Wiring up a Tdi Conversion

With Tdi conversions to SIIIs becoming ever more popular, it makes sense to give a dedicated guide as to how I wired mine up. It’s not the only way, but it works perfectly and reliably, and uses the standard loom.

My vehicle was originally a petrol model, and this affects a few bits of the loom and the ignition switch. Diesel models has some differences, particularly the ignition switch and glow plug circuit. I will deal with the petrol version first, and then the diesel. I will not deal with SII/SIIAs because their electrical systems were quite different and I have no experience of them. However, I’m sure that you could apply the SIII’s principles successfully, so the following may still be of help.

The SIII has a negative earth, 12V system just like all subsequent models. Its measured perameters are the same as on the SIII, but has one or two variations on ancilliaries controlled by the vehicle’s electrics, namely the glow plug system and fuel shutoff solenoid.


Fitting the electrical system to a petrol vehicle is relatively simple. The ignition switch and fuse box should basically be left as they are.

The battery will need relocation, and one possible location is under the front left seat like a Defender. From the battery, a heavy positive cable is routed to the stud terminal on the back of the starter motor’s integrated solenoid. This stud becomes the battery bus for the vehicle’s electrics, with the dash and fuse box supplies (heavy brown wires) connecting to this stud. The alternator output wires also connect to this stud, feeding the battery and the dash/fuse box when charging. The W terminal of the alternator (the thin wire) is connected to the dash charge warning light, which was controlled by a thin wire in the original alternator harness.

The starter solenoid control wire from the ignition switch is simply re-routed from the now defunct SIII solenoid on the old battery mount to the blade terminal on the integrated starter solenoid, just above the battery connection.

The diesel engine does not use an ignition system, so the coil’s feed wire (white) seems redundant. However, the Tdi uses an electric fuel valve (a solenoid in the back of the injection pump) to permit it to run or to shut it down, and this can be controlled by extending the coil LT lead to the solenoid. If you can’t locate the coil feed wire, you can simply run a new one from the fuse box, using the terminals already attached to white wires.

The oil pressure warning light and temperature guage work in exactly the same way. The oil pressure light wire (white with brown stripe) is connected to the Tdi’s oil pressure switch near the oil filter. The temperature sender unit needs to be replaced with the SIII’s to be compatible with the SIII gauge. Use the brass adaptor for a 2.5 petrol engine if the SIII sender doesn’t fit the brass fitting in the Tdi’s thermostat housing. This is connected to the standard SIII temperature wire (green with blue stripe).

The remaining job is the glow plugs. The ideal is to use the donor vehicle’s glow plug timer relay and harness. This is a big black relay on the bulkhead with a plug attached to the bottom. The plug has six wires. All the terminals on the relay itself are parallel, except one. This will be our reference terminal, with the relay held so this terminal is lowest, terminals pointing towards you.

The power for the relay circuit comes directly from the starter motor solenoid stud, through the brown wire if you have the harness. The output to the glow plug on cylinder 4 comes from the top terminal (thick yellow and black wire). The relay is activated by a feed from the fuse box (black and white wire, lower left terminal) which needs to be connected to the white circuits on the fuse box. The relay is deactivated by a timer with no further input, but also by activating the starter motor (to ensure maximum power for the starter motor), and this is done by fitting a new wire to the relay’s upper left terminal and the ignition switch’s starter motor terminal (or splicing it to the starter motor solenoid’s control wire – white, red stripe). Like all relays, this one has to be earthed, and that is done with a black wire to the upper right terminal. The small, lower right termiinal feeds the dash “cold start” warning light through the thin black and yellow wire. If you do not have the donor harness, you can make yor own using 50A wire for the power supply and glow plug connections, all other wires can be 8A. Using the correct colour coded wire will make the job and future maintenance much easier.

If you do not have the timer relay and don’t wish to buy one, a heavy duty switch in the dash, with a 50A feed wire from the battery or starter solenoid stud and a 50A output direct to the number 4 glow plug will suffice. Ideally, use a spring loaded switch that will cut out on relaease. The cold start dash light can be connected to the number 4 glow plug.


Follow the petrol instructins, except:

Remove the glow plug ballast resistor and connect the glow plugs directly to the feed which ran to the resistor. Ignore the glow plug timer relay – you don’t need it.

You will now have everything except fuel solenoid control, which was cable operated on the Series vehicles. You cannot use any outputs from the ignition switch or fuse box to control the fuel solenoid
because when the starter motor is activated, all other circuits are isolated (unlike on the petrols). You can avoid transplanting a petrol model’s electrical system by having a dash mounted switch to directly control the fuel solenoid. This switch must take its power directly from the battery or starter solenoid post, and its output runs directly to the fuel solenoid in the injection pump. The switch could be concealed as a security feature, but is best made easily visible and operated from normal driving position for safety (so that you can shut the engine off quickly in an emergency). You can always add a second concealed switch for security.

The rest of the vehcile’s electric are unaffected.

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  1. Hi Nick, great stuff finding your notes, I was looking for some info to guide me as to where the battery leads connect. I have a 1974 Norwegian military S III 109 FFR with a petrol 2.28, I just cannot believe my negative earth lead is bolted to (steel bolt about 1/4 in or 5/16) the front edge of the battery box which is between the two front seats. The box houses the two 12 series coupled batteries. Just below this earth bolt is a grommeted hole where the positive terminal lead dissapears.
    I guess this + lead is what appears to be connected to the starter solenoid mounted on the bulkhead at the rear of the engine, this sounds similar to what you were describing in your article.
    Each of my battery leads is accompanied by a smaller diameter lead from the terminal connectors, the positive small lead also dissapears into the grommeted hole but I have not yet located where it is connected after leaving the battery box. In your article you describe a brown lead which feeds the ignition switch, any other possibilites ?
    The negative terminal connector also has a smaller lead connected, this too follows the positive leads into the grommeted hole but I have not located where this is connected, could you guide me on a good possibility as well please.

    As I stated in the beginning I can’t belive the main thick negative lead should be connected to the battery box frame as earth, I cannot even tighten it without someone holding a spanner on it from under the vehicle and it’s in a very inaccesible place from underneath. If I should run an extra lead where would you advise it be earthed, on engine block or ?
    Your advise and any comments would be most welcomed.
    Best regards from cold old Norway.
    Let me know if you would like to see some photos of the vehicle, it has some unique heating equipment which could be useful if your winters get any worse !

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