| "Magneto
Tips and Tricks" #4
- Dynamos and cut-out September 2001
Most
of these tips are generated following a specific problem. This time it follows
an unusual problem encountered on an overseas rally when a Lagonda’s dynamo,
recently rebuilt by a company specialising in dynamos and starters, suddenly stopped
charging. An ohmmeter showed that the field winding was intact, likewise the armature.
The owner suspected the dynamo, but with about 1.5 volts measured on the armature
with the engine at a fast tick-over with the dynamo switched ‘off’,
and with an indicated field in good order, it ought to have started charging when
switched ‘on’. Holding the cut-out armature ‘in’ with the
engine running produced a greenish spark at the cut-out points, but still no charge.
The thought occurred that maybe the cut-out shunt winding was open circuit, the
dynamo had then produced an output but with no cut-out pulling in, the voltage
would have risen to the point where the field fuse should have melted. The field
fuse was a rather heavy piece of fuse wire – probably about 10 amps –
so the diagnosis was leading to a damaged field winding caused by too high a voltage
caused by the dynamo being activated without the battery load connected. But the
field appeared OK when checked with a meter! Further checks showed a resistance
of a thousand ohms or more between the cut-out’s field fuse and the dynamo
– more than would be found even if a summer/winter charging system was in
use (which it wasn’t!). Further investigation showed that the removable field
fuse had been wired carelessly and the wire was just touching the contacts when
the fuse was plugged in, an intermittent resistance being left in circuit –
sufficient to stop the dynamo charging! So,
the usual advise of ‘suspect a simple fault’ was absolutely right –
but it took some finding! The tip is to make sure that your field fuse is rated
at between 3 and 5 amps, and if a wire fuse rather than a cartridge, that it is
carefully wired such that the connections are good when it is plugged in. A simple
general-purpose electrical test meter, costing about £10, enables checks
to be made to help diagnose this sort of problem. One
entrant to the rally was castigating dynamos as ‘useless’, he said.
Modify to use an alternator, he said. He even suggested modifying the magneto
use it as a distributor with a coil. Sad, really, because our old motor cars ran
perfectly well when built with dynamos and magnetos and part of the enjoyment
of running an old car should be in appreciating the way it was designed to work,
and retaining as much as possible of the original systems. 
#3
- Low Tension Ignition March 2001 Three
times in recent months we have been asked to sort out low tension magnetos, and
three times we’ve found that virtually nothing needed doing, so here are a few
ideas about low tension ignition systems, triggered off by today’s inspection
of a very early Arrol-Johnston magneto. First,
an overview of how the system works: A low voltage (about 75 volts peak) is applied
to an igniter electrode projecting into the combustion chamber. A rocking arm
shorts this electrode to the chassis causing current to flow. When the spark is
required the rocking arm is moved away from the electrode, and this causes a spark
as the contact breaks. The source of current is usually a low tension magneto,
but in emergency can be from a battery providing a suitable inductance (coil of
wire on a soft iron core) is included in the circuit. The
usual problem that we encounter is the erroneous idea that the LT magneto is a
dynamo. Unfortunately this is very wrong – a dynamo gives a continuous output
while the LT magneto produces peaks of output twice per revolution. This means
that timing the magneto is just as important as in a high tension system.
Fortunately this is a simple task if covers are removed to expose the armature.
The output peaks occur just as the magnetic field reverses through the core of
the armature, and this is just after the pole piece of the armature leaves the
fixed pole piece. This may sound complex but is easily seen if the armature is
turned. So,
the sequence of events to set up an LT system is: Find
Top Dead Centre (TDC).
By adjusting the camshaft timing, set the LT igniter
contact to break at TDC with any advance/retard lever set to full retard. Allow
sufficient clearance on the ignitor adjusting system for the contact to snap closed.
Say 1/32nd inch free movement after the contacts have closed.
Adjust the magneto
timing such that as the igniter breaks, the armature pole is about 1/8th to 3/16th
inch away from the carcass pole, having just left the pole in its normal direction
of rotation.
If
the system is fixed timing it probably makes sense to start as above to get the
engine running. Then advance the ignitor break, and the magneto timing so that
the relationship between the armature position and the igniter remains the same,
until the engine kicks back on starting. In the case of my 1904 Berliet this is
equivalent to about 40° maximum advance. Remember
that this is a current operated system so the igniter needs to make a good contact
to the igniter arm. Point contacts are no good. Use a multi-strand cable, similar
in size as used for headlamps (28 strands of .012). If rebuilding igniters useful
sources of mica are electric iron or toaster elements or the old Lodge spark plugs
which use mica washers as their insulation. The ignitor arm will spark erode its
way into the igniter electrode so watch the gap on the operating arm but don’t
adjust the electrode too often. I use a high Nickel content stainless alloy to
make the electrodes and arms, and it must be getting on for 5,000 miles since
they were last dismantled! Finally, low tension magnetos are simple and reliable
and probably the last thing to suspect when the engine stops, but when mine failed
in Ireland due to a vibration fatigue break in the armature output wire we found
a modern ignition coil from a scrap dump and wired the primary in series with
the 12 volt battery and the igniter supply – it got us 80 miles to the docks!
The HT output was kept out of the way, and the scrappie could not understand why
we wanted any old coil, working or not!
#2 - Care of your points Dec 2000
Proper
working of the points is essential to maintain efficient working of the magneto
– and coil ignition too! Most
of the earlier magnetos need Platinum/Iridium contact tips for proper operation
without excessive sparking at the points, and it is surprising how many magnetos
are found with the incorrect material, usually Tungsten. The problems resulting
will be a lack of power at speed, probably misfiring and evidence of serious spark
erosion of the contacts. Starting and very low speed operation is largely unaffected.
If left unattended it is possible for the complete point contact ends to be eroded
away, and this will be accelerated if any trace of oil finds its way on to the
contact surfaces. So the tip is to keep the points in good condition by occasionally
checking them and, only if needed, cleaning and polishing them. Most magneto contact
breaker assemblies are easily removed by undoing the centre screw. Remove the
whole assembly and carefully dismantle. The pivot bearing needs to be free of
any stiffness, but at the same time not floppy. A smear of petroleum jelly acts
as a good lubricant, and on the cam surfaces minimises wear. The points should
be faced with the lightest rub of a very fine Carborundum stone, holding the stone
in one hand, use gentle pressure and a circular motion with the point in the other.
Platinum tips are expensive, so avoid cutting away too much material, and NEVER
use a file! Even the so-called ‘magneto points files’ do more damage
than good. Tungsten points can be similarly cleaned up, but take more effort.
#1
- Wire termination We
often receive magnetos with HT wires nearly frayed through or falling off their
terminals. One lead off the magneto will have two effects - engine running on
three (or 5!) and magneto seriously over volted. Most magnetos have a 'safety
gap' for the spark to jump under these circumstances, but these gaps are very
wide and the voltage reached to make them spark is well above the normal running
voltage of the magneto so, premature death likely! (see the picture below of the
cut open windings from a dead BTH magneto showing a massive breakdown) A simple
but effective solution is to purchase brass screw eyes from your local hardware
store - the type used in net curtain spring rails - and simply screw them into
the ends of the HT cables - a neat and tidy termination, no loose strands of wire
and the outer insulation taking all the stress, If your HT cable is perished,
we stock black neoprene insulated cable, unobtrusive semi-gloss appearance, resistant
to hot manifolds and remaining flexible at lower temperatures.
| 
