battery cut off switch

[!kid]

I just picked up a new kill switch since my last one faild at the track.
before I wire it in I want some opinions.

does this look like a good way to install it?
would you do things another way?

[jlevie]

A kill switch has two normally open NO) switches and one normally closed (NC) switches. Schematically the connections are:

Battery > main lugs (NO) > engine bay
|
Resistor > small lug (NC) >------

Small Battery wire > small lug (NO) > small wire to engine bay (DME power)

The usual cause of a kill switch failure is that the small NO contacts fail. They are only rated at 20-30A and the engine management system draws more current than that, eventually causing the contacts to fail. A fix for that is to place a 60-80A relay in the DME power circuit and control the relay with the small NO contacts. The power to control the relay should come from switched ignition unless the kill switch is the ignition switch. Otherwise the relay will drain the battery.

[!kid]

So on the small pin I have for DME, use it to power a 60-80amp relay to essentially power the DME?

[!kid]

I found this on spece30.com

[Cabriolet]

does it not reset the DME when the power is getting cut on it all time?

[!kid]

does it not reset the DME when the power is getting cut on it all time?

it does, is that a bad thing? I am not sure.


Hey Jim, should I do somthing like this?

[jlevie]

does it not reset the DME when the power is getting cut on it all time?

Yes it does, but that is no big deal on a race car. You tune the engine on a dyno and before every pull you reset the DME so as to always tune against a known condition. Very little adaptation will occur on track because the engine spends most of it's time with the WOT switch closed

If I designate the terminals of the low power NO kill switch 2a & 2b, the relay coil terminals as R+ & R-, and the relay power terminals Ra & Rb the connections would be:

switched ignition > 2A
2B >R+
R- > ground
Battery > Ra
Rb > DME

The diagram on that you found on spece30.com is wrong. The NC contacts used for the resistor have one side to the resistor and the other side needs to go to the main power cable, not to the DME power. The reason is that once the kill switch is opened the DME power is isolated, so the resistor does nothing. But if connected to be main power lead to the engine bay, which the alternator is connected to, the resistor will pull off any power surge from the alternator as the engine spins down.

If you will post a link to the tread when you found that circuit, I'll post a correction on spece30.com

[Jparkr]

Run the sub circuit to kill power to the coil. not the DME. Mine is a little different but ill grab a picture for you.

[Jparkr]

We are about to remove the engine, so this is cut.

simply cut the original wire (we choose the 10mm connector side) run it back...

to the secondary kill switch:

[jlevie]

Run the sub circuit to kill power to the coil. not the DME. Mine is a little different but ill grab a picture for you.

A kill switch is more than just something to make the engine stop. The point of a kill switch is to sever the battery from the engine bay to eliminate the possibility of a fire sparked or supported electrically and to bring the engine to a full stop.

[Jparkr]

A kill switch is more than just something to make the engine stop. The point of a kill switch is to sever the battery from the engine bay to eliminate the possibility of a fire sparked or supported electrically and to bring the engine to a full stop.

And you think this isn't accomplished by cutting all power to the junction block because...

[jlevie]

Cutting all power between the battery and the engine bay suffices. But if in the stock configuration there are two power supplies to the engine bay, the main battery cable and the smaller wire that powers the engine management system. Both have to be severed.

[!kid]

The diagram on that you found on spece30.com is wrong. The NC contacts used for the resistor have one side to the resistor and the other side needs to go to the main power cable, not to the DME power. The reason is that once the kill switch is opened the DME power is isolated, so the resistor does nothing. But if connected to be main power lead to the engine bay, which the alternator is connected to, the resistor will pull off any power surge from the alternator as the engine spins down.

If you will post a link to the tread when you found that circuit, I'll post a correction on spece30.com

http://spece30.com/forum/41-electrical-gauges-and-sensors/10203-kill-switch-again

[!kid]

We are about to remove the engine, so this is cut.

simply cut the original wire (we choose the 10mm connector side) run it back...

to the secondary kill switch:

I've got an M52b28, this set up wont work for me.

If I designate the terminals of the low power NO kill switch 2a & 2b, the relay coil terminals as R+ & R-, and the relay power terminals Ra & Rb the connections would be:



switched ignition > 2A

2B >R+

R- > ground

Battery > Ra

Rb > DME

okay, think I got it now. let me know if its wrong.

[Jparkr]

Cutting all power between the battery and the engine bay suffices. But if in the stock configuration there are two power supplies to the engine bay, the main battery cable and the smaller wire that powers the engine management system. Both have to be severed.

Ah, I see where you were coming from. I have the little one spliced into the larger for full disconnect to junction box. The picture I took doesn't show that.