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MG Midget and Sprite Technical - Master switch wiring for dynamo

Hi

Anyone installed a battery cut out master switch on a positive earth dynamo car?

All the advice and wiring diagrams for FIA type switches are for alternator equipped negative earth cars, and all the advice is fit the FIA type Autolec switch (includes a resistor and swich for ignition as well as battery switch) to stop frying the alternator when the master switch is used when the engine is running (e.g. by a motorsport marshall when a car is crashed at a race circuit).

Can a simple battery master switch (non-FIA) be operated on a positive earth dynamo car when the engine is running witout causing electrical damage? And would this pass for motorsport usage in the UK, as dynamo could still be turning? I am planning on keeping the orig ignition switch in addition to putting in a battery master switch. I am wanting to sprint/hillclimb the car. Car is Mk2 948cc Sprite.

Thanks
Mike
M Wood

Well, I am an expert.....

At setting stuff like this on fire !!!

So that said, id think the main issue with a cut off switch to a dyno, is you would probably need to figure out how to re-flash the dyno, once its been de-activated

One option might be to have several spare dynos and replace them as needed and just rebuild them... just diods is probably all that would be needed. Agian just guessing, postive earth cars always make me nervous if modified from factory orginal

Im hungry for some Hot dogs and marshmellow ...let us know when there ready to eat

Prop



Prop and the Blackhole Midget

I used to race a midget with a dynamo but with a negative earth system. I had an autolec , the basic one with just the two battery terminals. My friend wired it in and I believe he had to do something at the control box, maybe join two terminals to make it stop the engine. I am sure someone will be along with some more specific advice but I am confident it can be done

Mike
m fairclough

Use the FIA switch.

If you don't wire the ignition through the circuit breaker (easy to do with the FIA switch) the engine will keep running after the switch is thrown because the dynamo provides power to the engine. You don't need the resister because the dynamo doesn't have a diode pack to fry.

I get a couple of cars in scrutineering each year where the engine runs after the circuit breaker is thrown because the ignition hasn't been correctly, or at all, wired through the circuit breaker. It's a bring the car back when you've fixed type of issue.

So, no, the basic switch won't see you pass scrutineering if the circuit breaker test is done with the engine running. And, in the event of any kind of accident (driver unconcious) you do want the marshal to get the engine shut down promptly (along with the rest of the electrics).
Daniel Stapleton

However, it's probable you don't need an external cut-off if your car was built before 31 Dec 1961 or licensed for road use - S10.6.3 which refers to specifically refers to K8.1-4.

Whereas for circuit racing it's mandatory by virture of Q19.11.1 (with similar age related exemptions unless the SRs refer to K8.
Daniel Stapleton

Hi Mike, Prop and Daniel

Thanks for replies, helpful and hilarious!

Daniel - thanks for the scrutineetr's insight. I'll go for an FIA switch, as I want to be able to isolate the battery for storage reasons, but don't want to fail scrutineering for a speed event for a switch I don't have to have being the incorrect spec and operation. It's a 1962 car so wouldn't be period exempt for racing - not that I will ever be circuit racing anyway.

Cheers
Mike
M Wood

Sorry to hijack - but am I right that the ignition switch to coil wire (white) should be the one across the cutoff terminals?
Philip Dodd

Philip

No problem - you are very welcome to thread hijack.

I'll be able to answer this when I wire it up tomorrow!

Attached is the wiring diagram that came with the FIA battery and ignition master switch - note that it assumes negative earth and an alternator; hence my original question.

Is you car positive earth and what kind of master switch are you going for - FIA, or the simple battery master switch?

Best wishes
Mike


M Wood

This thread was discussed between 22/09/2013 and 28/09/2013

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