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Old 07-14-2019, 10:39 PM
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Mallory ignition coil test

I posted another topic in this forum labeled "fuel system questions" since my 31 ford model a wasn't firing (good voltage from battery, fuel going into carb, just not firing). After doing some more troubleshooting, I believe the problem i'm having is ignition related.

I have a mallory promaster coil 29440, with a MSD 8360 distributor.

For the coil, the specs say:
primary: 0.6ohms
seconday: 11.3kohms

The readings I just took show:
primary: 1.5ohms
secondary: 12.75kohms

Does the higher resistance on both primary and secondary indicate a major problem?

Also, what's the role of the ballast resistor? there's one mounted above the ignition coil - i'm still trying to figure out what all the wires go to, but only one side of the resistor is wired to something.

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Old 07-14-2019, 11:56 PM
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The resistor is a left over from points ignitions. 12 volt point systems have short point life due to arcing. So the system is really a 6 to 9 volt system. I say that around the fact that the key switch wires the coil in two different paths of cranking and another for running.

During cranking the starter is pulling a lot of amps which pull the B+ voltage from 12 to around 10 to 8 volts. At this time the key switch feeds the coil with full B+ voltage.

Once the engine fires and the switch moved to the 'Run' position the now 12 to 14 volts on the B+ with the generator or alternator on line is now shunted through a resistor to reduce the running voltage at the coil to somewhere between 6 to 9 volts so as to extend points life. The resistor can be in the form of a typical resistor mounted usually on the firewall, or it cam be a length of resistive wire from the key switch's run terminal to the positive side of the coil.

Most electronic distributors like GM's HEI use a full 12-14 B+ line voltage all of the time. So when switching these into an older points vehicle the resistor or the resistive wire needs to be rewired so the run terminal of the key switch provides the full B+ voltage to the HEI if integrated distributor and coil or to the coil if it is a small cap HEI and standalone coil.

If you're reading a different resistance across coil terminals, assuming the coil is not wired to anything, you can be seeing normal manufacturing tolerance, a coil not the same as specified by the original manufacturer, or a coil with some corrosion at the internal terminal connections. the coil can be simply checked for life buy putting a wire in the high voltage terminal that has a gap between if and the ground terminal. Then applying 12 volts the positive terminal with a jumper from the negative terminal to ground of the 12 volt source. Then when you break the ground connection the coil will discharge a high voltage spark from that terminal to the negative terminal as the magnetic field collapses.

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Old 07-15-2019, 05:28 PM
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Well, I tested the coil and it appears that have 12.63 volts to it. I put my positive lead on the positive primary terminal and then negative lead to engine ground....turn ignition switch on and read 12.63 volts.

Are there any other tests I can do before I have to rip out the distributor? Reason being is that I'm gonna have to remove the rear carburetor to really get any access to it. I'd like to rule out all other possibilities before that. I can reach the plug wires and potentially remove the cap without removing the carb but that's about it.

Is there anything else I can test or is it likely a distributor problem at this point? I have an msd 8360. If it turns out to just be the module, can that be replaced without having to purchase a new distributor?

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