Subject: [Mgs] heater fan and wiper motor current draw?

From: Aaron Whiteman <[email-masked]>
Date: Sat, 23 Mar 2013 18:28:01 -0700

I'm planning on migrating my '75 away from the original wiring design for the
heater fan and wiper motor to a setup that is relay based. I know the normal
done thing is to tap the purple circuit for that, but I already have a spare
pair of slots available and plan on dedicated circuits for each (with purple
wires, of course).

I've got the relays, but I don't know what an appropriate size fuse and/or
wire would be.

Anybody know what these devices draw in normal use? More importantly, any
clues as to the startup draw?

I am thinking individual 15A fuses and 14AWG wiring should be more than
adequate, but would like to confirm before I go forward.

--
Aaron
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From: "PaulHunt73" <[email-masked]>
Date: Mon, 25 Mar 2013 09:20:33 -0000

I take it you will only use the purple circuit to feed through the relay 
contacts, and not to operate the relay as well, as that would mean the fan 
could still be running while the car is parked.

The purple circuit is already fused, so you won't need another fuse .  The 
fuses in an MGB are designed to protect the wiring and the standard rating 
of 17amps, 35 amp blow is all you need, that rating is used in the green 
circuit that supplies the fan from the factory, as well as the purple 
circuit.

FWIW the fan takes a surprisingly high current considering what air movement 
it results in.  An ammeter will tell you that, and a resistance measurement 
of the fan motor when stopped together with Ohms Law will tell you the 
instantaneous current.  But as far as typical automotive fuses go that can 
be ignored.

Can't see any reason to use other than the same gauge of wire as used in the 
majority of the rest of the wiring.  With the relatively small amount of 
wiring in the MGB compared to a modern car having different grades of wire 
according to the load, and consequently different ratings of fuse to protect 
them, only means you will have to carry multiple ratings as spares.  Keep it 
simple, and the two spares in the fusebox lid can be used for anything else 
you add.

You say 'relays', but there is only any point in using a relay in the 
circuit for the 'high' speed.  The 'low' speed is obtained by putting a 
resistance in series with the motor, and any additional resistances in the 
original wiring will have a negligible difference on the resulting fan 
speed.  Leave the low speed as it is, and use a single relay operated from 
the fast speed contact of the switch to power the motor bypassing the heater 
box resistance.

PaulH.

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