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Re: Fuse box upgrade - MGA

To: Eric Stephen <>
Subject: Re: Fuse box upgrade - MGA
From: Barney Gaylord <>
Date: Tue, 04 May 1999 02:44:44
At 09:44 PM 5/3/99 -0400, Eric Stephen wrote:
>I've been thinking lately that I would like to convert my two fuse box in
the MGA 1600 (35A & 50A) to a multiple fuse configuration.  Does anyone
have any suggestions in this regard? .... I'm getting jumpy about the
accessability of the 50A fuse, ....

Okay, I'll take a shot at this one.  Start by point a web browser here:
You will find simplified, color coded wiring diagrams for the MGA broken
down into nice subsets for easy thinking.

First of all, that 50 amp fuse is live at all times and powers the fused
direct circuit.  As it so happens the only thing on that circuit is the
horn, so no need to split anything off there.  That fuse should be a 25/50
slow blow fuse that will melt down slowly at 25 amps continuous current, so
it will take a surge current at that level, or it will blow instantly at 50
amps if there should be a short in the wiring.  This is a good reason to
buy the original Lucas fuses if you may not otherwise know what you're
getting.  And there is no way that the (small) wiring for the horn will
carry that much current for very long, so this circuit is intended for
intermittent duty only.  The factory workshop manual even tells you to
short circuit the fuse so it won't blow when testing and adjusting the
horn.  Scarry thought isn't it?

And there are several circuits in the MGA that are not fused at all.  The
starter circuit is not fused, but then that wiring is very heavy cable, so
that should be no problem.  The charging circuit is also not fused, and
that has about 12 or 14 gauge wiring to continuously carry the maximum
ouput current of the generator, so we hope that these circuits will never
short the battery to ground.

And all of the lights except the brake and turn signal lights are not
fused, as well as the ignition coil and fuel pump, presumably because a
failure of one of these circuits could be a safety hazard.  Appearantly
automotive philosophy can change over time.  Would you rather have your
lights or engine quit unexpectedly or have a fire in the wiring harness if
you should get a short?  All of these circuits are fed by one brown/blue
wire from the A1 terminal of the control box to the ignition switch before
it splits up, and this is not even connected to the fuse block at all.
Here I think it may benefit from some more fuses.

The switched side of the ignition switch is wired back to the fuse block A3
terminal, where it then is split off to go the the ignition coil and to the
fuel pump (all white wires).  You could add one or two fuses here if you
like, right near the original fuse block.  The same A3 terminal feeds the
second fuse in the original fuse block.  Similar to the first fuse, this
fuse should be a 17/35 slow blow fuse to satisfy the original circuitry.

The brown/blue wire on the unswitched side of the ignition switch also runs
on to the lighting switch before splitting up into separate circuits on the
switched side.  Here you could add a fuse block under the dash in the
vacinity of the lighting switch with as many as four new fuses.  From the
lighting switch you could have one fuse for the headlights, another fuse
for all of the dash illumination lamps and the map light, and a third fuse
for the fog lights (if fitted).  The fourth fuse could be powered from
either the switched side of the ignition switch or from the unswitched side
of the lighting switch (same brown/blue wire) and be used to feed the
blower switch for the heater motor.  This last circuit is normally a green
wire fed from the switched fuse in the original fuse block, coming by way
of a comon power terminal on the back of the petrol gauge, and that's the
fuse circuit you're most interrested in splitting up.  Tagging the blower
circuit onto the ignition switch would not add any extra current draw
there, because that current already goes through the ignition switch to get
to the original switched fuse.

Now there is one more place you could add a fuse to split the load on the
green circuit.  You could take the screenwiper feed wire from the A4
terminal of the original fuse block and run it through a new fuse fed from
the A3 terminal. Removing the heater blower and the wiper motor from this
circuit leaves only the turn signals and brake lights on the original fuse,
and the petrol gauge which draws very little current. 

So there you have it, as many as three new fuses near the original fuse
block for the ignition coil, fuel pump, and wiper motor.  And as many as
four new fuses near the lighting switch for the headlamps, dash lamps, fog
lamps and heater blower.  Wow!  Imagine an MGA with nine fuses?  Or maybe

My MGA has a few extra in line fuses.  One tapped from the starter switch
is used for a dual cigar lighter type power jack (20 amp fuse and 14 gauge
wire) which I use for charging a portable battery pack and camping lantern,
an electric impact wrench for changing wheels, a small air pump, a hand
held spot light for night rallying, and for a DC to AC inverter to power
the laptop computer on the road, and as per last month's discussion now
also a 12 volt soldering iron.  I also have a separate fuse for a dedicated
circuit for the rally computer to be on full time live power tapped from
the live side of the ignition switch.  And there is (or was) another
in-line fuse (or two) for a radio.  I guess that could be up to 12 or 13
fuses if you count all the possibilities.

Now when it comes to sizing the fuses, just add up the maximum current draw
of all the devices on any given circuit, multiply by 1.5, and use the next
larger standard fuse value.  Or if you use slow blow fuses, use a
multiplier of 2.0.  And if you're installing a 200 watt stereo amplifier
you had best check the wiring size for maximum continuous current carrying
capacity (and maybe also swap out the generator for an alternator).

Regards to the circuit happy side of us,

Barney Gaylord
1958 MGA with an attitude
  (still running a generator but converted to negative earth)

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