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AH 100 wiring challenge

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Luegolover Avatar
Luegolover Steve L
London, London, UK   GBR
I have a standard wiring loom bought from A Head 4 Healey’s which I am about to install in my car. However, there are a few things I now I want to change with regard to the wiring and I am not sure how best to do it.

I want a 12V cigarette lighter under the dash so that I can use things like a tyre pump.

I want a pair of USB sockets to keep our phones charged

I want to run Halogen bulbs

I want the option of having a radio.

Seriously now, I want to future proof the car, so any suggestions please tell me.

These all need a 12V spur off the loom but I am not sure where to take it from.

I know that the car is going to be negative earth.

Any help warmly received,


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filospinato Avatar
filospinato Jake Voelckers
Manassas, VA, USA   USA
IIRC, brown wires are generally hot, green wires are keyed. I'd run the lighter from a brown, same with the radio, so they can work with the key off. I'd run the USB sockets (5V BTW) off of a green wire so they will charge when driving but won't drain the battery if left plugged in. I think you can pick these wires up at the fuse box, I'd run them fused even if you can't. Make sure to verify that the amount of amperage you're planning on pulling won't be too much for the current wiring.

MarkBD Avatar
MarkBD Mark D
LLandassillio, West Wales, UK   GBR
I wanted the same as you steve, so i ran a new 50A cable from the the starter solenoid (battery side) to a new 6 way fuse box under the dash out of site and i run all my new items from this. To make things easier i used a blade type fuse holder with led indicator of a blown fuse.


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Penfold99 Avatar
Penfold99 Dave Buchner
North Hollywood, CA, USA   USA
The other replies are good, the only thing I would add is to have decent headlight performance, add relays for them. There are easy to install kits for this...

‘54 BN1

Luegolover Avatar
Luegolover Steve L
London, London, UK   GBR
Thank you for the advice. I will add a new 6 way fuse box as suggested and a relay for the halogen lights.

I need to work out where to put the box so that it won't be seen.

I also need to work out how to get the power to the lights. Can I use the existing wire in the loom for this and add the relay in the dash area? I'm not sure that is a clear statement so I'll go again. I have a brand new and lovely looking loom from AH4H and am reluctant to run a new wire alongside it to the front of the car. Can I take the feed from the light switch to a relay and then the output from the relay to the existing wire that was designed to go from the switch to the lights and thereby avoid running a new wire alongside the loom? I ask because the only info I have read about the need for relays says they are to protect the switch. Is the existing wire in the loom capable of carrying the power required for a halogen bulb?

As ever, thanks in advance.

MarkBD Avatar
MarkBD Mark D
LLandassillio, West Wales, UK   GBR
Hi Steve
I mounted my extra fuse box and relays on the cold air box section on the driver side (as per pic). It is advisable not use the existing light cable in the loom to power H4 halgen bulbs as current is higher (H4 60w/55w = 5A / 4.5A and original bulbs 45w/40w = 3.75A / 3.4A).

Option 1.
Mount the 2 relays at the front end of the car and use the existing loom cables to switch the relays but a new power lead needs to be run from the fuse box rated at 22A to power the lights.

Option 2.
Mount the relays under the dash, disconnect the dipped head light lead from the lights switch, connect new lead from light switch to first relay and a lead from the main beam warning light to the second relay. Run a power feed from the new fuse box to both relays (12A each) and then run two cables to the head lights from both relays (12A each).

If you need any help laying it out message me and i will draw it out

Mark D

Fuse box.jpg    31.2 KB
Fuse box.jpg

Luegolover Avatar
Luegolover Steve L
London, London, UK   GBR
Thanks Mark,

I decided to fit a little fuse box under the dash and run new leads to the lights. The wires in the loom are very thick so I suspect they could have handled the power anyway but I decided that I wanted to have an extra feed in case I decide to fit an electronic fan at a later date. I bought some cloth tape which isn't too far away from the loom wrapping material so it won't stand out and all the relay/fuse box wiring will be well hidden. I'm not too sure why I am making a fuss about this other than I always think simple wiring in engine bays looks good and sets the tone for the whole car.

Trevor Parker Avatar
Trevor Parker Silver Member Trevor Parker W
Sidney, BC, Canada   CAN
1954 Austin-Healey 100 "Healey Beep Beep"
1955 Austin-Healey 100
Hi Steve I agree 100% at the wiring under the dash sets the tone of the car. When I decided to add electric fan and driving lights I thought what would the dealer do. So I used the same relays as they use for the overdrive. And also original style cloth covered wiring. And I mounted the relays where the BN1 overdrive relays are mounted. So to my eye it doesn't look out of place at all .cheers Trevor

IMG_1269.JPG    36.5 KB

Luegolover Avatar
Luegolover Steve L
London, London, UK   GBR

I am really impressed that you thought of that, had you posted 2 months ago I would have copied that. As it is I went down the road that Mark did and whilst I am not unhappy about it I am conscious that it does look out of place. Damn. If it weren't for the fact that I have loads left to do on the car I might even go back and replace what I've done but I want to drive the car now and so going backwards is not an option. It never occurred to me to think of the original relay's as just that, relays. Oh well.

Well done,

IMG_9330.JPG    36 KB

O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
Why hide it? When doing something smart, best let others know! This is like a $20 solution. I love the lights that tell you at a glance that a fuse is blown. Or not.

I have long suspected that the original fuse scheme will relies on the heat from the car burning to the ground to melt the fusible link.

The only other caveat I foresee is that a generator might be pushing it's limits to supply your power budget. A one wire ( self exciting) alternator is a very simple upgrade.


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