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MG MGB Technical - Wiring a new, original or modern?

Well the wiring in my 70 B started smoking under the dash area and I think it is time to rewire. This is about the only thing that I have not replaced in the past 8 years as it seems an overwelming job. The first thing I need to decide is whether to go with original wiring from Moss or British Wiring or to go with a new modern Power Block system from Advance Auto Wiring. Has any one used the advance product and how much harder do you think it is than using a original style product. I have done all work on my car to this point and feel comfortable with about everything, But electricity is my weakest point !!!
Bob R.

Bob. British Wiring has an excellent reputation for both the quality of their wiring harnesses and the quality of their customer support. They used to have a website which had their telephone number on it.

I would assume that Advance Auto Wiring also has a website and a telephone number.

I would contact both and see what they had to say, then, go with the one who made the most sense to me.

As for myself, my 68GT has the original wiring harness still installed. A very simple system which, when I have the necessary bodywork done and the car repainted, I will replace with an original style system. It has worked well for the last 38 years and I find it much easier to deal with than the wiring system on my later model cars. Like you, there are other things I would like to work on rather than electrical systems. Thus, I prefer the most simple system available over something which might be, under some circumstances "better".

Les
Les Bengtson

British Wiring:
http://www.britishwiring.com
708-481-9050

If you choose to go with a stock replacement, I would recommend them. The quality of their products are high, tech support is excellent, and they are super nice folks to deal with.

Advance Auto-Wire:
http://advanceautowire.com
865-982-9373 - tech questions
770-926-2213 - ordering info

Our kits are a bit harder to install, taking on average about 40 hours. Installation is well within the capabilities of the average MG owner. We feel that our kits are a major improvement over stock. We've sold over 100 kits so far, with no negative feedback.

I am the tech support for our company, and tech support is just a phone call or an e-mail away. I prefer phone calls, Because I believe I can better answer your questions if I can also ask you questions as we go, rather than an endless round of e-mails.

If I can answer your question in five minutes, fine, but if it takes an hour or more, as I walk you through trouble shooting and/or diagnostic steps, that's fine also. I'm available from 11:00 am to 10:00 PM EST, Sunday through Saturday, so if you want to get some work done in the evening after your regular working hours or on the weekend, I'm here.
Dan Masters

Bob, it may be worth identifying why your wiring overheated in the first place.

I have had the loom out of my car as part of a LHD to RHD conversion. There are signs of the lighting circuit and the charging circuit having overheated in the past. The worst damage was to the wire that runs from the starter to a 4 way juction to feed the lights, fusebox, ignition switch etc. This is one of the most important wires in the car.

When I got the car 14 years ago only one of the large tabs on the back of the altenator was in use. So I put a second wire in between there and the starter motor. The overheating of the charging cicuit happened before that. Later I fitted relays for the headlamps which reduced the load on the wires going to the light switch and back.

If you buy a new standard loom it might be worth supplementing it with headlamp relays. The is a great deal of comment on this in the archive.

In a new loom for a '70 B there should be 2 wires from the altenator to the starter already.

If you want to understand the wiring on your car a little better it is worth looking at the colour coded diagrams on Dan's web site.

http://www.advanceautowire.com/mgb.pdf
David Witham

UPDATE: Today I pulled the dash and all the wiring from that area. The wires that caused the smoke seem to be the 2 white wires from the Tachometer, however the fuse that blew when the problem happend was the one for the brown wires. I think the problem started when a brown wire in the area of the radio shorted out while doing some work there with the battery still connected, DUMB I KNOW !!
Is there a way to test that the tachometer is still OK???
After seeing what a mess everything is behind the dash I am leaning towards just going with a stock wiring setup, as even this seems to be a very tough job. But I am still open to ideas from anyone who has done either change...

Bob R.

Bob,

The brown wires are unfused, hot all the time. The fuse that appears to be for the brown wires is being FED by the brown wires. Look at the color of the wires on the other side of that fuse to see which circuit actually blew the fuse (purple?). The white wires are also unfused, the only difference being that they are controlled by the ignition switch.

The folks at British Wiring are super nice! ' Suggest you contact them. This problem does not necessarily mean you need a complete new wiring harness. Perhaps just a new dash wiring harness, at the most. Wires inside harnesses can last for decades without problems; problems occur at bullet connectors and other connections (i.e. at the ends of the wires). Of course if the bare end of a brown wire, a white wire, or a white/green wire were to come into contact with just about anything metal under the dash, smoke is released. And it could have cooked one of these wires inside a harness, in which case ignore what I said about wires inside harnesses.

The white/green wire is the accessory wire from the ignition switch and it too is unfused - although it should lead to a line fuse (not on the fuse block). The downstream side of that line fuse is green/pink and that should run to the radio, the heater blower switch, and the windshield wiper switch. The only brown wires you should see under your dash run to the ignition switch and the headlight switch. Yes, your headlights are unfused - intentionally, as Abingdon didn't want your headlights failing due to a blown fuse.

This is just a guess, but if your tach was fried, the problem was IN the tach. I rather doubt it; ' think you're safe in presuming it's OK until you get the wiring fixed. Is there a clearly visible spot where the short (smoke, burning, etc) seemed to start?

Of course it's amazing how bad a rap Lucas takes for DPOs who do totally stupid things. On one of my Bs, the DPO had connected the radio with a safety pin penetrating the radio lead and the brown wire. The safety pin was resting on the rectangular channel brace that runs across the car under the dash. There, but for a coat of paint, was a car fire waiting to happen.

See if you can determine where the worst damage occured and report back. One of us should be able to figure out what happened. Don't blame your wiring harness - yet!

FWIW,
Allen
Allen Bachelder

Absolutely what David has said. The harness may need replacing anyway, but unless the fault was actually in the harness (i.e. chafed insulation which on its own is quite rare) then you run the risk of smoking the new harness as well.

If the two whites by the tach are burnt this implies a short out towards the coil +ve on the white, and this is the critical thing to check out very carefully. The tach itself will be undamaged (except possibly by heat!) as there is no *electrical* connection between the whites and the tach electronics at that point, only magnetic.

The brown wire doesn't have a fuse as such, but if you are talking about the fusebox it *feeds* the fuse for the purple circuit. If you are talking about an in-line fuse then that will be for the hazards and is brown both sides. As such one brown feeds the fuse, and the other brown takes the fused supply on to the hazard flasher. If a fuse blew because of a short on the load side then that would have been all that happened - that is what fuses are for. But because the whites are damaged that will be the real problem as I say above, the brown fuse will have been purely co-incidental. However because one of the whites at the tach is damaged that is fed from a brown via the ignition switch, and the excessive current will have been flowing right through this circuit all the way back to the battery. As such it may have damaged the ignition switch and the brown feeding it.

The green/pink circuit Allen mentions is only applicable for the 71 model year to the end of chrome bumper production, after that the wipers etc. reverted to the ignition circuit. Of course if Bob's car were a 71 model manufacturered in 70 then it would apply.
Paul Hunt 2

Thank You Allen & Paul: It was the fuse for the Purple circut that blew but all of the purple side wires seem fine so guess the fuse did it's job. After taking apart the Wiring harness I have found that the white wire from the from the coil to the tack and back out of the tach to where four whites come together was burnt, inside the tach the white wires were hot enough to melt the wires little coil thing that they go around, so I will be looking for a new tach. I tested the ignition switch with an ohm meter and it seems to at least let current flow between conections when the key is in the right possition. I do have doupts about the pink/purple wire as I don't have connectivity there ( bare with me as I do not know the proper electrical terms) The alternator warning lamp still will works. There was also a brown wire coming out of the wiring harness that I think was going to the Hazard switch that was very melted and burnt and I believe this is the place that started the problem. Anyway, that is were I am now so if there is anything else I should check before started a rewire let me know how !!!!
Bob R.

Purple/pink? Or green/pink? The former was only on 76 cars for the ignition key buzzer, the latter should show 12v in the accessories and ignition positions.

The hazard switch shouldn't have a brown on it, a *fused* brown goes to the hazard flasher, so that shouldn't have been burned either (except by physical proximity to something else very hot). From the hazard flasher to the hazard switch is light-green/brown, other wires on that switch being two greens, green/white and green/red, and light-green/purple.

Any burned brown will be nothing to do with the burned whites as a *cause*, but could be as an effect but only from the ignition switch. Only a short on the white between the tach and the coil would cause the whites to burn the way that have, and that is the prome thing you must ensure is not still present, although with a replacement harness only a short-circuit coil (either to the -ve terminal or the case) could cause a reoccurence. If you have another burned brown then either it happened at a previous date and the cause was removed, or there are two problems.
Paul Hunt 2

Paul: Thanks so much for being such a big help. Yes it is a Green/Pink wire, would this part of the ignition switch only cause the buzzer not to work when the door is opened. I disconnected the wire to the door switch years ago. When I do rewire the car I would like to just get rid of the buzzer all together.
Let's say that a short somewhere caused a wire to melt and in turn melted the white wire just enough to cause it to short out. Is that not possible? At any rate when I rewire I plane to use a new coil and tachometer, besides the ignition switch what else should I check and how.
Bob R.

Whoa, this is getting confusing!

I was wrong saying the purple/pink was only from 76, it was from 1970. With the key in the ignition the ignition switch connects a ground to this wire, which goes through the buzzer, then a special (insulated from the body) door switch to a purple wire. This causes the buzzer to sound if the door is opened with the key in the ignition.

The white green carries 12v with the ignition switch in either the accessories or ignition position. On a 70 it was used for an optional radio, left hanging if no radio fitted. For the 71 model year and the remainder of chrome bumper production this wire went to an in-line fuse then a green/pink fed the electric washers, wipers and heater fan.

The purple/pink and white/green circuits are completely separate.

The only thing that could have caused the whites at the tach to melt in the way you describe is if *those* whites were carrying excess current, which could only happen if the white from the tach to the coil shorted to ground somehow, so that is what you must concentrate on. If the white at the coil shows no signs of damage then possibly the white itself got chafed somewhere, like where the harness passes through the bulkhead and inner wing. However it is important to remember that you will get more heat build-up and hence more heat damage where the wire is in a confined space like the tach than in open air like in the engine compartment, even though both parts of the wire are carrying the same current. Another possible cause is the coil going short-circuit, either between the two spade terminals, which would tend to cause similar damage in the black white to the points, the points themselves, and the ground wire inside the distributor. Or the coil +ve terminal could have shorted to the case, which wouldn't affect the black/white etc. Or a PO could have attached some other wiring to the coil +ve feeding something else, and it is *this* that shorted out and damaged the whites.

When rewiring, before finally connecting the battery ground strap (which should be the very last thing you do), connect a 35 amp or higher rated fuse in its place. Then check each circuit *in turn* (not all together) and make sure nothing blows that fuse. You should be able to do everything except crank without blowing the fuse.

This checks everything except the starter relay and solenoid circuits. If you disconnect the white/brown from the solenoid you can also check the relay and wire down to the solenoid by turning the key to crank (all you will get is the relay click).

And if you can somehow disconnect the link between the solenoid and the starter motor, or remove the battery cable and browns from the solenoid but connect the cable and wires together, and reconnect the white/brown at the solenoid, then you can also test the solenoid (you will now hear the solenoid clonk). But that is all rather a fiddle.

If nothing has blown the fuse then remove it and reconnect the battery ground strap.
Paul Hunt 2

Paul: thanks, if you think confusing, just imagine what I'm thinking. The white wire has the worst damage at the tack and at the coil the other wire from the coil to the distributor is also very damaged and goes to an ignitor replacement to the points, the little ground wire inside the distributor seems undamaged, everything else in the distributor looks to be undamaged. The white wire also has the least damage inside the wrapping and shows no sign of damage at the connectons to alternator warning lamp, overdrive,fuse box or ignition switch. I am printing out all of your instructions and am sure they will come in very handy when I get to that point. putting a fuse between the battery and the ground strap for testing is a great idea...
Bob R.

Damage to the wire between coil and distributor indicates the coil went short-circuit between its two spade connections. This would have passed through the ignitor and possibly damaged that, a good excsuse to junk it and retrofit points and a ondenser. I wouldn't expect any damage to the warning light or OD whites as coil current doesn't flow through them. It does however flow through the whites at the fusebox.
Paul Hunt 2

I've retreated to the sidelines on this as Paul knows much more than I, but is it possible that the white wire was accidentally connected to the wrong side of the coil at one time? If the engine was at rest with the points closed, this would fry both the white and the distributor primary the moment the ignition switch was turned on, would it not? Since many of these coils have double spades on each connection, that does not strike me as an implausible error.

FWIW,
Allen
Allen Bachelder

Indeed it would, but at least one of mine has a male spade one side and a female the other so you can't get both wires on the same coil terminal. You could connect the white and the black white together though!
Paul Hunt 2

Paul, I suspect your spade arrangement is probably more like an original set-up. A lot of aftermarket coils sold on this side of the pond come with double males.

FWIW,
Allen
Allen Bachelder

Paul & Allen: I still have not figured the cause of the problem. I have decided to do a complete rewire of the car and replace the distributor, coil, & tachometer before even trying to put a load on anything. Will then follow the guide you have given me when testing the new wires. This will all take some time so wish me luck. If you are not tired of my questions, do you see any reason why I could not eliminate the intire circuit from the PK wire on the ignition to the buzzer and LH door switch, as I don't want the buzzer anyway ??
Bob R.

Bob - absolutely not. Breaking the circuit anywhere at the ignition switch, insulated door switch (2 wires purple/pink plus purple/green) or buzzer will silence it.
Paul Hunt 2

Bob

You might want to consider using the buzzer and door switch as a warning when you manage to leave your lights on. I have found that when we turn the lights on in the dark AM to go to work and then the sun is well up when we arrive we (at least me) occasionally manage to leave the lights on. A buzzer connected to the headlight power lead and the door switch will give you an audible warning to turn the lights off.

FWIW

Larry
Larry Hallanger

A good idea, you would need to change both ends of the circuit - if you connected one side of the buzzer to a lighting wire (parking light I suggest, rather than headlight) you would have to connect the other side of the door switch to ground. Or vice-versa.
Paul Hunt 2

I did this on my 73 GT by buying a 12V chime from Radio Shack. +ive goes to the white/red dash-lamps and -ive goes to the door switch for the courtesy lamps. Here, we are required to run headlights whenever the wipers are needed, so the chime has saved me from a dead battery many times!

Allen
Allen Bachelder

Bob R,

Good luck with your re-wire. I used one of Dan Master' setups on my MGBGT V8 conversion last winter and it has worked well, although as Dan says, it's quite a lot of work - maybe more than 40 hours - I gave up counting!

However, after my diversionary ramble, the real point of this post is to pass on some advice I was given. If you are replacing an old damaged loom, cut off the old wires about 2" away from the component they serve and remove the old loom. When you put your new loom in place the old wires will show you how the connections you need to remake were fitted originally.

Before you start recommissioning the car after completing the wiring change, it is worth reading the last section of Dan Masters' instructions on how to recommission a car after installing one of his wiring systems. Many of his ideas are good, basic engineering and apply to any rewiring work.

Regards

Peter
P L Hills

I did a British Wiring re-wire and it was 12 hours or so being very careful and I am no tech or electrician....just was following instructions...though I had fixed a few wiring probs on the car previously. Had color wiring chart plus the Bentley wiring chart that was the precise one for the car which helps if you get confused or something conflicts.

Be very thorough. I melted down my dash wires and found after pulling the wires going to the back of the car that there was a short where the wires went through a couple of fold over soft metal brackets near the clutch slave cylinder. All three wires rubbed raw and shorting....the white hot wire among them. So don't skip the wires to the back and all of them in the back as well as the wires in the front.

Oh...I forgot I didn't have to do the ones in the back or the front as I had already rebuilt those myself earlier so might take a little longer. Just do a section at a time and be sure of each thing you do and don't hurry.

One approach is to cut the wires off on the gauges, lights, etc. and leave about 2 inches attached to the component to know what color to re-attach. This assumes not a lot of DPO re-wiring with wrong color wires. If you use Masters or British Wiring they can give great tech help. British Wiring was great about that.

Do test circuits before hooking up battery with low power battery charger.
J.T. Bamford

"-ive goes to the door switch for the courtesy lamps"

If you use the switch for the courtesy light and the courtesy light is still connected you need to add a diode or the dash lights will slowly discharge the battery, see http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on "Lighting" and "Interior lighting and 'lights on' warning".

It also needs to be from the red from the main lighting switch rather than the white/red to the gauge lights or if the gauge lights are turned off or dimmed right down the buzzer won't sound. However in that case it won't discharge the battery without a diode!
Paul Hunt 2

This thread was discussed between 23/10/2006 and 02/11/2006

MG MGB Technical index

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