Posted by Alan Bowes on September 06, 1998 at 16:23:54:
I'm currently rebuilding two M37 distributors (at least they're SUPPOSEDLY M37 distributors). Both appear identical, except that one has a coil that is stamped "24V." The other coil looks identical, but does NOT have a voltage stamp. Checking these coils with a meter shows that they are NOT the same. The "24V" coil reads 12.2 ohms across the primary terminals and the unmarked coil reads 6.1 ohms (exactly half). This leads me to believe that the 6.1-ohm coil might be a 12V coil that was designed to be used with a 24V system in conjunction with an external ballast resistor that is bypassed during starting to avoid a weak spark while cranking the engine.
I'm not aware of any M37s that originally used a ballast resistor circuit. I've looked at M37 and M37B1 wiring diagrams and can't find such a circuit. So, I'm thinking that one of these distributors may have come from a different M-series vehicle that used an ignition resistor circuit. Any ideas? I'd like any information that anyone might have on this subject. I'm writing an article on M37 distributor overhaul and I'd like it to be as complete as possible.
Incidentally, if a bunch of 6.1-ohm coils have been mixed up with 12.2-ohm coils at some parts suppliers, it could explain things like coil failures, points that burn out too quickly, or why one coil might produce a better spark than the other (the 6.1-ohm coil would probably produce a hotter and higher voltage spark if used with the same primary voltage, although I wouldn't recommend it).
It also suggests that those of you who are interested in 12V conversions might be able to install a 6.1-ohm coil and keep the stock distributor.
By the way, both of these coils seem to produce a good spark when tested, and there is no sign of any damage, corrosion, or leakage, so I assume that these are good coils and that the resistance values are correct. However, I'd like to verify this, so if it's convenient, perhaps some of you could check a coil or two. Disconnect either (or both) of the coil's primary leads, then read the resistance across the primary terminals with a good meter (preferably a low-impedance digital meter, but any good meter will work) and post the results.
FYI:
The number on the marked 24V coil is CT-4002 24V 7G (Auto-Lite). This is the 12.2-ohm coil.
The other coil has no numbers at all (Superior Magneto Corporation). This is the 6.1-ohm coil.
Both of the distributor bodies are marked "Ignitor 24V", but one is by Auto-Lite and the other is by Bendix.
Thanks,
Alan