After
the vacuum driven wiper motor a strange electrical device was fitted to Austin
Sevens
which used a pulse motor to drive the wipers. In this type of motor there is no
continuous flow of current through the coils of the armature ( that sausage bit
in the middle that spins round, the same as in a dynamo ) but a contact breaker
( very similar to the one in the distributor ) which puts a pulse of current
into the coils around the armature creating a short pulse of magnetism. This
rotates the armature sufficiently that the contact breaker closes again and puts
another pulse into the armature. To set all this going the driver spins the
armature by hand using the on-off knob. Once running the wiper motor keeps
spinning, or rather should do.
If there is insufficient power in the pulse to keep the armature moving
another quarter revolution the whole thing stops; the two normal causes are the
contacts of the contact breaker are dirty or the are incorrectly adjusted.
So how do we go about keeping the wiper running smoothly?
Firstly
we need to make sure that the contact breaker points are clean, they open and
close four times for every revolution of the armature, which in turn revolves
many times for each sweep of the wiper arm. Clean them with a little slip of
fine abrasive paper between the points. If you revolve the armature so that the
points are open there is enough room for a slip of emery paper. Whilst the paper
is between the points, rotate the armature a little so that the points close a
little and then drag the paper from between the points.
Don’t be too rough, they are quite delicate. You will
notice that the contact closes to the cam on the armature is attached to a leaf
spring which in turn is attached to two posts by small screws. This can be
carefully removed if the points are in bad condition and the surface of the
points can be carefully cleaned up. It’s a bit fiddly so it is actually better
to leave all in place.
Once you have cleaned the
contacts, connect the motor to a power supply, loosen the adjusting screws and
slide the whole bakelite (see arrows on diagram above) plate so when the
armature is spun it rotates freely and at its maximum speed. Lock the two
adjusting screws when you are happy with the setting.
A little lubrication on the cam (very little) will help as will adequate
grease in the gearbox under the wiper motor which is accessed by removing the
plate underneath the motor as it appears in the picture.
This article, written by Malcolm Watts, originally appeared in CA7C Seven Focus
in May 2001 pp 10-11.
After
the vacuum driven wiper motor a strange electrical device was fitted to Austin
Sevens
which used a pulse motor to drive the wipers. In this type of motor there is no
continuous flow of current through the coils of the armature ( that sausage bit
in the middle that spins round, the same as in a dynamo ) but a contact breaker
( very similar to the one in the distributor ) which puts a pulse of current
into the coils around the armature creating a short pulse of magnetism. This
rotates the armature sufficiently that the contact breaker closes again and puts
another pulse into the armature. To set all this going the driver spins the
armature by hand using the on-off knob. Once running the wiper motor keeps
spinning, or rather should do.
Firstly
we need to make sure that the contact breaker points are clean, they open and
close four times for every revolution of the armature, which in turn revolves
many times for each sweep of the wiper arm. Clean them with a little slip of
fine abrasive paper between the points. If you revolve the armature so that the
points are open there is enough room for a slip of emery paper. Whilst the paper
is between the points, rotate the armature a little so that the points close a
little and then drag the paper from between the points. 