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24v To 12v Conversion

24v To 12v Conversion

I have recently completed a conversion from 24v to 12v on my SIII 109" FFR. I converted to 12v as I was fitting a V8 and found this was probably as good a time as any to do the conversion. The ignition had been converted by a previous owner so it was neither 12v or 24v

This task will be made easier if you have a donor vehicle for the parts and of course a multi meter to trace cables.

Straight swap (replace with 12v Part)

Wiper Motor
Washer motor
Heater Blower
Starter Motor
Starter Solenoid
Light Bulbs (inc dash bulbs)

Flasher Relay

I used a flasher relay from a TD5 90. But any 4 pin flasher will do. The existing flasher relay is a 6 pin Hella unit which is used for hazard lights and indicators, and found on the bulkhead behind the six way light switch. The hazards operate when the ignition is on or off, but indicators only work when ignition is on.

Disconnect the existing wiring from the six way hella plug and connect to the new relay as follows:

Pin 49 – Green/Brown (Hazard) AND Light Green/Pink (Indicators)
Pin 49a – Light Green/Brown (Output for exterior lights)
Pin C2 – Light Green/Purple (Trailer tell tale)
Pin 31 – Black/Red (Flasher tell tale)

Lamp Relay

This is a relay that enable the horn and headlight flash when the ignition is on. It is found under the 24v flasher relay behind the six way light switch. I was going to replace this with a relay as it's a 4-pin 24v relay. But I by passed it instead

To bypass just connect the White Wire (Ignition Live)to the Brown/Blue x2 (Horn and flash).

If you wish to fit a standard 12v relay in its place, unplug existing relay and connect replacement as follows

Pin 30 – Brown/Yellow (Constant Live)
Pin 87 – Brown/Blue x2 (Horn and flash)
Pin 85 – White (Ignition Live)
Pin 86 – Black (Earth)


I had a 24v Alternator fitted, but this should be a similar solution if you are removing the generator. You will probably have to create your own wiring. On my 2.25 petrol I used a civy alternator mount which replaces the fan belt idler.

A stock Lucas a127 alternator will bolt on directly mine is wired straight to the battery with a switched ignition live going to the sensing terminal on the alternator via a charge warning lamp on the Dash board.


The Speedo will work fine assuming all the light bulbs are changed for the warning lamps and illumination. The combined fuel/water/oil temp gauge will not work and needs to be swapped out for a 12v unit. You can use separate gauges if you wish but this is a neater solution.

You will loose the oil temp gauge, but as I was fitting an engine without an oil temp sender this was not a problem. You will also need a 12v temp sender for your water temp, again not an issue for me. The wiring remains the same for the water temp and fuel gauges, although you will need to include a voltage stabaliser/regulator as the gauges work on 9v.


You can discard the filter box that sits ontop of the rocker cover, these sell well on ebay. I took a new switched ignition live from the fuse box straight to a new 12v ignition coil using a heavy gauge wire. I took this from the un-fused side of the fourth fuse.

Hope its been helpful Cheers
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