Most of the good coils I have put a meter on show between 1 1/2 and 4 ohms of
resistance on the primary or low voltage side. A ballast resistor is used
somewhere in the circuit to limit the current. Some coils have a built in
ballast resistor. My M51 has an external ballast resistor to limit the current
from the 24 volt source. I have found that coils typically fry on the high
voltage side of the coil and often times you cannot tell that it is bad via a
ohm-meter unless the hv coil actually burns open. Coils are hard to test
unless they are actually driving a spark plug.
Dave
Richard Notton wrote:
> ----- Original Message -----
> From: "Lisa" <jeepcj8@home.com>
> To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
> Sent: Sunday, November 19, 2000 11:28 PM
> Subject: [MV] 24v Coil ohms test
>
> > Hi all,
> > Just had to replace a bad coil in the M37. Using an
> > ohmmeter, the bad coil showed 0.000, the replacement coil
> > borrowed from another distributor showed 0.402. Does anyone
> > know the proper range for a good coil? This one works, but I
> > would like to be able to test the ones I come across at swap
> > meets.
> >
> I know nothing of M37s, however, surely something wrong here. I've never
> seen a 24V coil either as the norm is to use a 12V one with ballast
> resistors allowing a useful boost on start-up.
>
> 0.402 ohms is not sensible since with a running engine and therefore
> charging battery some 28V + can be expected as a supply and this would
> result in a primary coil current of some 69.65 Amps, this can't be correct.
>
> Either the measuring instrument is way off or more likely perhaps the
> measurement has been done with the coil in-circuit which would mean the
> measurement includes all the parallel circuits of the vehicle. One or other
> primary lead must be disconnected to effect a correct reading.
>
> Maybe a typo also, 4.02 ohms is entirely believable and typical of a
> so-called 12V "sports coil", all examples of RR B Range Mil (gas) engines
> from fours in Champs through the sixes in Ferrets to the eights in
> Saracen/Saladin/Stalwart/432 have 12V coils of 4 ohm nominally and 4 ohm
> ballast resistor (in reality a pair of 8 ohm devices in parallel).
>
> Richard
> Southampton - England
>
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This archive was generated by hypermail 2b29 : Sun Dec 03 2000 - 20:29:56 PST