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MG Midget and Sprite Technical - negative cut off switch

Gentlemen,

I change from positive to negative earth and from dynamo to generator. The dynamo did not like to be revved above 5500 rpm….. I like to maintain the cut off switch. I prefer to place the switch between the battery and the mass. I don’t like to toast the generator diodes. Where can I find a schedule? A photo of a schedule wit the cut of switch in the positive track is included below.

Flip


Flip Brühl 948 frog 59

Yes, you can located the switch between your battery's negative post and the body.

One thing though, after fitting an alternator you must make sure that the engine is off before you disconnect that switch or the alternator diodes will be fried. Only disconnect the battery after the engine is off.

As long as you observe that rule you will have no trouble with your plan.


Norm

Norm Kerr

Norm,

If I connect the main electrical feed via switch1 and the resistor to the positive pole of the battery wil the circuit be protected?
I dont like Murphy's law.... but i believe in it.

Flip
Flip Brühl 948 frog 59

No. The negative side of the battery will not be connected to anything, so connecting the resistor to the positive side will serve no purpose.

The resistor needs to ground the alternator output.

You should wire it the way it is in the diagram.

Why do you want the cut-off in the ground/negative?

I can't think of a simple way to wire it with the cut-off in the negative lead and still protect the alternator.
Dave O'Neill2

I agree with Dave, if you want to use this switch to shut off your motor (like in emergency racing situation) you need to connect it to the positive side as shown in the switch manufacturer's diagram, along with the secondary circuits shown to the coil, etc.

If you connect it to the negative side then you must only operate the switch after the engine is off to avoid alternator damage.


Norm

Norm Kerr

One advantage of cut off on the negative side is the convenience when you need to weld!!
Allan Reeling

How often do you need to weld?

;o)
Dave O'Neill2

If I connect the output (+) via the switch 1 and the resistor to the battery- and the main switch to the ground wil the alternator circuit be protected?

Flip
Flip Brühl 948 frog 59

If I understand you correctly, you will effectively be connecting the resistor directly across the battery. Not a good idea.
Dave O'Neill2

Flip,

Did you also change over your coil?
G Lazarus

More to what Dave and Norm have said. On an alternator-ed car, if you put the cut off in the earth line you run the risk of the switch NOT shutting the engine down owing to 'bleed through' in the alternator windings powering up the LT circuit.

It's a HUGE no no on race cars!
Deborah Evans

I will follow the original instructions (it is a lot more work as I already had an cut out on the ground side)
thank you all

Flip
Flip Brühl 948 frog 59

"How often do you need to weld".
Well in the last 6 months I've, welded brackets to the axle, modified a battery box, stiffened up the hatch lock plate, welded the crack of doom, welded gussets to the rack mountings and done a chassis repair. And this is a very sound car!!!
Allan Reeling

This thread was discussed between 10/04/2012 and 14/04/2012

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