PC December Issue

WORKSHOP BASICS
Swap from positive
to negative earth
How to change the polarity of your classic in a few hours
With Ed Hughes
DIFFICULTY RATING
TIME
(HRS)
3
MONEY
0 (£)
Wipers
Wiper motors and flasher
units should function
correctly without
modification.
Radio
A period radio
sometimes has a
positive/negative
earth switch on the
back, which can be
changed over.
Label it
Place a label in the
engine bay to state
the car is now
negative earth.
Relabel the coil
connections.
You will
need
Battery clamps,
screwdriver, small
spanners, pliers,
length of stout
wire.
Many older British classics – plus
a few others – were built with
a positive earth electrical system.
The positive terminal of the battery is
connected to the chassis or body.
The current flows to the various electrical
components connected to the shell,
through wires to the switches and controls,
back through the fuse box and finally to the
negative terminal of the battery.
It’s a complete reverse of the negative
earth system that’s become the modern
standard. Positive earth isn’t a bad thing in
itself. However, problems arise when you
connect accessories or upgrades such as
a radio, an alternator or electronic ignition –
or perhaps try to use a phone charger in the
cigarette lighter socket. These won’t work
and could be damaged in the process.
Converting to negative earth is a simple
job. You don’t need to touch the switches or
the wiring loom. The only vaguely technical
operation is repolarising the dynamo, but
even this is easier than you might think.
128 DECEMBER 2015 // PRACTICAL CLASSICS
www.practicalclassics.co.uk