If you have a 20amp switch then you don't need a relay, but you will have to
use pretty stout wiring due to the long runs. A better option is to use a
heater fan switch, which will look better as well as only need standard
gauge wiring, and a relay mounted somewhere between the fusebox and the rad.
If you are going to operate the fans manually and *not* have a thermostatic
switch then I strongly recommend that the fans do *not* run with the
ignition off. If you park in a noisy location you could walk off and leave
them running which will flatten the battery pretty quickly.
You can wire a ground to one side of the switch, and the other side of the
switch to one side of the relay winding. Wire the other side of the relay
winding to the green circuit at the fusebox. This means that the relay is
fused, and cannot remain operated while the ignition is off. Wire a brown
from the fusebox, via an in-line fuse (a standard 17amp rated, 35amp blow is
fine and you will have spares for it in the fusebox), to one of the relay
contacts. Wire the other relay contact to one side of the fans. Wire the
other side of the fans to a good ground. Use stout wire for the brown and
relay contact/fan wires.
Metal can Lucas 6RA relays have the windings on terminals W1 and W2 and the
contacts on C1 and C2. Modern relays have numbered terminals; 85 and 86 are
the winding and 30 and 87 are the contacts. In both types it doesn't matter
which way round the winding is wired, or the contacts, but don't get a
winding wire on a contact terminal or vice-versa. However on modern relays
the convention is to connect the 12v feed (i.e. the brown from the fusebox
in this case) to terminal 30. Modern relays are also available fused and
unfused, the fused relay will eliminate the need for a seperate in-line,
but then you will need to carry different spare fuses for it.
Be careful when using spare Lucas 6RA relays you may have lying around.
There are many different types and some, like the starter relay, are only
intended for intermittent use whereas as other like the ignition relay are
intended for constant use. If you use an intermittent-use type in a
constant-use application i.e. for cooling fans the relay gets very hot and
will eventually burn out either the winding, the contacts, or even the mica
base.
----- Original Message -----
From: "MonteMorris" <mmorris@nemr.net>
To: "MG list" <mgs@autox.team.net>
Sent: Saturday, August 31, 2002 4:14 PM
Subject: electric fan switch
> I have searched for an aftermarket, adjustable fan switch to replace a bad
> (aftermarket) one that controls the fans in the 79B. No luck.
> So I think I want to do as Jim Schulte suggested and wire the fans so they
run
> all the time the car is on. I bought a 20 amp, 2 circuit toggle switch
that
> has a place for three wires (it says ON-OFF-ON) that I'd like to wire to
the
> fans so I can turn them off if I want. I think I also have an extra
> starter/ignition relay sitting around. How do I wire this? I have a parts
car
> that I can rob of the correct color wires. Please explain s--l--o--w--l--y
> since I'm a greenhorn at this:-). If I need to put another relay in, I
will
> definitely need SLOW explanation due to the 4 or 5 wires that go to it.
> BTW, I have looked at Paul's electric cooling fan site and have printed
off
> the diagram, but I still don't understand how I can wire this up; not due
to
> Paul's explanation of fans but wiring, but due to my ignorance of the
> subject.
> Thanks,
>
>
> Monte
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