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MG MGB Technical - Spliced and mish-mashed electrical

A lovely thing happened the other day when I pulled the engine out of the engine bay for its rebuild. I was glancing around inside the engine bay to do abit of damage assessment on the leaking brake system, and came across a few dead end wires. I knew they were far from original from the different colors they were hooked into, so I followed them closely into the passanger compartment and through the main harness. Turns out some nut job went through and totally destroyed the wiring harness. It's in such bad shape that I can't really figure out which wire is for what, they're all over the place.
I haven't a clue as to whats what anymore, so here's my question does anyone have a wiring diagram for the 72 MGB model? I'm royally buggared on what goes to what anymore on this thing. I've found the headlight switch had been cut up so many times before that I'm not sure what in the heck the other owner was trying to do with it. This is where all the weird loose, and unknown wires end up and I'm loath to go trying to replace any of it till I have a wiring harness diagram or a really competent automotive electrician look at it. What a bloody mess... http://www.planetfurry.com/forums/download.php?id=635
CJD Dark

You can find a wiring diagram here:
http://www.mgbexperience.com/#toc

You may end up ahead of the game if you replace, rather than repair, the harness. British Wiring ( http://www.britishwiring.com/ ) has a very good reputation for its products....

HTH!
Rob
Rob Edwards

This wiring diagram should should be right for your car.
http://www.mgbexperience.com/view/?service/wiring-7172.jpg
You can download and print it. The Official MGB repair manual by Bentley is a copy of the MGB shop manual and has wiring diagram. You can buy them at Amazon for $35.00
http://www.amazon.com/exec/obidos/ASIN/0837601150/themgbexperienc/002-2182357-8722414
If your wiring is as bad as you say it may be best to get a new wiring harness from British Wiring.
http://www.britishwiring.com/
You can also get wiring supplies from them if you want to repair your wiring. They can supply correct wire size and colors for your car. Look through their on line catalog for information about sizing and other parts information. Good luck, Clifton
Clifton Gordon

Well, I didn't check to see if you had a response before sending my reply. I didn't mean to repaet Rob's reply. Clifton
Clifton Gordon

Do yourself and your car a big favor and buy a new harness. Most old cars are not destroyed by accidents but electrical fires. I always, when possible, replace all the harnesses and plan to do the same with my 67.
Mike MaGee

Ditto on British wiring. They are a GREAT company with a great product. Save yourself the misery and buy a new harness to start out with. You'll eliminate so many of your wiring problems it will shock you.

Be like Nike and "just do it"
Justin
Justin

Being in the process of restoring a 73 MGB and having just finished repairing/replacing the wiring in my car, I also agree that you should replace rather then repair the wiring harness. One big problem with the wiring diagrams is that NONE of them are totally accurate. I have three different wiring diagrams for my car and all of them are only "partially" correct. It took me many long hours agonizing over the three diagrams trying to figure out the wiring. Go new, and you will greatly reduce your hassles. I am glad I finally did.
Robert Browning

Just a side note:

This is one of the many reasons, (and almost responsibilities of ownership) to keep the MGB as close to original as possible -- so as not to be labeled as a
"*^%@*!%^ previous owner".

As to your current situation, a new wire loom or harness is a tad on the expensive side, but - if you plan on keeping it and desire less trouble down the road, replace it.

You may consider re-wiring the loom from say, under the right side dash, (around where the wiper motor is), where the loom splits to engine/dashboard, but I believe you'll find the amount time of tedious work would pay for a new loom.

And the end result, when all electrical connections are new, like love, is never having to say you're worried or sorry.

glg

Picking up on Roberts comment about the accuracy of wiring diagrams, it is usful to have a lucas wiring key. This enables you to have the fundamental knowledge of what colour gets used for what application. Also it enables you to make ammendments in the colours that would have been used had the factory done it.

David
David Witham

One of the things that should be a requirement of all perspective MG owners is that they turn in their wire cutters until they can demonstrate their ability to make electrical repairs in a sane and responsible manner. I have owned numerous MGs from a 36 PB up through a 73 MGB GT and on every single one of them I have had to follow a trail of repairs by someone who is probably a great plumber or carpenter, because they sure as heck had zero ability in the area of automotive electrical repairs. I think that the one that tops everyting was on our TD when I bought it. Someone had cut out the entire front portion of the harness and rewired it following the plans for the shipfitting of a super carrier or something. The first thing that I noticed was that when I stepped on the brakes, the side lights came on! Cheers - Dave
David DuBois

David Witham: Where can I get a Lucas wiring key? Thats sounds useful.

David Dubois: Amen to all you say. The area behind by dashboard is a cats cradle or wires. The only way to sort it out seems to be to rip the whole thing out and start over.

Andrew H

Andrew,

There are two sites I would recommend:

Firstly, for the basic Brown, White, Purple, Green, Black logic I recommend you visit Paul Hunts site www.mgb-stuff.or.uk there is lots of other interesting information there as well.

Secondly, for detailed colour codes relating to the individual items in the car I recommend www.teglerizer.com/mgstuff/wirecolors.htm

David
David Witham

Andrew, CJD
http://www.mgbexperience.com/service/lucas-colours.html

Also...details about each section of wiring at http://www.mgbexperience.com/electrical/

There is also one someplace (thought it was MGB Experience but can't find) that cross references from component (headlight, headlight switch, etc.) to wire colors



Hey...anyone besides me not able to access the wiring diagrams on the MGBExperience web site?

JTB

J.T. Bamford

Although Haynes tends to have many minor variations in a single diagram and only has 9 diagrams covering all years my Workshop Manual (and hence probably Bentley) doesn't and has 19 diagrams covering just up to 1974 for North America and to 1977 for UK. Each diagram in both Haynes and the Workshop Manual is accompanied by a key, which may cover one or more diagrams. As well as generic information on the Brown, Purple, White and Green circuits http://www.mgb-stuff.org.uk/wn_electricsframe.htm contains detailed diagrams of many circuit elements like lighting etc. and listings year by year of which colours connect to what under 'Wire Colours and Fuses'.
Paul Hunt

It must be fun to cut up the dash harness in '72 B's- mine had the same problem. I cured it by cutting the dash harness from a parts car and splicing it into the harness in my car. It was easy to connect (colored wire on the donor harness to its match in my harness). Fortunately the donor car had a complete dash assembly, all I had to do was label each wire as I disconnected it then connect it to the matching item in my car.
william fox

I'm trying not to become a ****** PO at the moment. The courtesy light switch was mangled when I brought the car and the wire leading to it is discoloured. It's showing 12 Volts but there is essentially zero current. I'm finding it difficult to source the correct purple and white wire, in case the entire length of wire rather than the junction is at fault. I'd resigned myself to waiting and hoping someone at Stoneleigh would have it. Is there anywhere I could try in the interim?
Thanks

Vic
V Todman

Ordering the wire harness from british wiring, the whole thing front to back. Buggar's gonna cost me but at least it'll save me some time. I'm still having problems figuring out why the owner did this nightmare? Oh well its a mote point now. This is gonna cost me, but in the long run the Vixen'll be better for it. Btw she's only got 79,149 miles on her. :) I've decided against replacing the gauges for now and opting to have them rebuilt. I'll use the midget for my custom work for now.

Neat bit of info here I found out recently. AutoZone sells the front calipers for the MGB for 50 bucks, and a core charge of 40 bucks. They also have the master cylinder, Clutch Master Cylinder, and Slave valve (sp?) on their computers as orderible parts. I should've gotten prices on those while I was there earlier. It'll take a few days to get in but it's all good. The Proportional valve for the duel line braking system can be had from O'riely's for 86.99 new, not rebuilt. If you have an Allen & Kerber near ya'll they can bend brake lines for a small sum with the original mushroom flanges on the ends (In steel, or stainless steel), same for fuel lines. They also make stainless steel braided hoses for the brake system, and can make braided stainless steel heater hoses, and radiator hoses provided you have the original hoses on hand.
And doing some price comparisons between Moss and Victoria British can offer even more savings.

CJ
Hope this might help someone else out there!!
CJD Dark

CJ, I just checked online at:-

http://www.partsamerica.com/

Brake Master Cylinder, Remanufactured, Power....$41.19
Brake Master Cylinder, Remanufactured, Dual Cct.$54.28
Wheel Cylinder: Rear ...........................$24.99+
Clutch Master Cylinder: Single Brake Line......$107.99
Clutch Master Cylinder: Tandem Type............$126.99
Clutch Slave Cylinder: Single Brake Line.......$119.99
Clutch Slave Cylinder: Tandem Type.............$115.99
Derek Nicholson

With descriptions like that it would be a cold day in hell I'd order anything from them. Since when did any clutch have tandem lines?
Mike MaGee

Vic - I can't see a whole length of wire being damaged, although like anything it is possible. Is only one side faulty? The wires from both doors having the same damage is even less likely. Are you sure the connectors are clean and making a good connection? There is a four-way about mid-way behind the dash where the two loose wires from the door switches plug in and a third wire which disappears into the loom and appears at the courtesy light. Does the light work from the manual switch? OIf not do you still have 12v on the purple at the light when grounding the purple/white at the doors or operating the manual switch?
Paul Hunt

Hi Paul,
The N/S switch is intact and works the light ok, as does the manual switch.The other circuits protected by the fuse are also OK. I've also tried a test-lamp in series with the O/S wire and ground. A very faint glow is just discernable on the filiament, so some current is just getting through.

You are correct that the problem could be the junction. However, I've cut the purple and white wire back wire back about 3", to the point where there is almost no slack left, and the core is still blued. The test lamp connected here is no brighter. So I'm wondering if the switch came out because the screws worked loose and it was broken when the door was slammed. If it shorted, perhaps the wire overheated before the fuse blew.

I've been hunting under the dash for the junction and by a process of elimination discovered it is not quite inaccessible. Perhaps no wonder the PO didn't fix it. I am reluctant to go through the contortions required to reach it more than once, so wanted to have the correct coded wire available if the original was damaged. I was prepared to cannibalize the speaker connection, but in the course of the checking I found this was breaking up between the body and the door.

I could use another colour wire and alter the wiring diagram, but don't really want to.

Thanks

Vic



V Todman

Mike, I think you'll find that Advance Auto/AutoZone are quite correctly offering two different clutch master cylinders. One for early cars with single cct brake and the other for later cars with dual cct braking. Moss does the same. The choice of two different slave cylinders is a bit puzzling.
Derek Nicholson

CJD,
I have a real horror story to add. A good friend fried his harness through means that I won't get into, but he decided to let his insurance company handle the repairs. He bought a new harness from Moss and sent the car to a large auto electric company here in Jacksonville. When he got his car back two weeks later, he discovered a softball-sized lump of wire and tape at the firewall. You guessed it, the idiots at the shop cut his new harness and solder-spliced it into the old one where it goes into the cockpit of the car. I encouraged him to raise a stink and get it done correctly but he has not done so.
David

Crude what a mess there David, I hope your friend gets it back there before the warrenty expires for the labor, else he'll be screwed on getting it redone, and the shop could also claim the owner did that to completely avoid the having to redo it all.

Found something interesting tonight. http://www.planetperformance.com offers some parts for the MGB, and Midget, not much but better then nothing. I found a four pronged headlight switch there I can use as a temp if my head light switch ever buggars out on me again.
CJD Dark

Ah, it was a misunderstanding the way they listed the parts descriptions.
Mike MaGee

Vic,

That wire is getting its 12V via the filament of the interior lamp - if you earth it out, the lamp should light.
Chris at Octarine Services

CJD,

With all this great advice you're likey to have already proceeded to a solution but I must add that the wiring from Moss is a good product if you want to compare pricing. I have now replaced my harness for the third time (mice damage each time) and though a diagram is paramount, if you take your time under the hood and dash you'll start to see the pattern by the wire lengths themselves. Line things up with gauges and ancilaries and then cross check with the diagram. You'll see that it all goes neatly and logically in place.

Also, I took the haynes manual to the local copy shop and had both the diagram and the key enlarged to 18 x 11 placed back to back and then lamminated. Very usefull for some of our 40+ year eyes and handy when doing contortions under the dash. Rewiring of your car with the engine out can be done at home in a day with a good workspace. Also, make sure all your firewall grommets are on the loom before install or you'll have to undo major chunks of work or cut gromments.

Paul
Paul Hanley

Thanks Chris
But whilst the replacement switch was on order I fitted an accessory shop alternative. This should have earthed the wire when the switch was open but the lamp didn't light , which is where this question began. I'm off on a trip tomorrow morning, but the MG switch arrived today so as soon as I get back I'll try earthing the wire.

Vic
PS would have got around to fitting your zinc plated studs if not sidetracked by this.
V Todman

As Chris implies the function of the switch is to ground that wire so lighting the lamp, so if the wire shorts out on the body somewhere it will just permanently light the lamp and not blow the fuse. The 'blueing' is not a sign of overheating from shorting out but actually corrosion of the copper from dampness creeping its way up the insulation from the bullet in the back of the switch. You will have to clean to shiny copper before making a new connection, someone (Chris?) mentioned insertion in a mixture or oil and vinegar, maybe I was using the wrong flavours but it didn't work for me. When connecting your test-lamp between ground and the purple/white if the test-lamp glowed slightly but the courtesy light showed nothing, and given that the LH and manual switches are OK, then the problem is 99% certain to be in the connector I mentioned.
Paul Hunt

Thanks again for your help Paul and Chris. I went and shorted the wire as Chris suggested and bingo the interior lamp came on. Then put in the new MG door switch and lo and behold light on with door open and off with door closed. Finally checked the resistance of the temporary switch I'd fitted and found it stayed open circuit with the plunger extended. If the original switch had been in stock or I'd been more suspicious of a new part out of the box I wouldn't have wasted your time.


However the colour of the wire is definitely from not corrosion - trust me I was a chemist in an earlier life. I know the wire is designed to carry the current to earth, so should stand the load, but if this happened in normal circumstances, keeping the courtesy light on would drain the battery in 2/3 days. The PO used to drive the car only 2/300 miles per year and keep the battery on an intelligent charger. Suppose he broke the switch after the MOT in September and didn't notice until getting the car ready for it's trip to Silverstone in the summer?

Anyhow I am really pleased I asked for help before tearing out everything behind the dash and thanks again for helping out.

Vic
V Todman

This thread was discussed between 20/11/2003 and 25/11/2003

MG MGB Technical index

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