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MG MGB GT V8 Factory Originals Technical - Wiring and peace of mind

1 when diving into the morass laughingly known as a MGB loom be aware that 32 years ,several POS and a conversion to V8 mean as the Chinese say 'May you live in interesting times '.
2 Logic has little or no ability in sorting/adding /deleting ancient electrical equipment
3 prayers, imprecations and deep sighs are normal behaviour patterns
4 Wiring diagrams do not refer to your car ,instead there would appear to be several different versions .
4 Previous POS are colour blind and have perfected the art of hiding two wires of different coloure joined together under miles of black tape
5 If in doubt earth it preferably with 60 amp wire ( in black )- if I ever meet the PO who used a bit of yellow .......
6 All this to add headlamp relays,and second switch for the fans
7 It is cheaper than going down the pub and the glow of virtuous self satisfaction is only equalled by the ache in my back
Lucas did not do a bad job by the standards of the day-we just have moved on .

ap lestocq

I feel your pain and have some understanding. I am presently involved in a project as the guy who has to sort out the wiring for a car that was disassembled totally, the original wiring harness removed and replaced with another used harness which had been modified by some other DPO. Splices where color code changes, wires cut off and left hanging, all the usual suspects, but it's about done now. Just take your time and hook up everything that matches by color code to the schematics and then sort out what ever remains. One of the most confusing areas is for the cars that used the factory electronic ignition and have been changed to either points type or some other type of electronic system. Those wiring changes can drive you towards insanity. Here's what I was given to start with under the bonnet.

Bill Young

PO attentions aside (as Bill's comments testify) the factory harnesses are perfectly logical. Spurs come out by their components, and in many cases you don't need to refer to the wiring diagrams. But then I have been playing with electrics longer than I can remember so I suppose I would say that. What did catch me out for a while on a late UK harness was that it came from the manufacturer with two in-line fuses cross-connected, brown to brown and green to green, instead of brown to green and green to brown. There are at least two common sources for diagrams - Haynes and the Workshop Manual. Haynes gets by with seven diagrams whereas the Workshop Manual covers the same range of years with no less than nineteen. Haynes can only do that by combining various alternatives into one, which does cause confusion.
Paul Hunt

In order that the headlights( served by relays )and the radiator fans (ditto) receive current by the shortest route I propose taking the large brown wire from the alternator and putting connections to the h/lights and fans from it .And of course continuing the brown to the starter terminal .Thereis no spare terminal on the alternator . The R/Fans will have a Kenlowe thermal control and a remote switch controlling a relay . I believe this is viable but before altering anything ask advice
ap lestocq

There are very good diagrams shown in a file of the page of

You can print them out by modell and year and they are coloured, so have a look there.
They are based upon the actual Bentles Manuals

If you are going to add relays (which should definitely be done if you are fitting uprated lighting that draws more current) fuse the individual filaments as well. If you use the existing bullet connectors by the right-hand headlight you can avoid cutting into wires. Run two wires from the blue/red and blue/white that come from the dip-switch to operate the relays, then run two wires from the relay contacts to four fuses, and four wires from there back to the original connectors again to join to the blue/reds and blue/whites that go to the individual headlights. Whilst running an additional 'ring main' brown from the alternator to the brown at the fusebox (preferable to the solenoid as it is closer to what uses power) will probably have slight benefits, if that means cutting into brown wires at the alternator that's not so good. Simply running a brown up from the solenoid for the relays will be very nearly as good and less hassle. You could fuse that as well, but it will only be protecting a few inches of wiring and the relays (unless you put it right at the solenoid which is inconvenient). If you do decide to do that use 30 amps minimum for the main fuse and 15 amps for the filaments, which is about double a typical headlight. I've done this myself, see and click on 'Lighting' and 'Uprated Lights'.

Give the cooling fans their own earths to bodywork as close as possible to where they are mounted, don't use the headlight earth. In fact run another wire from the black bullet connector by the headlights to the same ground and that will benefit the headlights as well. I gained about 2v to my fans by utilising the spare spade on the alternator and local grounds like this.
Paul Hunt

I fitted a stand off terminal on the ledge alongside the heater.It's connected with battery cable from the starter solenoid terminal & also directly from the alternator. In other words, it's an extension of the solenoid connection. The brown lead for the fuses is connected as well. It's a convenient source for any direct future connections plus the fact that it is an ideal connection to give or receive a jump start. I bought the terminal from the electrical dept. of a marine shop. they were available with 5/16" or 3/8" studs. Barrie E

B Egerton

This thread was discussed between 16/03/2009 and 31/03/2009

MG MGB GT V8 Factory Originals Technical index

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