Alan Bowes wrote:
> The M37 doesn't use an external ignition resistor. The correct resistance
> across the coil primary terminals is (if I recall correctly), about 12 ohms.
> I'll measure a couple later today to be sure.
I checked two known-good M37 coils. They read 12.2 and 12.5 ohms across the
primary terminals, with the leads disconnected. This coil type is correct for
use with a 24V system without an external resistor, which is exactly how the M37
is wired. The M37 TM wiring and distributor diagrams do not show a resistor,
which corresponds with my own experience. The M37 did not use 12V coils. In
fact, one of these coils has "24VDC" stamped into the can; the other coil does
not have anything stamped into it at all; it's completely unmarked. The tags on
the distributor housings (which contain the coil) both indicate "24VDC." See
picture: http://todacosa.com/html/figure-11.jpg
As mentioned, the primary resistance test does not necessarily indicate if a
coil is good, since there are various types of potential problems, such as
secondary shorts or arcing within a coil, or intermittent shorts that may not
affect the primary resistance. However, it will tell you if you have the correct
coil or not, and whether there is an open or shorted primary winding.
If your coil seems to be working and you only obtained a 0.4-ohm reading, as
Richard had mentioned, the points may have been closed when you took the
reading. Even with the points open, having a capacitor (condenser) in the
circuit will also affect the resistance reading, since the capacitor has a
certain amount of internal resistance, which changes as it builds up a charge
from the voltage from your meter. So, to obtain an accurate reading, one of the
coil's primary leads should be disconnected (it doesn't matter which lead you
disconnect if you're measuring across the primary terminals). Picture of leads
and terminals at: http://todacosa.com/html/figure-19.jpg
A lot of identical-looking (but 6-ohm) coils were made for several other
applications, but these were either used in 12V systems or else they were used
in 24V systems in conjunction with an external resistor that was bypassed during
cranking. On one particular 24V distributor/coil unit that I purchased a couple
of years ago via mail order, there was a "hockey-puck" style resistor installed
underneath a 6-ohm coil inside the distributor housing. However, this unit was
NOT out of an M37. I don't remember what it came out of (perhaps an M151?) but
I'll look through some TMs for other vehicles to see if I can find out what it
came out of.
I have not yet run across an M37-style coil that used an internal heat-sensitive
resistance wire, although many American civilian vehicles were fitted with such
coils, especially during the 1960s and 70s.
If you wanted to use one of the 6-ohm look-alike coils instead of the original
12-ohm coil, you could modify the wiring to include an external ignition
resistor, in which case you would have to design the new circuit to bypass the
resistor just while cranking the starter motor. This would give you a hotter
spark while you're starting the engine. Then, after the engine starts, the 24V
(probably closer to 28V with engine running) to the coil would be diverted
through the resistor, which would drop the voltage to approximately 12V (14V
with engine running) at the coil to avoid overheating the coil while the engine
is running. This was a common approach for many vehicles, but not the original
M37.
If you're going to be doing any work on the distributor, you might find this
article helpful: http://todacosa.com/html/distributor-overhaul.htm
Allow a couple of minutes to download all of the images.
Have fun,
Alan
This archive was generated by hypermail 2b29 : Sun Dec 03 2000 - 20:29:57 PST
