We have an intermittent problem. Mallory dual point with mopar coil and correct ballast resistor. We have 12 volts at ballast and 6 volts out to coil.
Checked resistance at coil...all good.
When it is running we have 9 to 10 volts on positive coil terminal and 5 volts on negative but when it runs bad it has 13 plus volts on negative side of the coil.
What is causing this?
Ballast coil points condenser are all new.
Please help as it is driving me crazy.
All the best frank.

it had this problem & you replaced the parts you mentioned & no change? or did the problem develop after you got it running and in the process of getting it running those (ign) parts were replaced? or were there any part changes immediately or pretty close to the onset of the issue? We're missing something as there could not be 13V at the coil neg primary especially with it running. Holler back when you can & we'll continue on

It had the problem and after changing these parts one at a time it came good for three days and then the problem came back. I thought I'd fixed it with the new ballast resistor but problem came back after 3 days. Now that we've looked into it more we found that it runs fine for about 5 to 10 minutes and then the negative side of the coil reads battery voltage and it starts to stumble. The timing light shows really irregular flashes when it goes bad.
What do you think is happening?

The points are the path to GROUND. Have you made sure you have a good ground path from the breaker plate to the engine block ?

it kinda sounds like the coil is shorting (even tho its new/it does happen). I hate to suggest shotgunning parts onto a vehicle but do you have another coil in your stash or even one from another running car that you can sub in for a quick test (wouldn't cost anything just some time). EDIT kind of WAG on this one but I wonder if a condensor could do that. MORE EDIT JohnRR may have nailed it. holler what it is, I love these toughies

Tried three different coils...two new ballast resistors now going to try voltage regulator and maybe another new condenser. We have good ground at distributor plate and housing. Can ignition switch be an issue if it's intermittent in the run position?

it ain't the reg. I would swap the condensor. Do you have another point dist you could toss in for a quick test? EDIT just a side note, with some thought I think the 13V reading is just the slow reacting characteristic of a digital volt gauge

Fixed...tachometer could be faulty and sending power back to the negative side of the coil. ..disconnected the tachometer and it hasn't reaccured...anyone know if this is possible or had a same issue with tachometer.
Remember that it was intermittent so crossed fingers.
All the best frank.

Yes I've heard of that being it one time. You might open up the tach & see if you see how it is bleeding over improperly cuz if it was just an open (at least on the coil sense wire) it'd be the same as disconnecting the tach which you just did. keep us posted

I hope you got it as I have seen tach's cause problems but usually what I see is a tach shorts to ground which would drop your volts and keep the car from running. You have to remember that when the points are open you will see 12 volts so the voltage you read at the coil with the eng running is an average of the points open and closed. If you turn the ign key on with the eng off and the points closed and then read the voltages in the ign system you should get 12 volts at the ign side of the ballast. About 6 or 7 volts on the coil side of the ballast and the coil + terminal. It should 0 volts on the coil negative terminal and at the points. Now tap the eng over until the points are open and it will read 12 volts at both sides of the ballast and 12 volts at the coil pos and neg terminals and 12 volts at the points pos side. Thats because it has no ground with the points open so when the eng is running and you read the volts at the coil terminals its reading and average at the pos side of the 6 or 7 with the points closed to the 13 to 14 with the points open and eng running. When you get 13 on the coil neg side it tells me either its loosing its ground or maybe the tach is shorting voltage to the coil neg side and putting 12 volts to the coil neg terminal. Lets hope that was it and you have it fixed. Ron