Military-Vehicles: Re: [MV] Ignition Coil

Re: [MV] Ignition Coil

Andreas Mehlhorn (
Sat, 18 Oct 97 23:44 MET DST schrieb:
> I was told that all coils need a resistor, either an internal one or
> external. Both my WC and M-37 have an external ceramic resistor.
> I've noticed this drops the voltage across the primary from 12v to 6v,
> although the coil is listed as 12v.
> Can anyone tell me why the guy who sold me the 12v coil insisted I not remove
> the external resistor?

The resistor helps to get a better spark when starting, but therefore you
need a starter switch with an additional contact. It works as follows:

In a normal 12 V system the ignition coil is made for 12 V. When you start
the engine, the starter motor takes a lot of current and the voltage goes
down to 8-10 Volts. If you feed a 12 V coil with only 8-10 V, the spark
becomes weak.

Now comes the high performance coil with the external resistor. This
coil is made to make
a good spark with 6-8 V after the resistor and 12 V before the resistor.
In normal use you connect 12 V to the resistor, which makes 6-8 V for the
coil. You may not remove the resistor and connect 12 V direct to the coil
for a longer time, this will heat the coil and damage it.

When you start the engine, the voltage goes down. A special contact in the
starter switch connects the power supply as long as the starter runs direct
to the coil (behind the resistor). Now you have a strong spark, because the
coil without resistor is built for a lower voltage. Starting is only a short
time and the voltage is reduced, so the coil gets no problem with the heat.

To use the better starting of these high performance coils, you need a
special starter switch (or a relay) and a second cable for power supply.
If you only have one single power cable to the coil and a second cable to
the contact breaker, the coil works only like a normal coil without
starting spark enhancement.

Andreas Mehlhorn

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