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Battery Master Switch
Big Stu - 14/12/05 at 06:04 PM

Hi all,

I know that there is a previous post on this subject however I did not managed to get a definative answer from reading it. It basically consisted an arguement then lots of hugging and back patting.

Anyway here is my question. I require to fit a battery master switch to my Land Rover project for competition reasons. The switch has six connections. The two large connections split the battery to starter cable.

The other four connections are the small blade type. Two of these split the coil from the 12v supply, and the others are for..... well that's my question. How do you connect this side. I think that you connect the resistor to one spade and the altenator output to the other. With the resistor going to earth?

Is this correct?

Sorry for waffiling, but I have had a couple already.

Cheers

Stuart

Mr G - 14/12/05 at 06:34 PM

.

[Edited on 14/12/05 by Mr G]

iank - 14/12/05 at 07:14 PM

There is a diagram here (Rorty's site) - top right.

http://www.rorty-design.com/content/buggy_electrics.htm

It's protecting the alternator not the coil.

One comment it isn't just the starter thats isolated, everything on the ignition needs to shut off (fuel pump etc). Probably best to shut off everything including any bits that are permanently live normally.

Don't wire it up p*ssed though coz car batterys can melt big chunks of metal when shorted

[Edited on 14/12/05 by iank]

lsdweb - 14/12/05 at 08:39 PM

Just make sure that the small terminals that break when you knooc the switch off disconnect the live ignition feed from the coil and fuel pump (if electric).

Don't worry too much about protecting the alternator - just make sure you use the ignition switch before knocking the master switch off in normal use. If you, or marshalls have to use the master switch then you're in big trouble so damaging your alternator will be the least of your worries!

Wyn

skydivepaul - 14/12/05 at 08:46 PM

have a look at this, i wired one up recently and this works great. Just be sure to get the n/c and n/o contacts the right way round. I didnt after a few drinks!! and the permanent live on the resistor makes it get very hot
Burnt through the tie wrap holding the resistor to the chassis leg. No damage done though once corrected Rescued attachment fia mater switch.jpg
Rescued attachment fia mater switch.jpg

iank - 15/12/05 at 10:36 AM

quote:
Originally posted by lsdweb
Just make sure that the small terminals that break when you knooc the switch off disconnect the live ignition feed from the coil and fuel pump (if electric).

Don't worry too much about protecting the alternator - just make sure you use the ignition switch before knocking the master switch off in normal use. If you, or marshalls have to use the master switch then you're in big trouble so damaging your alternator will be the least of your worries!

Wyn


The whole point of the expensive 6 connector switches is to protect the alternator diodes. If you just want to shut everything down and don't care use one of the cheap 2 pole ones and cut off the battery from everything.

I doubt the small contacts can handle very much current anyway.

Big Stu - 15/12/05 at 02:25 PM

OK I think I have got it sorted I am going to wire it similar to pauls layout above, so that when the switch is pulled it will short the positive supply to earth via the resistor.

The other side of the switch will disconnect the coil and fuel pump.

skydivepaul - 15/12/05 at 10:32 PM

remember to connect the resistor to the positive load side not the feed from the battery.................its easy done

[Edited on 15/12/05 by skydivepaul]

lsdweb - 15/12/05 at 10:48 PM

One of the points of the 6 terminal (3 pole) switch is to protect the diodes. But, the main point of the cut off switch is to cut off the engine! Scrutineers won't give a monkeys about your alternator and, when you're upside down with the engine still running, the marshals won't either! All they care about is that the switch will kill the engine, that's why you need to isolate the ignition / management and fuel pump as just diconnecting the positive feed from the battery will not kill a running engine with an alternator!

Wyn

iank - 15/12/05 at 11:45 PM

quote:
Originally posted by lsdweb
One of the points of the 6 terminal (3 pole) switch is to protect the diodes. But, the main point of the cut off switch is to cut off the engine! Scrutineers won't give a monkeys about your alternator and, when you're upside down with the engine still running, the marshals won't either! All they care about is that the switch will kill the engine, that's why you need to isolate the ignition / management and fuel pump as just diconnecting the positive feed from the battery will not kill a running engine with an alternator!

Wyn


There was a touch of sarcasm in my comment that could probably have done with a smiley re-reading.

My point is it's as easy to protect the diodes as not, so saying not to bother seems pretty pointless. The scrutineers don't care so long as the engine dies, but they also don't complain if you wire them up as intended. The first time a mate (or a scrutineer testing it works) hits the switch and destroys your alternator 10mins before the race is really going to p*ss you off - no? I certainly don't treat alternators as consumables.

And as for the single switch not killing the engine - the sarkie comment was intended to imply it will when the diodes go pop on the alternator coz there won't be any more power going into the system. I believe they blow quickly (in milliseconds), but I've never bothered trying to destroy them, I wire it up as recommended, which also has the advantage that you can use them as a quick kill switch while working under the bonnet.

Have you ever seen the diodes survive? I don't imagine their are engineered to survive that kind of treatment?

lsdweb - 16/12/05 at 08:24 AM

Hi IanK

I should have spotted the sarcasm- it's my greatest 'strength'!!

I'd agree that wiring in the resistor etc is easy - they do seem to break on a regular basis though!

A lot of scrutineers appear keen on testing the cut out switch these days so, as you said, wiring it in makes sense!

Regards

Wyn

RichieC - 16/12/05 at 09:35 PM

The issue is more to do with completely stopping the engine rather than damaging anything.

By only disconnecting the battery ala 2 pole, the engine will run on (knacking your CDI etc). If you only want to isolate the battery for anti theft purposes, there are easier and neater ways of doing that, but, if you want it to work as it should and must in motorsport, you have to use the correct switch wired up correctly.

It was always the very first test by the scrutineers, drive into the bay and they operate the switch to check the car shuts off.

Rich

[Edited on 16/12/05 by RichieC]