of testing this system would be using a solder iron to trigger the ECU with the key on, simulating the engine running. Sort of a load test for the ECU and other components.

Not exactly sure what the soldering iron is supposed to do. The reluctor and pick up coil assembly coil assembly generate a negative going pulse, refined by the electronic components in the ECU to turn OFF the coil's switching transistor in the coil primary creating the spark. Do you mean soldering gun?? A soldering gun is a one turn secondary winding to the soldering tip causing it to get hot by high current and 0.01 AC volts. If you were aware of how the ECU operates, you would understand......trigger pulses turn the switching transistor OFF, creating the spark and then turning the transistor ON to recharge the coil's primary winding for the next spark event.....it's really not a mystery.....the ballast resistor LIMITS the coil's primary winding current and voltage to prevent overheating and limits the current the ECU's switching must handle during the turn on time......
BOB RENTON

 

[Black_Sheep]

First thing I would check is voltage into the ignition switch. If it shows battery voltage then I’d check voltages at the outputs of the ignition switch in the crank and run positions. Trace the circuits and check for voltage drop at each connection. There should be full voltage at the coil + while cranking, and less in the run position.

 

[LowBikeMike]

The pickup coil and reluctor ring is a simple signaling device turning the primary on and off, building and collapsing a magnetic field. You can simulate that action with a soldering gun.

 

[Black_Sheep]

The pickup coil and reluctor ring is a simple signaling device turning the primary on and off, building and collapsing a magnetic field. You can simulate that action with a soldering gun.

So you can trigger the ecu to create a spark at the coil wire? I assume that the ignition system would have to be powered up. I’ve never heard of this but it sounds plausible. I’ll have to look into it…

 

[RJRENTON]

The pickup coil and reluctor ring is a simple signaling device turning the primary on and off, building and collapsing a magnetic field. You can simulate that action with a soldering gun.

I'm very much aware of how the reluctor/pickup coil operates in conjunction with the ECU. The strength of the induced trigger pulse, if too great, will damage the ECU's timing network that turns off the coil's current....you've been lucky thus far by not ruining the ECU with this method.....just thought you might like to know....
BOB RENTON