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MG MGB Technical - wiring harness

I am new to the restoration business and have a new wiring harness for a 75 b. Any hints on where to start? thanks
Don

Do you have the old wiring harness out of the car? If it is out I hope you made notes on which wires go where, photos would be better. If it is not out remove the old harness making notes, drawings and photos as you progress. It would be a good idea to buy a box of small marking tags with strings on them. Tag and mark everything. You will remove lots of clips, clamps and screws. Use small plastic bags to store them, mark each bag so it can be identified later. After you get the wiring out lay the old wiring and the new side by side and compare them. Make tags for the new harness so you have a general idea of where each runner will go. It's best to remove the seats and dash for rewiring. There will be other parts that need removing. I don't know of any publication that gives a step by step sequence. That said, there is someone on this BBS that can handle any question about wiring as many of the group have rewired their cars. If it takes longer than expected to do the job, remember, it needs to be done right or a short circuit can easily burn up the new harness. Make a check list for final inspection before applying power, it will save you time. Good luck. I'm sure others will respond with additional suggestions.

Clifton
Clifton Gordon

Don - After getting your new harness installed in the car and all hooked up, reconnect your battery through a 10 or 15 amp fuse, then go through and turn everything on individually, including the ignition and make sure it all works and nothing blows the fuse. Only then should you connect teh battery directly and try to start the car. For aditional help with what goes where, go to the Advance Auto Wire web site at: http://www.advanceautowire.com/ and click on Stock Schematics. Scroll down to the drawing for the year of your car for a color coded electrical drawing. Thi drawing can be printed as large as 11 X 17 inches and laminated at Kinkos. Good luck - Dave
David DuBois

If you haven't yet got the old harness out when you have all the 'tails' in the engine compartment disconnected from their components tape them to the main harness some pointing backwards and some forwards, so as to keep as even a thickness as possible. If you leave them hanging then when you try to pull the harness out (from the cabin) they will all point forwards and you can end up with a thickness too great to get through the holes in the firewall - the later the harness the worse the effect. Do the same before pulling in the new harness (feed it from the cabin) and put split cable sheathing over the edges of the firewall holes so as not to nick the wrapping and insulation.

Use all new bullet connectors, and clean up the spades on the components and bullets on the tails particularly in the engine compartment. Assemble bullet connectors with a little Vaseline, this lubricates as well as helps seal against moisture. Because the bullets are only crimped onto the wires, unlike the spades which are spot-welded, moisture can creep down the crimp and along the wire, causing corrosion for several feet. I drilled a conical depression in the end of each bullet (new harness and headlight tails) to show shiny brass and copper, then used a hot high-power iron very briefly to fill the depression with solder. Briefly mind, or the insulation will be damaged. When assembling bullets you can push them in past somew resistance and think they are in, but they should be pushed in further past a second point of resistance. You can get bullet connector assembly tools, but I made my own by cutting a notch in the bottom of each handle of a pair of pliers just big enugh to go over the insulation. When you squeeze on the handles this then forces the bullets all the way in to the connector.

If there is any corrosion visible on or underneath the fusebox replace that too. This is handed, one end of two of the fuses is linked internally, this goes top front on the car.

In fact because of the tails coming off the main run by their component it isn't that difficult to work out which connects to what component.

Take the time to clean up the battery 12v and ground cable terminations at both ends.

When reconnecting the battery *make sure* you have it the right way round, in fact don't plug in the alternator until you have got everything else connected and tested, including the battery. Always remove the battery ground connection first and reconnect it last, and that goes for any time you want to disconnect the battery, not just for this rewire. Before connecting the ground strap for the first time I substituted a high-wattage 12v bulb, like a headlamp bulb, instead. This ensures that if anything *is* shorting out, the bulb will limit the current to a safe level and you will not damage anything. It also shows the presence of a short by glowing at full brilliance. With everything off the test bulb should not glow at all, and you can test each circuit in turn. As you turn on each circuit the bulb will probably glow a bit, the higher the load the brighter the glow, although some loads might be too small to make it glow at all. Where there is a glow it means the component being tested won't have the full voltage and current available, so things like wipers and horns probably won't work, but by the absence of a full glow from the test bulb you will know that a) they are connected and b) they are not short. The only thing that could give a full glow and make you think there is a short is if you turn the key to crank, so just look for the glow when testing this and ignore the full glow.
Paul Hunt 2

Don,

If you haven't removed the old harness yet, do so by cutting off the old one about 2" away from each element - lights, horn, switches etc. That way, you will then leave a colour-coded 'tag' on each item which will help you when re-wiring.

Good luck.

Peter
P L Hills

This thread was discussed between 02/08/2006 and 03/08/2006

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