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MG MGB Technical - changing car's polarity, and fitting speakers?

Hello,

Sorry I don't think that technically this is an MGB thread, but I am always impressed by the comments and suggestions you all come up with, and I am mechanically ignorant.

I am fitting a stereo in my Riley 1.5, which is positive earth, so needs to be changed. It has an MGB 1800 in it, but with a generator rather than alternator (I am assuming that is different from all MG B's?). I want to change the car's polarity by myself, after getting tips from my knowledgable yet pricey mechanic. Please tell me... is this procedure correct, complete, and safe for my car:

1. disconnect battery,
2. swap around the 2 wires on my electric fuel pump in the boot, and the 2 on the back of the ammeter in the dash,
3. reconnect battery other way around, with negative post connected to earth strap,
4. disconnect fan-belt and the big and small wire on the back of the generator,
5. run jumper-lead from positive earth of the battery, and hold other end over the 2 exposed spade terminals on the back of the generator, shorting it out so it acts like a motor.
6. put everything back together and check on the ammeter that it is behaving as before.

So, if anyone has done this before or has any tips or warnings, these would be VERY appreciated, as I plan to do all of this tomorrow night, and am nervous about blowing up something.

very unimportant second question: anyone made perforations in the car's vinyl to make a regular, profesional grille for the stereo speaker behind this? Any tools/techniques that helped you?

Cheers, and happy motoring,

Tom.
Thomas

Early MGBs were generator, and positive earth, so both are valid questions.

When disconnecting the battery *always* disconnect the earth strap first, and reconnect it last, and this is irrespective of which polarity you have. The reasoning is that if you accidentally short the earth terminal to the body while working on it nothing will happen because they are at the same potential. Once this is removed shorting the 12v terminal to the body will also do nothing, as the earth connection has already been removed so no current can flow. If you tackle the 12v terminal first and touch *that* to the body you will get a big flash, and possibly explode the battery.

In my experience batteries always have different-sized +ve and -ve terminals. If you have the cup-type connectors they will not fit the other way round and
will need to be reversed. But if doing that I'd replace them with clamp-type anyway. These are also different sizes but can be made to fit, although I'd rather swap them over. Cup-type are usually moulded on and would have to be cut off slightly shortening the cables, the clamp-type are usually connected to the cable with two bolts.

For an SU fuel pump, you only need touch it if it contains a diode for points protection instead of a capacitor. If so, then rather than reverse the wiring to the pump which will leave the pump body live, you should reverse the connections of the diode under the end cover. Can't speak for after-market pumps.

You are correct for the ammeter, and *briefly* flashing 12v to the D terminal of the dynamo although you don't need to remove the fan belt, and even manually operating the cut-out relay with the dynamo still connected may be all that is required. The battery must be reconnected its new way round to do this, and if you have a fusebox like an MGB in the engine compartment then you can use a wire from here (brown in the case of the MGB) rather than the battery. But then, the battery may be in the engine compartment in your car! In either case it is from the 12v hot connection, which should be positive, not earth.

If you have a heater fan try connecting it both ways round and see if it makes a difference, on later positive ground MGBs it does, and connect it whichever way gives the best airflow.

You may find the wipers stop in a slightly different position to before as the motor will be turning in the opposite direction if it has a magnet stator and only a wound rotor.
Paul Hunt

CAUTION

If your Riley has an electic tachometer you MUST change it's polarity before reconnecting power!!!

As I have no clue as to the Riley's fitment, I cannot advise how to go about it.

Holes in the vinyl are best made with a hollow punch like for making gaskets. Similarly, if no punch is available, you can use a ball hammer and a suitable piece of iron with a hole in it to pein against to "cut" itty bitty holes. Hobby shops have pliers-type punches for leather that work quite well on vinyl too.

Mike!
mike!

Good point. Should have checked my own check-list! Quite likely to be much the same as MGs of the era, the usual way is to open up the case and reverse the 12v and ground connections inside.
Paul Hunt

This is perhaps the most ignorant question you will see in a while, but is the tachometer the rev-counter? If so, what is an "electric" one? I am fairly sure it has a cable leading up to the back, does that tell me anything? Does an electric tach measure in the engine and send that to the gauge in the dash by wire?

It was too dark and cold in Auckland last night for me to do any work, so I bit the bullet and took it to the mechanic... means I get rid of the hated cup-type connectors!

Thanks all for you helpful advice. Tonight I am going to create my speaker holes.

Oh, and if anyone in NZ reads this, can you recommend a good place to get hold of kits, eg. for rebuilding my SU's?

Cheers,

Tom.
Thomas

Yeah, "rev-counter" = "tachometer."

If you have a heavy stiff cable leading to the back of the rev-counter, it is mechanical so no worries. Electric ones sense ignition pulses and convert them to mechanical motion of the indicator needle (couple different ways).

Dunno about carriage charges but Burlen Fuel Systems manufactures all the SU and Z-S parts from the original tooling so a quick look at their web-site is recommended!

Mike!
mike!

This thread was discussed between 25/08/2004 and 27/08/2004

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