Classic cars forum & vehicle restoration.

[therobbiesmith]

Ok all you experts out there. My expertise is limited to previous activities and I've never attempted this before, so I need your help.

I would like to upgrade my existing Positive Earth system with Generator/Dynamo to Negative Earth with an Alternator.

So, three questions:

What will I need to obtain? An Alternator, obviously, but what else?

What will I need to do?

After the upgrade, what will I need to change? The Rev Counter is positive Earth, so that will have to go, what else? The radio is switchable I think.

[peter scott]

Hi Robbie,

Welcome to the forum.

Are you finding that the dynamo and RB340 control box give you a flat battery?

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[therobbiesmith]

Thanks Peter,

No, no flat battery, that's not really the issue. I would like to upgrade the headlights, fit reversing lights and so on, so feel I need to boost the available power.

Also Positive earth is a menace if I want to fit accessories.

Robbie

[lowdrag]

I know that in the day there were positive earth alternators made. Aston used them for one. Have you searched for one of those? I spoke to a friends about changing the polarity and he said it isn't that difficult but I forget how he said to reverse the polarity of the starter motor. I am sure the MGOC or MGCC will have something on the specifics for your MGB

[peter scott]

The starter motor is no problem as it's a series wound machine so it will always rotate in the same direction regardless of what polaity is applied.

As Robbie has highlighted the elements that need attention a those where semiconductors have crept in. Electronic rev counter, radio, fuel pump if spark reduction diode is fitted.

The alternator is normally a self contained unit including its regulator/cutout so the only wiring to consider is to attach the ignition warning light to the appropriate alternator terminal. The dynamo control box can be totally removed and the alternator output connected to what was previously connected to the D terminal.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[therobbiesmith]

Thanks Guys, now we're getting somewhere. Reversing the polarity on the dynamo was the first improvement I considered, a very simple job, but I felt 'Why not go the whole hog and get the better performance of an alternator while you're about it.

The fuel pump is polarity-insensitive and will work on negative earth without bother.

Keep those tips coming.

By the way, Peter, I love your SS100 and really envy you cruising along empty Scottish roads in that magnificent machine.

[47p2]

The low tension wires to the coil will also have to be changed around
_________________
ROVER
One of Britain's Fine Cars

[peter scott]

[therobbiesmith]

Peter,

Thanks for putting me straight. Whatever it's called, you've got one and it's lovely.

Lucky you,

Robbie

[peter scott]

Looks like a rev counter from a later car will solve one problem..

http://www.middlebank.co.uk/Mechanics/negearth.html

http://mgaguru.com/mgtech/electric/ac101.htm
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[therobbiesmith]

Thanks Peter,

Have you seen this?

Only £275,000 - a snip.

http://www.carandclassic.co.uk/car/C76077/#

Robbie

[peter scott]

Hi Robbie,

Yes, even an Edinburgh car. I suspect that if I wanted to buy it I would need to sell at least 10 cars like mine.

Whilst you would get to 60 mph in under 10 seconds as opposed to the 17 seconds for my old girl you loose in comfort and carrying capacity and I'm
not sure the SS100 was designed to accommodate my 6' 1" frame.

No, I think I'll stick with what I've got thanks. (As if I had the option!)

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[therobbiesmith]

That reminds me of getting in and out of my Midget, not easy, hence I now have a B.