Hello Lisa: 24v Coil ohms test
From: Lisa (jeepcj8@home.com)
Date: Sun Nov 19 2000 - 15:28:59 PST
Re; Test or measured ignition coil resistance at .402ohms
Firstly, somewhere around I have a pocket tester, homemade device to check a
coil at a swap meet. It uses two 9volt batteries worked great. If imterested
I could look for building it instructions ... real easy two 9v batts, one
momentary switch, one condensor, 2 clip leads, one short coil spark wire.
Let me know for more info.
1] Re your testing (.4 ohms) that may have been accurate... however, read
below
There could be other "things across the circuit" that gave such a low
reading... as good 'ol King Richard Notting (from over the pond) suggested.
2]
If the coil was still in the vehicle there is a possibility that the wrong
side or the coil were measured showing a reading to ground ...also, through
a charged condensor (if points were open) or grounded points?
3]
We don't know what type of meter was used to measure the .4 ohms
I have found many meters to be surprisingly inaccurate... even depending at
what angle they are held at?
What does meter angle matter???
Well, In the old style meters there is a balance system. This is often
inaccurate!!!
To test your meter accuracy, try this....
Turn the meter from the off position to "any other position" ...not all
meters (cheapos) have an "off" ?
With the meter on the table very carefully, set the meter movement's zero
screw... to accurately point to read zero.
Slowly lift the meter (on it's side) so that the needle constantly STAYS
horizontal or parallel to the ground. If it is out of balance, the needle
will tip up or down off the zero point
Next , start on the table again... turn the meter so that you can lift the
meter up in front of you, and keep the needle totally vertical or
perpendicular to gravity. Does the needle move Off zero??? if so it is an
out of balance meter...
But depending on this information your reading could be "out" even 3/16th of
an inch like one of mine is.
THE FIX ...once set to zero, keep the meter in the same plane in respect to
gravity.
4]
Was the meter OHMS Zeroed, (little thumbwheel or knob) correctly for ohms
readings
Was the battery in the meter low or dead?
What was the scale reading? ...or,
Was it a digital meter? (usually very accurate)
The COIL
From my "coil" experience I have measured the current in numerous "12v"
coils over the years and found that when running, the current / amps flow
was around 1.5amps approx!
When not running the current with the points closed was around 2.5amps or
higher
I have also discovered that many manufacturers installed coils that were
designed to run on 8-10volts and if you replaced the coil, it must be with a
same operating voltage type ...possibly, most probably designed with a
ballast resistor in series with coil?
The BALLAST
Due to a design feature, the ballast "is" inserted continually "in series
with the coil" ...if the engine is running or not. However, when hitting
the ignition switch's "start" position... the ballast was "electrically
removed" from the circuit. Giving the full shot of juice from the battery to
operate the ignition coil & spark ...at a maximum voltage available, at the
time, from whatever was left or available at the battery terminals during
the "actual starting mode"
This design allowed for a "good or best availabe spark" an adverse
conditions like a very "cold start" or a "weak" or "dying" battery etc.!
Why does the Coil Get VERY HOT with engine not running?
This depends on the points being in their OPEN or CLOSED position. This also
depends totally randomally when the engine was last turned OFF, just what
position the points were left in ..."were they opened or were they closed?
When left in the CLOSED position higher amps flow into the coil... then the
coil can get very very hot indeed! Not too good!!!
COIL does NOT get too hot when engine runs! SO,
Why or how, would the coils resistance in ohms seemingly change from when
the points are "closed" (Ign. ON - engine sitting stationary)... as compared
to when points start "opening and closing" (engine turns over and runs)
Well, the coil resistance ...does not change. The coil itself does not
physically / magically change its resistance, measured in Ohms, ...or does
it?
What happens here, occurs at the "speed of light" nearly "too" fast to
imagine!
During the absolute instantaneous moment when the points close, a current
starts to flow through the copper wire which is wound around the steel core
inside the coil unit.
For this argument... lets say there are 200 turns of wire in the low voltage
coil within our Ignition coil unit.
At the exact "moment of time" that the points close... current (amps)flows,
and a magnetic flux occurs around the first winding / turn of the coil...
(hence, an electro magnet)
This current then flows into the next (2nd.) turn of the coil, and the next
and so on... all happening at the speed of light! Very very fast here, ok!
Now as the current builds in the windings in turn number 1... a magnetic
flux appears around that turn. Now, laying right next to turn number 1, is
turn number 2
A very strange /funny thing happens now!
Winding or Turn number 1 induces a tiny voltage into turn number two. "in
adition to, but regardless of the voltage coming into the coil from the
battery....
Turn number 2, NOW has its very own little voltage "induced" from turn
number 1
...and this little voltage is in the opposite direction of the voltage
coming in from the battery !
However, now, This little voltage "IMPEDES" the incoming voltage as the two
"tend", to meet head on... and now as the current builds and flows in turns
numbers 1 & 2 ...this Impedance now starts to occur in turn number 3 and 4
and 5 etc., right up through to the end of the winding at turn number 200
This action, is called ...induction ! Whereby one turn starts to induce a
voltage in the next... and so on! Again each one's induced voltage, is in
the opposing direction of the previous winding or turn of the coil. This
INDUCTION creates what is called "IMPEDANCE" and we measure it in OHMS just
like resistance.
The INDUCTANCE "only" occurs when the points close and, at any instant of
time, the magnetic flux builds within the coil.
However, it also "Only" occurs at a second instant in time, that is, the
moment current-amps flow from the battery, stops. Which is, when the points
OPEN ! ..and then again, the flux collapses. It's the changing flux that
does it.
So, it is the rising and falling of the magnetic flux in each turn or
winding, that causes the Induction... and this in turn causes a higher
Impedance (ohms) ...and this higher impedance (ohms) allows for the
lower amps in the coil when the engine is running !
Therefore, "normal" coil temerature... No excessive heat No breakdown !
The interesting fact here is proven that when you leave the ignition On
...and the coil get hot. Remember, that impedance Only occurs when the
magnetic flux rises and falls, all very quickly at the speed of light.
NOW next, & this is important for my description... once the flux in each
turn has risen to a stable direct current flow ...and there is NO points
opening and closing, then we lose the effect of the (higher ohms) IMPEDANCE.
So, this coil becomes quite Hot because there is now a constant higher flow
of amps through the coil ...no rise and fall... no impedance ...just a low
resistance ! Therefore, we revert back to the original term of resistance of
the coil winding, in ohms. This resistance is too low to maintain a normal
operating temperature, and therefore your coil will become quite hot.
Remember, a lower resistance at a given voltage means a higher current
flow - amps!
Sometimes, If you replace an old coil with any other "old" coil that
works.. later, you might be in for a failure down the road ?
Some vehicles were disigned without a ballast resistor. Using a 12volt coil
a vehicle "was" designed NOT to have a ballast resistor in series in the
circuit. If you used "any old coil" that works... this coil could be an
8volt design... with a ballast resistor supposedly taking up the rest of
the voltage. Remember also, a 12v. battery voltage is actually approx
13.8volts.
Finally, yep we're near the end, RESISTANCE (ohms) comes into play
"generally" when we have a smooth current flow from a battery = Direct
Current ...for instance flowing through a small coil to hold a relay
armature closed. While IMPEDANCE comes into play when we constantly
interupt the flow (Ignition points opening & closing rapidly) or as in house
current which is alternating at say 60hertz per second... the current goes
one way turns off, then goes the opposite direction, then turns off and so
on it goes causing a magnetic flux to constantly be in a state of rising and
falling change.
This causes work to be done through a LOAD.
This occurs in your Ignition Coil... in your electric welder... & in part,
in your stereo / tv... in your street's transformer... and these are known
as "Inductive" loads...
But, in your household light bulbs, electric stove ring, elctric water
heater or clothes dryer, the same alternating current is not induced to any
degree, ...and this is called a "resistive" load
Hey, this Resistance, in ohms ...and impedance, in ohms ...and inductance,
is something I find fascination, yet probably quite boring for others.
Hey, electrical guru's correct me I may have wronged or missed something
here ...hope not, but I'm just one microfarad away from sleep.... and will
send this later, I think!
Robbo
'42 MB
----- Original Message -----
From: "Richard Notton" <Richard@fv623.demon.co.uk>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Monday, November 20, 2000 3:16 AM
Subject: Re: [MV] 24v Coil ohms test
>
> ----- 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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