The Story of an MGB Rebuild



In my view, there's no point in building a new car and not fitting a new wiring loom. The chances of removing the old one intact are slim. I also decided that I needed to make a number of changes and finally decided to build my own. Modern standards for wiring components are much better than the old plastic tape wrapping and those awful bullet connectors. I also feel that the old style 4 way fuse box is hopelessly inadequate. I considered getting a new loom and modifying it but decided that a better job would be to do the job from scratch.

I started by reverse engineering the standard wiring diagram so I could work out what each wire did. Watch out for the common points which are buried inside the wiring. I then started to build up a set of wiring diagrams using MS Visio working in conjunction with an Excel spreadsheet. Virtually all the wiring components came from Vehicle Wiring Products in Ilkeston who provide a first class service.

The main parts of the loom were constructed in convoluted tubing with T-pieces at each branch. The tubing allows new wires to be added and this came in useful when I changed some of the wiring during the build. Other wires can be added reasonably easily in the future. Wherever possible, I kept to the original colour scheme but I also individually numbered each wire. Heatshrink number rings ( were fitted to each end of a core with a tube of clear heatshrink over the top. Bit OTT? Well, when I come back to it in 3 years time, I should have no problem in working out which wire is which.

Every connection is crimped and nothing is soldered (I find that solder fatigue snaps over time). VWP supplied the crimp tools and the connectors.

I started the actual wiring on the fuse and relay boxes. I did make the mistake of trying to add the flasher to the relay box but this wouldn't have provided an audible click when flashing - the fuse/relay box was to be sited in the engine compartment. I altered the design so that a flasher base connection unit was fitted inside the car on the bulkhead.

The convoluted plastic tubing worked really well. The T pieces are a bit bulky but they do allow access to the wiring. It would have been nice to have a proper finisher unit on the convoluted tubing; instead I used black heatshrink - this can be seen in one of the photos below.

I tried not to cut any wire until I had to and I was sure of the length. At first, this meant I was dealing with some long strands of wire; this was a bit of a pain with threading them through the T-pieces. In my usual way, I over-ordered on wire but I'm glad I did.

The console is fitted with an ammeter and voltmeter; the former is used as the 12V unswitched distribution point. Therefore the wiring to the console is the most complex part and is fitted with some 11 way connectors so that the console can be removed easily. These can be seen in the bottom photo.

At commissioning stage, I was pleased to find that the number of faults was minimal. I made a mistake of connecting the voltmeter to 12V unswitched - it should have been on 12V switched. I also should have ensured an unswitched 12V supply to the radio - otherwise you have to keep entering the presets.

Number plate bulb holders were unobtainable from MGOC or Moss - the Welsh MG Centre helped out here. I'm still struggling with a boot light switch. Again, these aren't obtainable and I've tried several alternatives without success. I mistakenly thought that bulb holders for the instruments would be easy to find. In the end, Speedy Cables helped out here.

Here's where I started the wiring with a new 16 way fuse box fitted with a 4 way relay box alongside. You can make see the Visio wiring diagram and the number tags on each core. This is going to be fitted where the pedal box goes in left hand drive cars.

The fuse box has an irritating problem in that you really want a busbar down one side of the fuses (or really 3 busbars - namely, 12V Unswitched, 12V Switched and 12V lights). This isn't easy to do as you only get a set of individual female connectors and I had to daisy chain along them.

The wiring started by fixing the fuse/relay box into place on the nearside of the heater and threading the large diameter tubing through to the cockpit. The standard bulkhead grommet can accommodate the tubing nicely. Then it's a matter of threading T-pieces and tubing onto the wires, making sure that the right wires branch off at each Tee. I started this on a lovely spring day and the wires were stretched out down the drive. It all had to be gathered up once the showers started! The plinth for the original fuse box will be used for a junction box with the rear loom.

Here the junction box is fitted, the rear loom is in place and the engine branch is ready for final connections to be added. I didn't fit these until the engine was in. The junction box was a bit of a headache as I couldn't find a suitable distribution system. In the end I used a stationary 6.3mm blade distribution bar but it was all a bit tight. Small tie-wrap tie downs are used to secure the tubing.

This shows the detail of connecting it all up. 16mm convoluted tubing is used here as opposed to the 28mm at the fuse box. The tubing is clamped by brass P clips - these are neater than the plastic ones which never have a big enough hole! At the termination of the tubing, 2 PVC sleeves have been fed up into the tubing. A piece of black heatshrink as been fitted over the top of the termination. You need to be careful not to melt the PVC! The wires are numbered individually and then fed into 2 or 3 way connectors for the lights. Watch the reverse lights. The lens securing screw also fixes the light housing. I managed to crack the lenses as I tightened the screw too much. Next time, I would use a nyloc nut..

The camera manages to highlight a lot of dirt that accumulates during the build!

Here's where the fun begins! 28mm tubing with big T pieces. Each of the 4 outlets are by function - from the right - steering column stalk, instrument binnacle/flasher/wipers, central console(instead of speaker grille) and finally switch panel. Lots of checking and rechecking with the multimeter to make sure they're going down the right branch.

Fast forward and the dashboard is in place. Multi-way connectors have been fitted to the steering stalk connections. The other wires in that area are for the instruments, wipers, etc. It's only when the speedo/rev counter and radio go in when you notice the lack of room. Up to then, there's bags of room so make sure they go in last.

This is where the headlamp/sidelight connectors are fitted and you can see the tools of the trade. The multimeter is an ex Ministry job from the autojumble at the Cumbria Steam Gathering - an excellent purchase. The crimp tool has easily paid for itself - I used it for both insulated and non insulated terminals. The 8 way multi connector is a big brother to the smaller ones used at the back of the car.

This shows how the console was connected up. The console has 3 gauges (ammeter, voltmeter and manifold vaccuum), 5 switches (fuel pump, aerial, interior lights, reverse and fog lights) and 4 warning LEDS (overdrive, choke, reverse and fog lights). The choke warning is a must as I was always forgetting to shut it properly before. 3 multi way sockets carry all the connections. The 2 11-way sockets are wired male/female and female/male to avoid connecting the wrong pair - that was a good design accident!

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