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MG MGB GT V8 Factory Originals Technical - V8 wiring...

I'm getting to the stage now where i can finally connect up the electrics to the V8 and get it up and running.

It's a '78 B with a 3.5 Rover V8 - i'm planning on using the existing MGB coil but i'm not sure how to wire it up? does anything need to be done differently?

Any info would be great as i don't want to wire it up wrong and damage anything.

Cheers
Gavin

Gavin Jewkes

This is really something for Pauls expertise, or someone similiar but I'll tell you what I know.
There is every chance that your current coil is a 6 volt coil. The later rubber bumpers had a ballast resistor in the loom. This ballast resistor is actually a long high resistance wire that is taped into the loom and runs forward, across the front of the car and then doubles back again. It is either pink or white in colour, mine had melted its plastic coating so it was hard to tell.
Normally (normal being other than MGB!)these ballast resistors are a short wire (sometimes looking like a small spring or heater coil) held within a white ceramic block roughly 2 by 2 by 5 cm. In most cars this is often found attached to the side of the coil bracket or nearby. The wire gets hot, hence the ceramic. When the engine is running normally the coil is fed through that ballast resistor so the 6V coil gets the correct 6 volts only and generates a normal sized spark. There is an extra terminal on your MGB starter so that when the motor is cranking the ballast resistor is bypassed and the 6 volt coil gets 12 volts, producing a very powerful spark which of course makes starting a cold engine much easier. 12 Volts will not hurt a 6V coil for the short amount of time it takes to start the car, particularly as the starter is also drawing alot of current and you may not be getting the full 12 volts anyway. However if you try to run your 6 Volt coil on 12 Volts all the time then it will get very hot and likely burn out. For obvious reasons (see above re: melting insulation) I strongly recommend you substitute a ceramic ballast resistor.
There is an additional complication which might mean you will have to go out and buy a new 12 volt coil. It is likely that you are using an SD1 starter motor. Thats the one with the solinoid on the bottom so it will fit in the MGB. Unless you are very lucky this motor probably will not have that extra, "bypass the ballast resistor when cranking" terminal.
How can you tell if you have one?. You have at least one small terminal for the wire running from your igition key that gets/activates the starter motor to crank the engine. The ballast bypass terminal will look just like that one (small).
When cranking that terminal has 12 volts on it, when not cranking it is dead.
Even if you don't have a ballast bypass terminal all is not lost, you can put an external version. A relay that is operated by the "crank the engine wire". I believe that Paul has details.
My engine is fuel injected and did not need a Ballast resistor so I used a 12Volt coil that I had. Starts evry time. Maybe when the engine get older I'll wire a bypass ciruit etc in.

Peter

My suggestion is that you find the resistor wire in the loom and eliminate it. Extend a wire from the start of the resistor wire to the coil and use a 12V coil. I have converted my distributers to Pertonics pointless and use the matching Pertronics coils, but any good quality 12V coil will work. The Lucas Sport Coil is a good choice, especially if you want to maintain the British "look".

If Zach's web page is still up, it contains simple instructions to convert your 4 cyl tach to 8 cyl by adding a $.25 resistor.

The only other wiring would be adding a circuit to power that electric cooling fan. Don't think that the original wiring is adequate to power a 16" aftermarket cooling fan, should you choose to use one. You can use part of the existing fan wiring, including the "otter" switch in the radiator as the signal wire for a relay to power the fan. Run an additional 12 ga wire from the starter terminal to a relay located close to the fan. Use a 12 ga wire to ground the relay, another 12 ga wire to the fan, seperate 12 ga from fan to ground. If you want to maintain "British", use the lucas relay in the metal box from an earlier chrome bumper car, or any of the Hella relays. I use 35amp relays, an wire a 35 amp circuit breaker in the hot line, rather than a fuse.

The factory fans could be used if you live in a cool climate, but they will function much better if you modify the circuit with a relay and ground each fan directly to the body
Jim Stuart

Thanks guys, i have already got a 12 volt coil on my MGB - so its just a case of wiring it up then?
Gavin Jewkes

Right just had a another look and i have the following questions,

Distributor/Coil,

There are two wires coming off the distributor on the v8 - but there seems to be only 1 wire coming off the coil? what do i do here?

Starter Motor,
On the starter motor theres two lives and a small wire to the solenoid and also a thick earth lead do i just put the earth lead to any ground? on the engine or gearbox?


Gavin Jewkes

Need to state- Fuel injected or Carby'.
Hot wire or flapper EFI.
What sort of ignition modual do you have. The one the side of the distributor, or the remote sort (remote Federal or remote hot wire).
If your's is a '94, Hot wire Side of the distributor modual type I'll go and unpick the loom and look. Otherwise not much point cause they vary a bit.
There is a lot of variation through the years, hence the stunning lack of replies, and it is a bit of a head ache to work out.
You might try
www.rangie.com
all sorts of stuff there. The '89 to '98 range rover manual is worst than usless. The earlier one OK.
For the starter-Thick wire from/to altenator, which also goes to battery. One of the little wires will be starter and the other will be the ballast bypass. Basically wire up as per MGB 1800.


Peter

Gavin,
Which distribtor do you have? DLMB8 which has the amplifier on the body of the distributor or if is it just has two wires coming out from the body itself you need the external amplifier. If its the former you should have a plug connector to the amplifier with two wires with spade connectors on the ends which go direct to the coil. You need to run a new feed to the coil from the ignition controlled side of the fuse box. I tried a new sports coil but the car would not run and was told that the KV output was not compatable with the electronic ignition module. If you have the original plug connector be careful as the connectors inside the plug can be loose and cause problems, I made up two new heavy guage leads and and ran them too the coil and this cured the problem.The other lead from the coil probably black and white is the feed to the tacho which will read double if not converted or swapped for a V8 one. Sorry I cant be specific re the polarity of the connections to the coil as my car is garaged a few miles away but I'm sure someone will come in with the correct info.
Kevin.
Kevin Jackson

This thread was discussed between 07/04/2006 and 09/04/2006

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