A Poor Man's Mag Tester
by Tom Carnegie
You've probably heard someone say something to the effect "my mag puts out 'x'
number of volts". You probably also know
that this doesn't mean too much. At what
speed does the mag put out 'x' volts? Does
the number of volts really even matter?
Another popular mag test is to attach some
load (such as a small lightbulb) to the mag
and then somehow judge its condition,
maybe by observing how brightly the bulb
glows or maybe by how many volts the
mag is putting out. Again, the measurements are useless at worst and subjective at
best. There are better ways to test the efficacy of a mag. One of these is with a mag
tester such as the one described in Dykes
manual. (see next page) This is a very
handy instrument if you can come up with
one. If you can't, there are ways to obtain
similar meaningful results.
The Ford magneto can be tested with a special magneto tester.
This instrument is nothing more than an ammeter, but one provided with a “reactance coil”
which enables the meter to indicate a constant current at all speeds. In other words, the
meter is so designed that it will indicate if the magneto is giving its proper output at
any speed of the engine, while testing. This instrument can be used to test the strength
of the mag.
Method One:
Items needed:
An extra model T ignition coil with a good primary winding
0-5 amp AC amp gauge
Simply short the points on the coil (or block them closed) and hook the coil into series with amp gauge from the mag post to ground. Start and run the T on battery. A typical T mag will put out three or so amps in this configuration. This is regardless of engine speed, once the mag reaches sufficient power.
Method Two:
If you don't have an AC amp gauge method two may be for you.
Items needed:
An extra model T ignition coil with a good primary winding 25 feet of 18 gauge soft steel mechanic's wire (about .045" in diameter) A piece of wood dowel. (such as an old broom handle, or a model T wooden spoke) Masking tape 0-5 volt AC volt meter 25 feet of mechanic's wire has a resistance of about one ohm. I wrapped the wire around a T spoke. When I finished making a layer I covered it with masking tape, then made another layer.
When you are done you have a huge one ohm wire wound resistor. A normal one ohm nichrome resistor won't work well for this as it won't be big enough to dissipate the current going through it. There will be upwards of 50 watts of power that the resistor will need to dissipate.
Again, while running the T on battery, hook the resistor in series with the T coil from mag to ground. Then with the AC voltmeter, measure the potential across the resistor. This value (registering as volts) will be very close to the amps flowing through the circuit. The amps will not change much with the speed of the motor.
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