12v killswitch advice please

[pauldoubleyou]

I want to run a dash cutoff switch on my Lupo. Now, I presumed that I could run the battery cable from the battery in the boot through it and then through the bulkhead to the engine.

 

Now, thinking about it this wont actually kill the engine - it will just remove the battery from the charging circuit when the engine is running.

 

The other way I have thought about is to disconnect the negative terminal, which should kill the engine however it will be a pain in the tits running a negative cable from the boot, to the switch, then to the chassis.

 

Any advice? Anyone got a track car with one fitted and could share how it was done with a battery relocate in the boot?

[scaryoldcortina]

It doesn't matter which terminal you lift, the effect would be the same if the engine is running. Every car I've seen with a battery disconnect does this, it's because it isn't a kill switch at all.

 

All you do is remember not to use it unless the engine is stopped tbh.

[castros_bro]

Kill switch or isolator ?

 

Putting a switch in the main battery cable uses an isolator switch as all the power to run the starter motor also has to go through the switch.   This involves heavy power cables. 

 

A kill switch is usually only on the ignition circuit (petrol cars) and involves smaller cables.  Here is a Lupo wiring diagram, at the top between IGN COILS in the 12v power feed with a picture of a key.  If you cut this wire here and extend to your dash mounted kill switch it will work finely to stop the engine running.

 

[SiC]

An isolator as said above kills the power feed, but will leave your alternator powering the car still. Which will damage the alternator.

 

A isolating kill switch as used in rally/racing cars have the above, but also a set of low current contacts that go to the field windings/charge light connection on the alternator. When switched to the isolate position, these extra contacts close and the field windings get shorted to ground through a suitably large wattage rating resistor. This will quickly stop the alternator charging.

 

The other contacts are then between the fuel pump power feed too, which will quickly cause it to loose fuel pressure and the engine to stop.

 

Its still preferable to isolate with the engine off, however it is unlikely to damage stuff when its running using this switch.

[pauldoubleyou]

Superb. I have 00 gauge wire which is 10mm actual wire diameter (12.5 with sheathing) so hoping for no voltage drop. Think that equates to 70mm/2

 

I dont actually have a lupo engine installed but i will put a killswitch into the current ignition wiring setup. Big thanks

 

Paul