The Fordson F Ignition System
1917 - 1927
By Anthony West
A fellow restorer contacted me earlier this year asking for some help and
advice on a model F that he was restoring. He had over a period of months
spent a fair amount of his hard earned cash on replacement parts for the old
"trembler" ignition. Sadly though all his efforts seemed to be a waste of time
and money as he still couldn''t get the temperamental old thing to run
If i said that this was a little frustrating for him that would be
"conservative" in fact the problem had reduced him to tears of frustration!!
The old Fordson Model F represented in many instances the farmers first
experience of a gas engine machine. This "mistique" seems to survive even to
the present day as many modern restorers and would be restorers, view it with
grave misgivings and tend to keep the magneto conversions on their machines,
without really getting to know the fordson ignition system or attempting to
restore it along with their tractor.
The magneto conversions to these machines, of which their are many, are in
their own right of historical importance. Owners of such conversions may wish
to retain these "working changes" to save the character of their tractors and
with this i have absolutely no problems.
It does though, without doubt serve to prove that the simple Fordson system
with its in built quirks of character was never really understood or
The Fordson F ignition system is probably the easiest to repair and "
gettable" ignition system used on early tractors, it had to be, ...in the days
when farmers had to be their own mechanics with little more to guide them than
an instruction manual and a prayer. Nowadays of course to many people it seems
unduly complicated...but this is not so...trust me.
It is usualy the simple things, like too much oil on the distibuter roller and
its track allowing the build up of dust which acts as an insulator, or maybe a
bad connection and "shorts" where the wires fit onto the back of the coil
box. With all these points serviced and clean there is little more that can go
You may say what about the coil units themselves?, but if they are kept dry
(remember the old time driver who took his coils in doors each night, as
religiously as he previously watered his horse before attending to his own
needs) there is little to fear. Even the points are simple to clean and
adjust, But never try to adjust them by bending the supports.
The most likely problem today is that they are very old and the insulation is
breaking down inside them. The symptoms are of course, no spark or, a yellow
spark which jumps a gap when not under compression, but when under compression
it won't jump the spark plug gap. Often when this is the case the plug gap can
be halved, this will enable the tractor to run but it leaves the plugs liable
to oil up.
In the old days it has been said that it was often neccessary to close gaps
to start and warm an engine, then clean and re-gap one or more of the plugs
and the engine would work okay....Untill it had to start from cold again!!
It is not unusual for all the coils to fail together, so if one cylinder
constantly misfires, change the coil to another cylinder to prove the suspect
coil unit...but mark it first to avoid confusion!! It is easy to forgett
which one was the original suspect.
The generator is the unit which is the most "trouble free" of all
components of the model F ignition (just as well as the tractor has to be
split to get at it!!) the unit can be seen in the original manual
There are two main problems, the first is man made...ie if a battery is used
for starting and the lead is not removed from the pick up on the fly wheel
there is a serious risk of the magneto becoming demagnetised....this has
disasterous results to the current generation!. The second is simply
wear,....whether by fair use or because the previous owners have neglected to
change the oil often enough, or even let it run too low.
It is obvious that if electricity is to be produced by coils and magnets,..
as on the Fordson there is an optimum position for them to be held apart. This
is 1/16" controlled by the end float of the crankshaft, which if wear has
taken place can be more, in which case, too much gap will mean very hard
starting due to poor current generation and too little will result in the
components touching and cause desperate damage. However the latter is
thankfully unusual as the heavy knocking caused by excessive end play usualy
results in a major overhaul well before the damage occurs.
This excess play provided the answer which puzzled many old time users who if
needs meant that they had to hand over to another driver, would caution them
to "be sure and stop her facing downhill, or her won't start!" but could give
no explanation for this.
A little thought now tells us that if the gap is too wide it would not be
possible to turn the engine fast enough by hand, to generate the current
rquired to start the machine.!
Facing the tractor downhill meant that the magnets were closer to the coil!.
This of course should be taken into consideration if your restoration project
has had its crank ground and new bearings metalled into the block!!
I wonder, how many tractors in otherwise perfect order are difficult if not
impossible to start without a battery due to the thrust faces on the rear main
bearing being incorrectly machined. thus resulting in a more than correct gap
between the coils and magneto?
Now regarding the distributor / commutator located on the front timing cover.
This will give little trouble if it is cleaned and lightly oiled everyday, no
oil will induce premature wear on the cam or roller. Too much oil will cause
dust and dirt to collect on the roller track and is likely to lift the roller
away from the four contacts, which is prone to cause bad starting and
To remove the commutator assembly, disconnect the control rod, take off the
retaining spring and remove the cap and leads. Remove the camshaft end lock
nut, and pull off the roller assembley.
Now inspect all the parts, look for cracked or perished leads, renew if
necessary, inspect the cap, being sure that the steel segments are sound and
that the fibre insulating ring is not undulating, which might cause the roller
to bounce at high revs.
Check that the fibre is not perished as that will cause loss of insulation,
make sure that the roller and its bearing pivot are not worn, then clean
everything before re-assembly.
When rebuilding this assembly be sure that No:1 exhaust valve is closed and
the roller is at the top, the commutator case lug should always be pointing
towards the right hand side of the tractor ( sitting on the seat).
Having made sure the spark plugs and leads are in good working order, turn
now to the coil box and coils, again it is the simple things that give the
If you have taken the coils indoors and kept them really dry. Will all this
effort come to nothing because the insulation for the contacts in the coil
box, wood or fibre is damp or rotting? Does the coil box rain in?...Did you
leave the coil box lid open and allow the damp to creep in, when the coils
were removed? If all this has been properly sheilded from the weather then we
can turn to the coils.
Each unit is seperate to the other three, but is interchangeable. When they
are removed from ther case inspect them individually to make sure the brass
contact disks are clean and bright, also that the wooden cases are not damaged
A good coat of varnish will often help keep the damp from penetrating small
cracks on the wooden case, modern new coils have plastic cases which are
undisputedly better for insulation but not as traditional in appearence.
Inspect the contact points (see picture no:2) make sure they are clean, use
a fine file to clean both faces of the contact points and be sure the spring
is free to work and not corroded thus stiff to move. Set the gap by using the
lock nuts, don't as previously mentioned bend or modify the flat strips upon
which the points are mounted. Remember it will be necessary to hold down the
vibrator armature to check the points gap.
Now it's time to look at a few places ignition problems occur, if you have a
misfirew that is difficult to locate proceed as follows: Have available a 12v
D.C battery (small car type) but never connect battery to flywheel terminal.
Before we start our check, let me draw your attention to the dangers
associated with the cranking by hand when testing or repairing this ignition
system...especialy when using a battery in place of the fly wheel magneto!
Always place your thumb on the same side of the starting handle as your
fingers!! that's if you like your thumbs that is!! and when starting with a
battery never "spin" the engine just pull up sharply with the handle,
Remember after working on the ignition system if you have not replaced
everything as it should be a rogue spark in the wrong place may cause a severe
kick back!! this can dislocate a thumb as i found to my misfortune!! so ...be
If you are lucky enough to have a belt pully.and even luckier to have two
tractors! it is well worth belting them up when starting an engine for the
A belt driven engine is usefull to check the voltage output from your fly
wheel. The manual states that the F should dispense 14 volts at 1000 rpm. Down
to 6 volts at 300 rpm. This lower voltage at cranking speed is why it is
doubly important to have and keep points, plugs, commutator etc clean and
Talking of the fly wheel generator, if you are able to motor your machine via
a pully get it up to speed about 1000 rpm, disconect the wire from the fly
wheel pick up and connect it to a voltage meter. As stated it should read 14
volts. If it is low stop, remove the nipple from the pick up point on the
casting, check that the spring is clean and not shorted out by dirt or pieces
of swarf, or metalic particles. If metal is found then one must assume that
the magnets are also contaminated and the machine should be split and
corrective repair work under taken.If it is just gunge and dirt contaminating
the nmagnets, try adding some deisel to the oil and flush the whole thing
out...not forgetting to refill with clean fresh oil!! and under no
circumstances should you attempt to run the machine with diesel in the oil.
There is another way which is very useful if you find or suspect the
generator is down on output even if clean. This may be evident by the fact
that the tractor cannot be started by hand without the use of a battery, but
will run once the battery is disconected.
The second way is outlined in a fordson service memo put out in 1922 and
forms the part of a directive for in field re charging of the magneto to avoid
splitting the tractor.
The directive recomends the use of a rectifier to produce a 24 v D.C current
to be connected positive to the magneto contact and negative to earth.
It aklso suggests a number of fully charged 12 volt units connected positive
to negative in series will do the same job.
Now the generator may be down on power due to dirt, but more like it is
because someone has used a battery to provide starting current without
disconnecting the lead from the magneto post. This is almost always fatal to
the magnetic field!
This being so...one may ask how 24 volts connected to the magneto post can
repair the situation? the answer is the positioning of the magneto in relation
to the coils and never turning the engine whilst the current is connected. To
carry out this operation proceed as follows:
First obtain two vital pieces of equipment. A sensitive compass and a 3 ohm
resistor to limit your 24 volt output to 30 amperes, this resistor can be
obtained from automotive stores. Next...if possible run the machine to get it
warm, stop the tractor and remove the plugs, with a colleague to turn the
starter handle hold your compass close to the crank case about 1&1/4 inches
below the magneto post, and slowly turn the engine untill the north seeking
hand of the compass turns to face the rear of the tractor, now place the
compass the same distance above the magneto post and the compass should turn
and face the front of the tractor.
A fairly sensitive compass is required, but if the magnets are completely
discharged it may be necessary to remove the sump to get a reading.
With the compass telling you that the magnets are in the correct position,
and the positive wire connected to the magneto post, it is now necessary to
firmly touch the negative wire to the tractor body for two seconds, wait 5
seconds and repeat the process ten times! Turn the engine a 1/4 turn and check
position with the compass repeating at each 1/4 turn untill a complete
revolution is made, the magneto field should now be strong.
Is all this trouble and effort worth it...well if the proceedure worked and
the coil is now up to strength, you should have trouble free starting for your
model F, just the way Henry meant it to be. On the other hand if it doesn't
then we are forced to consider that the coils are irreparably damaged and need
If like me you are willing to experiment and have as go at restoring your
machine to its former glory then the whole exercise has been worth it just for
the experience of getting to know your tractor a little better!!.....safe and
happy mendings from the UK.
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