[TR] TR2/3 overdrive relay.

[Randall]

> Can you substitute a generic 30 amp four connector relay for the o/d
> relay?

My experience was that the generic relays (and purpose-made reproductions)
would not last long in service, presumably due to the inductive kickback
from the OD solenoid.  However, adding a snubber diode seemed to solve the
problem permanently, and I have been using the same cheap generic relay for
many years now with no further problems.  Here is an old photo of the relay
and diode on my previous TR3A before it got wrecked.  The original wiring
was later updated, but I am still using that relay and diode today.  (The
diode is inside the black heat shrink tubing that you can see leading under
the relay and over to the mounting screw.)
http://goo.gl/HDHjb

The diode goes from the output of the relay (to the solenoid) to ground.  If
your car has been converted to negative ground, the stripe on the diode goes
towards the relay terminal.  If you are still running positive ground, the
stripe goes towards ground.

I also added an in-line fuse to the power feed from the ammeter.  In
addition to protecting the solenoid if the cutout contacts don't open for
whatever reason, it will also protect the wires if the battery is connected
backwards (causing the diode to conduct) or the diode fails shorted.

-- Randall