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MG MGB Technical - Adding fuses for extra protection


In the process of putting in a harness. As part of the process, I have added an extra in-line fuse to protect the overdrive. Any advice regarding which other inline fuses and where to add them to protect the harness in the future.


IA Jones

Fuel pump and electric cooling fan (if you have one).
C Harvey

Hi, thanks for that.

Anyone any advice on relays? I know that they are really good for fog lights and heaters etc.; however, I do think they look a bit ugly stuck on to the inner wing of cars. Maybe a holder would improve looks. Which one(s) would anyone recommend as being the mist useful?

Thanks in advance.

IA Jones

A fuse on the permanently live circuits is good, a short on any of these is a fire in the making.
Relays on the headlight circuits have two benefits, 1. It takes the load off the lighting switch and flasher unit, increasing their longevity. 2. By not passing the supply through all those bullets, as well as the switch, the lights end up brighter! Put the relay as far forward as you can to reduce the number of connections and take a fused supply off the starter or alternator brown wires.
Autosparks and Vehicle Wiring Products can supply multiple relay holders.
Allan Reeling

Factory electric cooling fans are fused.

Fuel pump is a must, both my cars came to me with that wire and the main harness damaged, and several I have worked on have been the same. Simplest place is where the white from the rear harness joins the main harness.

Overdrive where the manual switch is on the gear lever is also a must, but advisable for others. D-type may need two fuses as with the relay and vacuum switch toe supply is taken from two places, more info here

Headlamp relays and fuses are beneficial, especially if you have uprated them, but fuse each filament individually or you can be plunged into darkness if a problem develops from the relays out to the lights. Ditto for front and rear fog lights and spot lights.

Thanks All,

Paul I notice in your web site that you put in 35am fuses in the fuel pump in-line fuse and that 7am too low. Would 10amp be ok for the overdrive in-line fuse or best go up to 25amp?


IA Jones

The glass fuses used on MG's are current rated at 17amps, 35amps is the current which blows them.
Allan Reeling

Thanks Allan,

But ..... sorry, it still does not make any sense to me, I'm afraid you've lost me there straight away. I have to be told -in exact detail (i.e. do that there ... preferably with colour piccies)- when it comes to car electrics.

As I said in a previous post, I'm a bit of an electrical dumbo - my brain just does not want to understand. PaulH did try to explain, bless him, but its all to no avail.

It's not as if I was not mechanically minded or (god forbid) scientifically challenged.

Any web site tips for a numpty like me to start to understand car leccy? Or, at least old car leccy.



IA Jones

Simple formula; amps x Volts = Watts.
e.g. a 60 watt bulb on a 12 volt circuit draws 5 amps.......5 x 12 = 60.
A fully charged and healthy battery can deliver 13.2 volts. Whatever the current rating of any particular circuit, think in terms of the load (bulb, pump etc) acting as a restriction to the current flow. However if the load is short circuited by the wire going straight to earth, for whatever reason, the maximum amperage of the battery, well in excess of 70 amps, passes through the wire. A wire which is designed to carry a max of 17amps, will quickly heat up like an electric fire element and burn the insulation. The 35 amp blow fuse will quickly disintegrate before the insulation and anything combustable around it gets ignited. Hence any circuit without the safety of a fuse can be a potential danger if the wire gets it's insulation chaffed through. Not sure about the current draw of an SU pump, but can't see it being much more than 2 amps, but the pulsing puts that up for very short periods. But, as said, you want to protect against a direct short to earth so stick to 17amp.
Allan Reeling

This is how I took care of this on my 67 GT. I added Relays that bolt under the front sheet metal where the hood latches. I also added a period fuse block above the original. This fuse block covers the head lights, tail lights and OD. I also added a relay for the brake lights. (left side of the coil)

Bruce Cunha


That does look very tidy, I may well look copy your idea of a period fuse box similar to yours.

I was thinking of putting the relays in a specialised enclosed 'box' holder underneath the bonnet 'slam panel' (I think that's the name for it) so that the relays are generally out of view and as the box/holder is a marine application - it has covers to ensure water does not get in - should not be affected by water spray. Could even put a splash guard.


IA Jones

This thread was discussed between 09/08/2017 and 14/08/2017

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