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Magnetos: Common Faults

There are several common faults with magnetos

The Capacitor

Capacitors, which are short-term storage devices, often fail in older magnetos due to dielectric degradation (short circuit) or conductor breakage (open circuit).

A short circuit absorbs all the power, hence no output or spark. With an open circuit the device has no CAPACITY to absorb the reaction voltage (back EMF) from the coil, therefore this energy is dissipated across the timing contact which quickly burns away. These faults do not necessarily occur instantaneously, they may develop over time leading to a gradual deterioration of the spark output.

Coil Failure

The voltage generated across magneto coils is quite high, in the order of 20,000 volt if no spark plugs are in place. The coils are, of course, designed for this pressure but no insulation is perfect. A small number of electrons is ever present and over years or decades this flow carves out an ever widening pathway through the insulation. This expediential degradation may appear to be sudden when the fault occurs. Another problem, due to constant hot, cold, expansion, contraction, vibration or mechanical impact, is breakage of the fine winding wire. This will not stop the coil from working. The high voltage will simply arc across the gap. The gap will widen as the wire ends melt further apart and soon the adjacent turns will be affected so the coil will eventually fail. Magneto coils are usually a long time dying.

Contact Points

The contact points which are mounted onto the end of the armature shaft, used to time the breaking of the primary winding current, will become pitted and worn, especially so if the capacitor is faulty. The result in high resistance will reduce the current flow in the primary winding and reduce the magnetic flux which adversely effects the secondary winding output voltage.

The Magnet

The permanent magnet can be static, as in the classical horse-shoe form, with the transformer windings rotating between the poles, or sometimes the magnet is mounted on the armature and rotates within the core of a static winding.

Alnico, the material most often used to fabricate early magnets has a tendency to lose its induced magnetic field over time due to various causes. An importance cause of field strength reduction is the removal of the magnet from its normal situation without fitting a keeper, a device designed to maintain the integrity of the magnetic circuit. Another cause of atom disorientation is mechanical shock, hammer blows or dropping the unit onto a hard surface, etc.

The voltage generated in the primary winding is a product of its velocity through the magnetic field and the strength of that field, so the condition of the magnet is vital for the correct performance of the magneto.

Magneto tips

If you are having trouble with your ignition system we recommend that you first change your spark plugs and HT leads. Over time the HT leads can degrade and reduce the potential difference at the spark plug. On many Rolls Royce models there is a dual ignition system. This can mask a fault on these cars as the vehicle will continue to run until the other fails.


If you have identified the magneto as the source of your ignition fault send it to us for repair. We will check all the parts of the magneto for faults and will provide a firm quotation for a repair along with a description of the magneto repair required.

The original coil may be reusable, although it is susceptible to faults - as described in our section: common problems on magnetos.

If the coil has been in service since the magneto was new there is a good chance the insulation has started to break down, allowing voltage to track. We would recommend having the transformer coil rewound as modern class F or H wires and insulations are superior to the ones used at the time it was originally made and would almost certainly give you another 40 years of good service.

Capacitors are replaced with ones that have a longer life expectancy and greater reliability. We always replace the capacitor during a repair as experience has show us that the capacitor tends to be the most unreliable part. If it is an original capacitor then it will almost certainly fail because of its age and the materials and techniques that were used to construct them.

Check out our magneto armature winding service.