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The old coil seemed to be shot, so I bought a new 6V coil from the only place around that stocks them, Tractor Supply. It's a "CountyLine" made, of course, in China. The box says it can be used on a 12V system with a resistor.

Put the new coil on and the car won't start or even fire. Spark at the plug gap seems weak to me. I always thought a 6V coil is a 6V coil, all the same. However, things change over time and the last time I bought one was at a real auto parts store in the 1970s.

Are there differences now? Will only certain 6V coils work on our Packards? Maybe a tractor coil is different somehow? Thanks for any info.

I can't speak for that particular coil but the NAPA IC-7 is a 6v universal replacement that works well in our Packards. If that other coil is a dual voltage possibly there is an internal resistor and those will not work well.

Do a voltage measurement at the coil power side terminal as measured to ground just to verify it is receiving proper voltage. Sitting with key on and points open it should read within a tenth or so of battery voltage -- right around 6.3v. With points closed it may drop another couple of tenths but still close to 6v. If that is good then the wiring connections and ign sw is OK.

Just to rule out points or dist issues measure the other terminal too. Points open it should also read close to battery voltage and with points closed, should be almost 0v. If it stays high points are not closing or dist has lost its ground. If it stays near 0v there is a short somewhere in dist. if the low voltage is not near 0v there is a poor ground to the dist or bad connection in the points.

The important voltage is during cranking. Ideally the voltage at coil should not drop much lower than 5.4-5.5 v. If it gets close to 5v the spark will be very weak and may not fire the plugs. Possible causes for a large voltage drop is excessive current draw from a weak starter, too small of battery cables or dirty connections. If the battery is low or getting tired and not able to maintain the large current requirements of the starter it could also drop the voltage.

Thanks Howard, I'll check those voltages out tomorrow. I just discovered the coil wire must be a resistance wire as it measures 5000 ohms. Would that affect the spark? (All the plug wires show no resistance.)

A resistor wire will affect the spark. A 6v spark does not have very much to offer in the first place and any extra resistance makes it worse. Packard did not use resistor wires or plugs but some distributors did have a small resistor on the rotor. Ignition noise was controlled by capacitors in Packards so the wires could be solid conductors.

Interesting about resistor wire and 6V and Packard using capacitors instead for noise suppression. I figured 5000 ohms might cause a voltage drop but, since so many cars routinely use resistor wires, wasn't sure that would contribute to the problem.

I just went out and checked the voltages at the coil. Switch side is very close to battery voltage, points open or closed. Distributor side is battery voltage with points open. With points closed, the meter reads one volt. I connected the distributor body to ground with a length of wire and the meter dropped to zero volts. The grounding of the distributor is evidently not completely solid. Would this seemingly small resistance significantly weaken the HT spark?

Cranking the engine with the meter between the distributor side and ground, the reading drops from approx 7 V to 5.5 V. (Maybe I should be measuring this on the switch side?)

I'm using an old needle type meter, so readings are not as precise as digital.

The lack of a solid ground will affect the spark and should be corrected. The coil never reaches full saturation if the voltage remains at a high level.. The voltages sound good otherwise so fixing the ground and replacing the resistor wire should give a decent spark.

The cranking test should be done on the voltage terminal as you are probably reading a combination of reverse emf and capacitor charging on the distributor side terminal. The voltages mentioned do not sound too far out but I would reverify on the other terminal to be sure.

Cars will run just as well with resistance ign wires as with metal core. Delco equipped 6V Packards and other make had a resistor in the rotor in about 53 and 54 and all 12V 55-56 v8's. We have worked on hundreds of 6v cars, some equipped with radio resistance ign wires, some with resistor plugs, some with resistors in the rotor some with all, some with neither. They all started and ran fine in all conditions.

In my experience no real problems with resistor ignition wires on 6-volt but what you really want to avoid is having a combination of both resistor wires and resistor spark plugs; if radio reception is a problem you must overcome, then one or the other but not both. If radio performance isn't an issue, then neither.

It has been pointed out that I used a poor choice of words when saying a 6v spark does not have much to offer. That is correct - I did use a bad statement. A 6v coil and spark will be very satisfactory as long as everything else is in top notch condition. But, unlike a 12v system, there is no wiggle room to compensate for any problems with a 6v system.

The average 6v coil typically has a spark output of around 25K when operated at the proper voltage. Any voltage loss to the coil due to a bad wire or connection, a large voltage drop from a low battery, too small of cable, excessive starter current draw or, in this case what is appearing to be a poor ground at the distributor will seriously affect that output number. 12v systems are typically running about 8v at the coil because of the ballast resistor. With a higher voltage available up front and generally a higher 35K or so output from the coil they can tolerate some voltage drop and still produce an adequate spark at least equivalent if not higher to what a properly operating 6v coil would produce at its maximum best. To compensate for starter current draw most 12v systems also bypass the ballast resistor during starting to provide 12v to the coil for an even hotter spark when it is most needed.

The one resistance wire by itself may not affect the outcome and it could possibly be an equivalent to an inline suppressor Packard added in that cable when a radio was installed. Until the spark and starting problem is resolved I would change it for now just in case it has a problem with the resistance conductor. Once the starting problem is found and car is running you could reinstall it to see if there is any difference in starting or on the radio with and without it.

Don't assume things. Check the old coil to make sure it's bad. There's ohm readings, primary and secondary, and continuity checks to confirm the coil inside wiring is sound. Beyond that there's just some iron that helps induction. No moving parts. If it checks out, it wasn't the coil. Test the new one similarly, if old one fails.
There's no resister in/with the early coils and according to the TS site this coil apparently doesn't have one, as it works in either, so should work. Since it doesn't - probably not the coil, if wired correctly.
I'm concerned your "coil wire" that measures 5000 ohm, is simply burnt, not a resistance wire. V=I*R So V = 6 and if R is 5000, not much I. V*I = Energy/Power. 6*(6/5000)=0.0072 VA
If the primary doesn't get the proper energy, then the secondary can't boost the voltage high enough to produce spark.
ps: A 12V system with a ballast resister is essentially running a 6V coil. So that's why this would work for them too.