How To Rewire Your MGB: A Beginner's Guide To Wiring Harnesses And Ignition Switches

Most MGB wiring failures come down to accumulated age and decades of patchy repairs rather than any flaw in the original design. Time is what breaks these looms: brittle PVC insulation, moisture creeping its way into connectors and the occasional previous owner who treated the fusebox as a suggestion.

Our full MGB electrical range covers everything you need to sort it, from a single connector to a complete harness replacement. This guide takes you through the whole process: reading the right diagram for your year of car, choosing and fitting a new loom, replacing the ignition switch and getting the colour coding working for you rather than against you.

Why does MGB wiring deteriorate?

The MGB ran from 1962 to 1980, which means the youngest car is now over 45 years old. PVC-insulated wiring from that era turns brittle with age; it cracks and lets moisture into circuits that were designed to stay dry. Add in a previous owner who patched things with whatever wire was available, and pulling back the carpet often reveals a loom with its own layered history: splices where none should be, bypassed connectors and fuse holders that have been repaired so many times they've developed their own character.

A partial fix clears the immediate fault, but it leaves the underlying condition of the MGB electrical system untouched. A full loom replacement gives you a clean, known baseline; every future fault becomes considerably easier to trace because you know exactly what the system should look like.

How do I replace the MGB wiring harness?

Planning takes longer than the physical work here, and that's the right way round. The most important decision is the one made before a single connector is disturbed.

Which MGB wiring diagram do I need?

The MGB changed considerably over its production run, and the diagrams reflect that. A MGB wiring diagram 1972 shows a different fuse box arrangement, different connector types and different circuit paths compared to a MGB wiring diagram 1979. The rubber bumper cars, introduced as a mid-year change in September 1974, brought further revisions to the lighting and instrumentation circuits. If you're working on a late car, the 1980 MGB wiring diagram covers the production-specific detail that doesn't apply to a chrome bumper example. Working on a mid-production car? The 77 MGB wiring diagram is the right starting point for that model year.

Use the wrong diagram and you'll spend hours tracing discrepancies that are simply differences between production runs, not faults.

Moss Rimmer's catalogues include schematic images specific to each model year; PDF versions are available to download and print for bench reference. Pinning a copy of the correct MGB wiring diagram to the garage wall at eye level is one of those habits that genuinely pays back during a job. Schematic images give you the full-system context that a phone screenshot can't.

Understanding the MGB electrical system before you start

The MGB electrical system runs on 12 volts. Pre-1968 cars used a positive-earth setup; the system moved to negative earth from 1968 onwards, which is the configuration on the vast majority of surviving cars. That matters if your car has been converted, or if you're fitting an aftermarket component such as electronic ignition, because polarity affects how ancillary equipment is wired.

The system itself is relatively modest: a generator or alternator, a fuse box, a battery and circuits covering lighting, instrumentation, ignition and ancillaries. Apparent complexity comes almost entirely from age and accumulated modification. Work from the correct diagram and the MGB wiring logic becomes clear.

Choosing your MGB wiring harness

MGB wiring loom sections and harness interconnections

The MGB wiring loom splits into sections. The main wiring harness handles the bulk of the cabin circuits, running rearward from the dashboard through the transmission tunnel. A rear harness covers the tail light and boot circuits. On applicable cars there's a separate gearbox harness. Mapping the main, rear and gearbox harness interconnections before you start means there's no guesswork about which multi-plug goes where once the cabin end is already in.

We stock a complete MGB wiring harness set for most model year ranges, with individual section harnesses available if only part of the system needs replacing. You can shop MGB wiring harnesses by year and specification, covering rubber bumper and chrome bumper variants for both GT and roadster applications. The MGB wiring looms and fittings we supply are built to the original specification; if you're not sure which harness fits your car, our team can confirm fitment from the chassis number.

Before ordering, check whether an early car has been converted to negative earth and whether it's running an aftermarket alternator rather than the original dynamo. Both affect compatibility, and confirming that upfront avoids a return trip.

Removing and refitting the main wiring harness

Step-by-step: how to rewire an MGB loom

Disconnect and prep

Disconnect the battery before anything else: negative terminal first, then positive. Remove or drop the dashboard, lift the carpets and take the seats out for clear access.

Photograph and label

Photograph everything before disconnecting a single connector — each connector, each routing clip and each grommet. Label connectors as you go, using your own tape markers alongside the diagram.

Map before removing

Follow the main wiring harness from the dashboard rearward through the bulkhead grommet. Keep the old loom in place as long as possible; it's your reference for routing paths, clip positions and connector locations.

Remove and refit

Once fully mapped against the diagram, remove the loom section by section and replace in the same sequence, routing the new harness through its original clips and grommets — not against a body seam or heat source.

How do I get the best results when repairing MGB wiring?

Sound practice here starts with reference material and an accurate understanding of the colour coding. Whether you're undertaking a full MGB rewire or chasing down a specific fault, the principles below are what separate a job that lasts from one that sends you back under the dashboard.

MGB wiring colours and what they mean

How to use the MGB wiring colour guide

The MGB uses the standardised British colour coding system common across Leyland and British Leyland cars during the 1960s and 70s. Each circuit carries a primary colour; a tracer colour is added where multiple feeds or returns run within the same system. Understanding MGB wiring colour codes is fundamental to working accurately on the car, because it means you're reading the system rather than just following continuity.

A MGB wiring colour guide shows you that brown wires are main battery feeds, green wires carry accessories through the ignition switch, purple wires hold permanent live feeds and light green wires connect to the instrument voltage stabiliser. Once that logic is in your head, the whole system reads clearly. Moss Rimmer's schematic images include the full MGB wiring colours breakdown within the relevant sections of each model year catalogue.

When rewiring an MGB, annotate your diagram with any deviations you find in the existing harness before you remove it. That way, you know precisely what was modified and what was original, rather than discovering the discrepancy after the old loom has already gone.

Connectors, terminals and the ignition switch

Why MGB electrics fail at the connector, not the wire

Connectors account for more MGB electrics failures than the wire itself. Bullet connectors and spade terminals that have been mated and separated hundreds of times over decades lose their grip, and a loose connection produces exactly the kind of intermittent fault that occupies hours of diagnosis. When you're rewiring the MGB, replace the connectors rather than reusing the originals. Crimp joints should also be soldered; heatshrink is preferable to electrical tape wherever individual joints need protection.

Replacing the MGB ignition switch

The ignition switch is one of the most-replaced items across the MGB switches and controls range, and for good reason. The barrel wears with constant use to the point where the key no longer returns the mechanism cleanly. When you do a MGB rewire, replacing the switch as part of the same job makes obvious sense: the dashboard is already out, the connectors are already off and the additional effort is minimal. A worn switch on a fresh harness is exactly the oversight that puts you back under the dashboard six months later.

Earthing: the step that most MGB rewires undervalue

No account of how to rewire an MGB is complete without covering earth returns. Poor earthing accounts for more MGB electrics failures than almost anything else on these cars. The MGB relies on the body and chassis as part of the return circuit for most systems. Paint, corrosion or a loose connection between an earth point and bare metal adds resistance to the circuit; the symptoms that follow range from dim lights and erratic instrument readings through to components that won't operate at all.

During a MGB rewire, clean every earth point back to bare metal, fit a new ring terminal and apply petroleum jelly or dielectric grease before reassembly.

Tools and consumables for rewiring the MGB

What to have on the bench before you start

For any MGB rewire, you'll want a quality crimping tool, a soldering iron, heatshrink in a range of sizes and a multimeter. A test lamp is useful for quick continuity checks without the battery connected. Having the correct MGB wiring diagram at eye level throughout keeps errors out and removes the need to cross-reference on a greasy phone screen.

Route the new loom in the original path, secure it properly in every clip and test each circuit before refitting any trim or panels. The MGB's wiring asks for the right parts and the patience to do it once, properly. Get those two things right and the car rewards you with an electrical system that works exactly as it should.

Disclaimer

This guide is intended for general informational purposes only. Electrical work on classic vehicles carries inherent risks, including fire, injury and damage to the vehicle. Always disconnect the battery before beginning any electrical work, and consult a qualified auto electrician if you're unsure at any stage. Moss Rimmer accepts no liability for any loss, damage or injury arising from work carried out in connection with this guide. Product compatibility and specifications should always be confirmed before purchase; contact our team if you need help identifying the correct parts for your car.