The Ghost in the Machine - voltage drop in the ignition system?

Can anyone help?  I bought a 1966 38 Commander Sport Fisherman back in October.  The boat had been refitted with Twin rebuilt Chrysler 360s about 7 years before I bought it.  After the engines were installed, the boat was water tested, then removed from the water for cosmetic work.  It subsequently sat idle for the next 7 years.  Needless to say, I got a good deal from the marina I bought the abandoned Commander from.  After purchase, I had it moved to my local marina yard and began the process of getting it water worthy again.  All was going well until I couldn't get spark from the coil in the port engine.  Here's what I've done:

1.  I removed all the spark plugs and put a few drops of Royal Purple in each hole.

2.  I turned over each engine by hand.

3.  I compression tested all 16 cylinders.  All tested well.

4.  I changed the oil to new Royal Purple.

5.  I replaced all the spark plugs and wires.

6.  I replaced both fuel pumps and filters and all the fuel lines from the tanks to the carbs.

At this time, I test fired both engines.  

The starboard engine fired right up and purrs.  

The port engine had fuel leaking out of the carburetor in several spots.  To save time, I bought a new, tested, Edelbrock 4 bbl carb and replaced the original.  The only difference between the carbs is the new has an electric choke which I wired directly to an unused switched fuse.

Then I test fired the port engine again.  NO SPARK from the coil.

This is when things got interesting.  Since that time I have tried the following:

1.  New cap, rotor and magnetic pick-up (properly gapped).

2.  New ballast resistor.

3.  New Coil.

4.  New solenoid.

5.  I broke out the wiring diagram from the M360 and from the mallory magnetic pick-ups and made sure everything was wired correctly.  I found a couple discrepancies and corrected them with no results.

6.  I have tested voltage at every location I think of and here's the quandary:

        When I disconnect the ignition wire from the coil I get 10.5 volts to the wire with the key in run and 12.5 volts when cranking.  If I reconnect the wire, the voltage to the positive post on the coil is about 3.8.  If I disconnect the wire from the negative post of the coil that runs to the ignition module the voltage on the positive side jumps back up, but with that wire disconnected the engine won't crank at all.

So, I replaced the negative wire to the ignition module with a brand new 14 gauge wire thinking it was causing resistance.  No change.

So, I tried taking the ignition module off of the starboard engine (that runs so nicely) and putting it on the port side.  No change.

So, I tried getting a high powered cranking battery and blasting it with amperage hoping to overcome the voltage drop.  No change.

I've run this by a few mechanic friends and nobody can figure it out.

Has anybody else ever run into a mysterious voltage drop in the ignition system?  If so, did you figure it out and how?

Or, does anyone have any suggestions or troubleshooting tips?

Thanks,

Baron

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Does this system have a ballast resistor? If so, you would see voltage differences between the run and start positions. I assume at this point you know you are not getting spark? Just had to ask. 

I just reread your origianal post, I thought I saw ballast resistors on your list of replacement parts. Also read that you were dopping from 12.5V to 3.8V. The resistor should only drop voltage down to 7-9V. Just an observation from n old Mopar nut. I suspect your new bllast resistor might be bad.

Check this out: https://www.forabodiesonly.com/mopar/threads/voltage-drop-at-ballas...

Some of the same troubleshooting applies. I would first hot wire it at the engine to confirm all is OK at that end. I would be looking at any connections between the ign switch and the coil. 

You mentioned that you have Mallory magnetic pick-ups. I recently went through a situation where the engine would only run for 30 minutes or so and then start running rough and stall. The final resolve was first to make sure all the components, resistor and coils, were the right match for the Mallory pick-ups. This took a trip to a local speed shop - Mallory dealer. We learned that the resistor should be .8 ohms cold and 1.6 ohms hot and the coil primary windings should be 1.4 ohms. The total resistance of the system ahead of the pickup should be 3 ohms. We found that in our situation the resistor was at 3 ohms cold, so when it warmed up it went out of tolerance and shut down the engine.

Based on my experience with the Mallory magnetic pickup, I’d recommend that you check the resistance of the resistor to insure it is .8 ohms cold and the coil has 1.4 ohms on it’s primary winding. I don’t know how many people we talked to that said resistors don’t go bad, but we found that’s not true as both of mine were bad. You surely could have gotten a bad resistor out of the box...

Great tip!  I may have never thought of that.  I'll test the resistance on both.  I'm betting they're high.  That would explain the voltage drop.  I'll let you know how it goes.

Daniel M Harrison said:

You mentioned that you have Mallory magnetic pick-ups. I recently went through a situation where the engine would only run for 30 minutes or so and then start running rough and stall. The final resolve was first to make sure all the components, resistor and coils, were the right match for the Mallory pick-ups. This took a trip to a local speed shop - Mallory dealer. We learned that the resistor should be .8 ohms cold and 1.6 ohms hot and the coil primary windings should be 1.4 ohms. The total resistance of the system ahead of the pickup should be 3 ohms. We found that in our situation the resistor was at 3 ohms cold, so when it warmed up it went out of tolerance and shut down the engine.

Based on my experience with the Mallory magnetic pickup, I’d recommend that you check the resistance of the resistor to insure it is .8 ohms cold and the coil has 1.4 ohms on it’s primary winding. I don’t know how many people we talked to that said resistors don’t go bad, but we found that’s not true as both of mine were bad. You surely could have gotten a bad resistor out of the box...

Even the mechanics at my favorite yard seemed to want to check things with the volt meter, looking at voltage readings. Yet in theory, the voltage readings are actually the result of the resistance in the circuit. When I saw the ohm ratings of the Mallory components in the Mallory book, it surely made more sense to go looking for resistance readings than looking at voltage readings and wondering what caused them.

Now all that said, if you find that your resistor is the correct .8 ohms cold and the coil primary is 1.4 ohms, then I would recommend getting good clean power to ignition right at the beginning of the resistor. There could be a large voltage drop being caused by other things in the circuit. I found large drops across the connector plugs in the dashboard and simply ran new wire directly to the input on the ignition key switch to eliminate it. Also, I had to, over the years, replace key switches themselves to eliminate voltage drop. Hopefully you'll have some success and get this behind you. I know it was quite a relief once we finally changed out the resistors and coils with the correct Mallory matching parts and the problem was solved for good! 

Is there an old Judson synchronizer light on board?  When I retrofitted my 454s with Delco EST ignitions this spring I had to remove the Judson, as it was wired in series with the ignition circuit and causing a significant voltage drop.

Daniel,

Your suggestion worked out perfectly!  I have spark!!  Now, I believe there must be a timing issue, because I have spark fuel, air and compression, but it doesn't start.  I've been nearly 4 months trying to get spark from the coil though, so this is a big day.  Thanks again for the tip!

Daniel M Harrison said:

You mentioned that you have Mallory magnetic pick-ups. I recently went through a situation where the engine would only run for 30 minutes or so and then start running rough and stall. The final resolve was first to make sure all the components, resistor and coils, were the right match for the Mallory pick-ups. This took a trip to a local speed shop - Mallory dealer. We learned that the resistor should be .8 ohms cold and 1.6 ohms hot and the coil primary windings should be 1.4 ohms. The total resistance of the system ahead of the pickup should be 3 ohms. We found that in our situation the resistor was at 3 ohms cold, so when it warmed up it went out of tolerance and shut down the engine.

Based on my experience with the Mallory magnetic pickup, I’d recommend that you check the resistance of the resistor to insure it is .8 ohms cold and the coil has 1.4 ohms on it’s primary winding. I don’t know how many people we talked to that said resistors don’t go bad, but we found that’s not true as both of mine were bad. You surely could have gotten a bad resistor out of the box...

Have you just tried a straight jumper wire from battery + to ign coil + yet? This is the easiest diagnostic to insure it’s not an engine issue!

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