MTFCA Forum

I know that y'all will be able to tell me what magneto output should be when hand cranking the engine. I am getting around a volt which seems like it is probably way too low. I haven't yet tried to recharge the magnets but I wanted to know what the minimum output should be at handcrank speed. By the way, I checked it will engine compression. I could certainly get a little higher reading with the plugs out.

I believe you are correct, Steve. A brisk pull ought to give a peak around 6 + volts. I assume that is with the ignition switched to MAG and the timer making contact. (?)

Since coils fire at approx. 1.3A regardless of voltage, voltage high or low at cranking speed is not an issue worth debating (but don't let that stop anyone). FWIW - Voltage created by hand crank will be around 2V and it takes a brisk idle to reach 5-6V.

If the car will run on MAG, then things are generally fine...many cars run on MAG and are pissers to start on MAG. That is due to any combination of 3rd main wear, magnet strength or lack of strength, and proper timing (or not).

If I had a question about anything, it would be to question the methodology of measurement that produced a 1V reading on a single pull in the first place.

Voltage must be sufficient to deliver 1.3 A .

Voltage must be sufficient to deliver 1.3 A .

exactly

hence, 2V at cranking speed mentioned above, which is pretty much what a good magneto will put out (at that speed).

6-7V is what you'd expect at a brisk idle (approx. 200 RPM per FORD) and quite frankly if you guys are spinning your car to start at the speed of a brisk idle, I am feeling quite emasculated right now.

I'm not about to try to spin mine with the ignition on.

By spinning, do you mean cranking over multiple compressions Pat? Single pulls over one compression are not a problem, unless the timing is incorrect.
How else can you hand crank start a T without the ignition turned on?

Allan from down under.

In my world, a "pull" is the usual method of starting the engine with the crank. I retard the spark fully on battery, or move the spark lever down several notches to start on MAG, and engage the crank and give a sharp upward pull on the crank from about the 7 o'clock position. "Spinning" involves grasping the crank and revolving the engine several full turns or more. I have not found it necessary to do that to start the engine.

The '26 engine I just pulled from my 16 would run on mag but would not start with properly tuned coils and 40psi compression across all 4 cylinders. Perhaps it is due to the .026" play in the 3rd main.

Less end play is better, but other factors also affect whether a T will start on magneto or not. The condition of the spark plugs and the spark plug gaps will affect starting, as will the condition of the timer and the adjustment of the spark advance lever. Dirty or thick oil will make starting more difficult. Any problems with wiring, the magneto terminal contact, or the ignition switch can make starting more difficult. Dragging bands or a dragging clutch will make starting more difficult.

colonelpowers wrote: ↑

Tue Feb 17, 2026 10:37 am

I know that y'all will be able to tell me what magneto output should be when hand cranking the engine. I am getting around a volt which seems like it is probably way too low.

If there are any other Model T owners in your area, see if you can borrow a Fun Projects replica "St Louis Magneto Tester" or Mike Kosser's ECCT with the software to check the magneto output. I can speak from experience, with the replica tester, that it is a great way to determine how healthy the magneto is on your engine. If the reading spikes to the 1914 level while doing a brisk 1/4 turn by hand, then the car should easily start on magneto by hand. I have used my tester when rebuilding T engines after I charged the magnets and got the magneto gap set within Ford limits to make sure that magneto was working properly BEFORE finishing the engine assembly.

With the car running at idle, I get a reading closer to the 1915 reading. I believe for 1915-1916 years, the magnetos put out more because the headlights at that time were also powered by the magneto. That is why there is a higher mark labeled 1915. I seem to recall that later cars that used the generator for lights had reduced magneto outputs, perhaps closer to the 1914 and earlier cars.

A number of questions have bugged me for years with respect to hand cranking a T. First, what current/voltage is required to create a spark. Second, what cranking speed is needed to attain that event. Finally, how could you ascertain what speed that is when starting with a quarter-turn. My observation while playing with the "Franken-Tester" is that a consistent spark is created at 2.5 volts on all four coils. I understand the statement that coils fire on amps rather than volts but it seems to me that a more accurate assessment would be that the combination of volts and amps (watts) would be more telling. 1.3 amps at 2.5 volts would produce a power of 3.25 watts. Again, my playing has suggested that a cranking speed of somewhere near 100 rpm would be necessary to meet that threshold which doesn't seem possible, let alone safe. A quarter turn start on mag would be out of the question, yet a good running engine will start on a quarter turn with some reliability. I suspect that the quarter turn begins at zero rpm, increases through 90 degrees and if the engine doesn't fire, ends back at zero rpm shortly after 180 degrees. I think that somewhere around the 90 degree mark the magnets are passing the poles fast enough to create the spark even though the speed at that point would be very difficult to calculate, and would only be long enough to create a single spark or maybe two as the pulses would only occur at 22.5 degree intervals. I would be interested to see what the results of an accelerometer attached to the flywheel would be but that is somewhat beyond my pay grade. Your opinions?

The St. Louis Magneto Tester or ECCT Magneto Test are proper ways to judge the health of the magneto to properly operate Model T coils at normal engine speed.

The discussion of voltage, current and power (watts) in regard to static (DC) conditions in insufficient. The detail omitted from the discussion of magneto actuated Model T ignition coils is the rate of change of coil current in the primary winding with respect to time (denoted di/dt). If the crank is turned slowly, the coil primary current (and associated magnetic field) both build up slowly (low di/dt). The magnetic pull down on the vibrator spring builds slowly where the vibrator spring tension upward and vibrator contact inertia are nearly balanced. The contact moves slowly (if at all), which increases the contact break duration of contact sliding effect resulting in arc (reduced spark energy and timing delay/retard when cylinder compression is lower and less likely to initiate combustion), higher contact erosion, and increased risk of contact welding (bad).

An abrupt rigorous crank pull, on the other hand, causes a rapid surge in current (high di/dt) producing an almost instantaneous, high-magnitude magnetic flux across the air gap pulling on the vibrator spring contact element. The fast rise generates maximum magnetic torque/force on the vibrator spring before mechanical inertia can counteract it. The fast current rise time (di/dt) ensures the vibrator spring reaches the contact separation threshold rapidly. This results in high acceleration of the movable vibrator spring contact, leading to a fast, clean break with minimal electrical arcing (timing delay and loss of energy). This is particularly important at normal engine speeds and the benefit of proper coil tuning for equal and consistent firing time by actually measuring the time to fire spark.

Voltage and current are not the same thing, but they are interrelated in most any electrical device, and the interplay of voltage and current can be complex in inductive and capacitive devices and circuits made up of inductive and capacitive devices.

You raised an interesting question of firing voltage & cranking speed. So off to my HCCT for a test. I know there is a resistor or similar thing built into the unit so results from an engine magneto may vary.
I have modern analog ac volt meter & ammeters on the unit.
Using a freshly tuned ECCT coil, Sparks started @ 1 volt. The meter is 10 V, so accuracy @ either end of the scale is not necessarily accurate.
Cranking speed was quite slow, about 1/2 of a brisk starting pull.

Early high tension magnetos were often fitted with an "impulse coupling" device, which was a spring-loaded trigger arrangement that compressed a spring or springs as the engine was cranked, and at the appropriate point, the trigger would release the springs and snap the rotating magneto armature rapidly forward, producing a surge of current to fire the coil. When the engine picked up a little speed, the impulse device would automatically disengage. Most reciprocating magnetos also have some sort of spring and trigger device to get the armature moving quickly enough for reliable starting. The design of the Ford low tension magneto precludes using such a device, so the operator is obliged to get the magneto armature (which is also the flywheel) moving fast enough to operate the coils satisfactorily. The large diameter of the Ford magneto assembly makes it effective for starting the engine at hand cranking speeds, provided the system is in good condition in spite of the lack of an impulse device. The older style lawn mower engine type magnetos were very similar to the Ford T magneto, with the magnet and primary coil located at the periphery of the flywheel.