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MG Midget and Sprite Technical - Sprite Rewire

I am preparing the ground work prior to rewiring my frogeye. At the moment I am working my way around the new wiring loom getting a feel for what goes where.
I shall use a couple of relays for the headlamps as opposed to the direct connections in the original circuits and I notice that the general suggestion is that the power circuit side should be fused. Typically, there are several unfused circuits in the original layout. Would it be wise to fuse all circuits?

Presumably one of the original concerns about fusing headlamp circuits was the sudden blackout as a result of a fuse failure whereas the fire resulting from a short circuit in a unfused supply provided sufficient illumination on its own until you ground to a halt!
Graeme Williams

Look at your new harness where the main harness connects to the rear section. On early cars the red wire just connects to each other. When there is a short in the harness, this red wire melts all of the wires in the rear section before the fuse finally blows. On later cars in the US maybe in the UK also an inline fuse was added. I didn't see this fuse listed in Moss so you would probably have to find it from someone with a later Midget or make up your own. I don't know what size fuse would work, but you could always gradually increase the size until it does work. This will save you much frustration and expense in the future.
J Bubela

Graeme

First question - have you got everything electrical working with your old loom, connections, switches and components? (And why are you replacing the loom?)

I found that trying to upgrade the spec of my wiring harness while trying to also fix problems was a pain. I found it easier to start with standard spec on my MkII 948cc Sprite and get that working first before modifying.

Second question - are you still positive earth? If so, you may want to consider a switch to negative earth and an alternator at this point as this is easier to incorporate into a new harness at this point, rather than modify a brand new one (I.e. Losing the control box, needing spade connections on the control box wires that you will reuse - Frogeye screw connections at control box. - plus the extra alternator wire). If you are bothered about under bonnet appearance you an get a dummy control box from Holden when doing an alternator conversion (the quality of new control boxes make them a bit like this!)

Finally Autosparks sell wiring harnesses for all the Spridget models and will also add in extras. So you can have a Frogeye spec harness, RHD, with neg earth and alternator wiring. Maybe also wiring for relays too.

There are BBS threads on installing relays.

J Bubela - good tip, I'll do this, thanks!

Cheers
Mike

PS witring and components from many sources, but can recommend Vehicle Wiring Products.
M Wood

I put labels on the old loom – not that I needed them – just in case. Copied the wiring diagram from the Haynes Manual and enlarged it; coloured the wires and wrote the names on the components. If the dip switch insulation hadn’t broken down it would have been plain sailing.
L Langley

gday Graeme if your putting relays in why not upgrade the fuse holder to 8 fuses. I used a circuit breaker (40 amp) to feed the headlight relays. I put in a horn relay and have an electric fan as well. HTH
Rod
R W Bowers

I cut the old wires leaving a short length on each item (switch/bulb holder etc) as a colour code of what went where.
Then laid /fitted the loom into position and refitted the new wires,one by one to each item replacing identical coloured tails.

R.
richard boobier

Lots of info!
Answering comments in general:
I have the correct autosparks loom with a couple of extra wires requested. The original loom has got very fragile and wires break as soon as you touch them. There is also some dodgy wiring as one fuse is live both sides (and the fuse is blown!). It is already neg earth.
So, I have an enlarged wiring diagram and my first step has been to work out groups of wires and get a sense on "which bit is which". I next intend to identify every wire with a label. Necessary because - did I mention this- I'm colour blind.
I had thought that removing every instrument and switch and cutting off each wire leaving just a tag would be helpful. I also thought I would put a temporary fuse in the brown wire off the battery so that for initial testing the unfused circuits would have protection. I am thinking that testing one thing at a time a 5amp fuse might do the job and then step it up when it comes to testing the lights.
Fingers crossed it doesn't look that big a job although working under the instrument panel is going to be hard going.
But should i attempt to fuse every circuit, including the lights?
Also, apart from the battery having been reversed, are there any other "cable swaps" from the oringinal pos earthe wiring diagram if I'm rewiring as neg earth?
Graeme Williams

When changing from positive to negative earth, you normally have to reverse the coil polarity.

I'm assuming that you have the mechanical tacho, so that won't be an issue.

If it has already been run as negative earth, the dynamo should have been polarised.
Dave O'Neill 2

Thanks Dave. I'll watch out for the coil colours although that's an area I have done a lot of fiddling with in the past (electonic ignition, electronic timing) so not unfamiliar with. Would be easy to slavishly follow the diagram and get it wrong!
Graeme Williams

This thread was discussed between 16/08/2015 and 18/08/2015

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