Making your own MG TD wiring loom

(Laurent Castel from France describes how he made his).

Would you guess that our little British cars feature 134 m of cables with 42 references ?

The electrical diagram is presented in the workshop manual. Different versions are commonly seen on websites. But manufacturing schematics are never seen. What diameter for each wire? What length of each colour? What length and diameter for each conduit? So I drew my own schematics based on the old loom and some measurements on the car itself. The goal is not to install a new original loom but a better one with the benefit of better materials and advanced knowledge and experience.

However, the fuse box with 2 fuses only remains because it is lovely. And I’m confident that short circuit with a brand new wiring loom seldom occurs, once tested.

There are two more reasons for building my own wiring loom. First, my car is RHD and I guess that “ready to fit” wiring looms are for LHD cars. (Wiring looms for RHD TDs are available – Ed).

Second, my car has some specific equipment (as any 60 years old car) that would require some additional wires to the standard loom. Lucas colour wires are available from Autosparks at reasonable cost. The use of these coloured wires leads to easy wiring. The risk of errors is almost zero. All wires with the same colour are connected together and never with a different one. Both sides of connectors must show same colour wires, pin to pin. I also ordered all connectors and terminals from the same web shop.

Designing the Loom (Figure 1)

Figure 1 shows the overall schematic, describes what is fused, what is monitored by the ammeter and what is switched by the ignition key.

The loom has to supply the following additional equipments:

– an auxiliary rear fuel pump with a switch on the right hand side of the dashboard.

– a heater fan switched by a command on the right hand side of the dashboard.

– independent rear and front flashers fitted on the bumpers. The command switch is under the dash on the rightmost part.

– a lamp in the glove box. Switch is included in the lamp.

– an oil pressure switch which will drive a buzzer.

– a reverse switch for reverse lights. Only wires are implemented since I don’t want to add these lamps right now.

– a single fog light.

The centre panel is removable thanks to usual 6 way spade connectors and 2 pole connectors.

Wire gauges are selected according to common rules depending on application.

Charging circuit: 3 mm²

Headlights: 2 mm²

Others: 1 mm²

For accessories, I took a measurement that showed 11 A per horn, 2 A for wiper motor and 1.1 A for heater fan. Fog light and reverse lights are wired with 2 mm² as headlights. Three Anderson 50 A connectors are used for high current wires. These are probably oversized but I don’t want to have voltage drop issues. Everyone knows those dim or varying lights or slow wipers when activating headlights or horns.

Some improvements were also fitted.

– Due to the high current of horns, and the inductive load they feature, they are both driven by a single relay to protect the expensive push button of the centre panel.

– All devices are grounded with a black wire to the chassis with a shake proof washer. The individual grounding was not original and is still probably not included in standard looms. Ground wires are same gauges as the related coloured wires. The shake proof washer allows a very good contact with chassis. The washer is installed between chassis and the eyelet terminal.

– A protection circuit with 470 µF capacitor and 100 ohms resistor is implemented on fuel sender to prevent any sparks inside the tank when the switch opens. I’ve never heard of such an accident but the cost is almost nothing. The loom is long hence inductive and this is why there could be an overvoltage peak when the switch opens. The circuit must be fitted close to the fuel sender.

The first schematic shows the global loom with colours, wire diameters and conduits length and diameter. The wire characteristics are always written above the wire when reading these characteristics.

The second schematic shows the centre panel and remote dash instruments wiring.

Ed’s note: Both schematics are shown at the end of this article and they will also be uploaded to the ‘Publications’ section of the website.

For a LHD car, I guess the chart would be very similar except for some conduits length. Mind the dipper foot switch for late cars and the conduits leading to the centre panel.

Building the loom

I first built the entire loom and then installed it on the car from underneath the dash board. Then, I connected and checked every equipment one by one. I used a power supply instead of the battery when testing an equipment. Thanks to current limitation of the power supply, a short circuit is not destructive. The conduits are braided ones except under the fenders where they are waterproof. The nylon braided ones are very easy to use because they feature a variable diameter depending on length. They are easy to adjust at the correct length when installed. When making the loom, I inserted one heatshrink at each end of each conduit. These heatshrinks are only heated on the car when everything is adjusted, connected and tested.

Inserting the wiper motor cable in the screen pillar can be quite frightening but eventually this operation was quite easy. I just had to remove the screws attaching the side support and the bottom and top corner brackets. Then I was able to insert the new cable using the old one to pull it in. Buy the special cable for this. It is expensive but really worth it.

Same technique for the rear number plate cable inside the spare wheel support.

Every wire is tin soldered to its terminal or before inserting it into a screw type terminal.

When wiring the centre panel, take care of the ignition and petrol warning lamps as they must be insulated from the panel itself.
It should be easy to add any specific equipment such as radio, cigar lighter or GPS to this basic loom. Why not a low power fridge ! But have a look on the maximum capacity of the battery, the generator charging current and the 2 fuses.

Be patient and work thoroughly and you will experience how rewarding it is when you first hear the horn sound as you push the switch on the dash.

Bill of materials

Black, 3 mm², 4 m
Brown/blue 3 mm², 2 m
Brown, 3 mm², 3 m
Brown/white, 3 mm², 2 m
Red/white, 1 mm², 1 m
Green, 3 mm², 2 m
Black 1mm²; 8 m
White/black, 2 mm², 1 m
Brown/blue, 1mm², 1 m
Blue/white, 1 mm², 2 m
Yellow, 1 mm², 2 m
White, 1 mm², 2 m
Light green 1mm², 2 m
Green/yellow, 1 mm², 2 m
Green/brown, 1mm², 3 m
Yellow/black, 1mm², 6 m
Brown/green, 1mm², 1 m
Brown/black, 1mm², 2 m
Green/blue, 1mm², 2 m
Green, 1mm², 6 m

(Click diagrams to view larger versions)

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