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Fia kill switch on the negative side ?
steve m - 26/12/13 at 11:33 AM

One of my winter mods is to tidy up under the bonnet, and having done so, I know need to wire in my Fia kill switch, as per

http://www.ebay.co.uk/itm/Master-Battery-Isolator-Cut-Out-Off-Switch-FIA-Type-Kit-Car-Race-Rally-/261080914202?pt=UK_CarsParts_Vehicles_CarParts_SM&am p;hash=item3cc9a2991a

I have two options, either quite long positive wires, which I am trying to avoid, or just wire it up directly on the negative side,
and only using very short wires,

Question is, is it ok on the neg side?

Steve

mookaloid - 26/12/13 at 12:04 PM

I do believe that they are more effective on the +ve side

theconrodkid - 26/12/13 at 12:25 PM

from memory,it has to go on the + side as the resistor goes to earth and lets the alternator down so the engine wont keep on running if the handle is pulled from the outside

Dave Bailey - 26/12/13 at 01:06 PM

And the resistor protects the diode pack I believe....

Dave B

redturner - 26/12/13 at 02:08 PM

I run a single seater without any charging system and have the kill switch on the negative (earth) lead . The earth lead must be marked with YELLOW tape.

bob - 26/12/13 at 02:10 PM

Ahhh so thats why my engine still runs when i pull the key LOL.

Actually mine was never intended as a kill switch, this was one of my anti-theft SVA parts.

coyoteboy - 26/12/13 at 04:46 PM

Also if you have it on the negative side and short ANY earth to the chassis you will keep the car live (and potentially set it alight). That's why the kill is on the +ve, so it works

MikeRJ - 26/12/13 at 05:29 PM

quote:
Originally posted by coyoteboy
Also if you have it on the negative side and short ANY earth to the chassis you will keep the car live (and potentially set it alight). That's why the kill is on the +ve, so it works


I don't quite understand this; unless part of the chassis contacts the actual battery -ve post, then it will remain isolated. All earths are connected to the chassis anyway, so how can you "short" an earth to the chassis?

The low current switch on the FIA isolators is electrically isolated from the high current isolator, so even with the isolator in the battery ground cable you can still use one pole to switch a resistor from the positive battery to ground to load the alternator, and break the ignition circuit with the other pole.

britishtrident - 27/12/13 at 06:21 AM

MikRJ is correct on this

steve m - 27/12/13 at 09:32 AM

Thanks Guys

Its going to be on the neg side, as that is the neatest routing,

also, it is just to immobilize / added security and avoid any battery drain (that I used to have, but seem to of fixed now)


regards

Steve

coyoteboy - 31/12/13 at 08:27 PM

quote:

I don't quite understand this; unless part of the chassis contacts the actual battery -ve post, then it will remain isolated. All earths are connected to the chassis anyway, so how can you "short" an earth to the chassis?


Depends on how you wire the isolation. I have seen a number of cars where the battery -ve and chassis are connected and all other electronics are "isolated" through their own ground through the isolation switch. This naturally doesn't isolate anything. If you link the only ground through the switch you have at least negated this issue, but it's not uncommon for people to do it the wrong way and the +ve isolation avoids the inherent problems when people subsequently start running their own unswitched grounds to items.

blakep82 - 1/1/14 at 12:36 AM

putting it on the negative side, is there a risk that things like the throttle cable may still provide an engine earth? possibly overheating it and perhaps melting? a bit like sticking a bit of wire over the terminals of a 9v battery?
or that the gearbox mount might still provide a route to earth? from my vague memory, the gearbox mount on my car is a metal mount, with a bolt through it to the gearbox, with a rubber sandwich in between, but still metal to metal contact through the bolt. ie, it may not be a reliable way to disconnect the circuit?

ReMan - 1/1/14 at 01:21 AM

But if the vehicle earth (chassis, engine gearbox, throttle cable or anything else) is not connected to the battery -ve terminal, then it is not earth and nothing happens
So in that respect it is no more likely for something to touch an isolated battery negative terminal than an isolated a battery positive terminal

blakep82 - 1/1/14 at 01:26 AM

quote:
Originally posted by ReMan
But if the vehicle earth (chassis, engine gearbox, throttle cable or anything else) is not connected to the battery -ve terminal, then it is not earth and nothing happens
So in that respect it is no more likely for something to touch an isolated battery negative terminal than an isolated a battery positive terminal


ah of course, yeah, i'm thinking of it being in the engine earth, duh.
erm, but isn't the point of negative earth cars to help prevent corrosion by keeping it with a healthy supply of electrons? i seem to remember something in chemistry about that. you'd lose that benefit,

steve m - 1/1/14 at 09:13 AM

The corrosion problem was caused by all the positive earth cars, hence the change to negative

Steve

[Edited on 1/1/14 by steve m]