The 24 Hours of Lemons Forums → Lemons Tech → Kill Switch in Neg circuit
Working on wiring the kill switch on our first Lemon car- Is there any problem with just breaking the circuit between the battery negative terminal and chassis with the kill sw (Battery neg post to kill switch, then wire other side of switch to chassis/engine block). Any downside here? Steve
A two pole switch is great.
The positive to the battery and the alternator charge wire are both broken simultaneously.
KT
Any downside here? Steve
It's likely the engine might still run off the alternator. Take a look at these diagrams and the discussion.
http://www.ssdiv.com/master.html
sr903 wrote:Any downside here? Steve
It's likely the engine might still run off the alternator. Take a look at these diagrams and the discussion.
Yeah, it does... still have to interrupt the alternator wire. If you just pop open that circuit, theres a good chance of frying the alternator too...
You need to GROUND the alternator. If you just open it you can damage the alternator AND any other electronic components in the computer. An alternator can easily generate 100v with no load connected to it.
That is what takes out the diodes.
The right kill switch comes with a resister to quench the alternator when you flip the switch off.
A kill switch must do 2 things in order to be effective -
1. Interrupt the 12V circuit from the battery to the chassis/engine/starter etc.
2. Interrupt the 12V supply from the alternator to the ignition system
#1 is the easy part. #2 can be accomplished a couple of different ways -
If you have a switch with 2 sets of poles that open and close together, the easiest thing to do is use the 2nd set of poles to interrupt the 12V going to the ignition switch or coil. This will prevent alternator current from getting to the ignition coil etc.
If you've got a kill switch that has a second set of terminals that are closed when the main set is open, you can tie one terminal to the 12V (sense voltage) going into the voltage regulator on the alternator and the other side to ground (through a resistor). When these contacts close, the sense voltage goes to zero, causing the field inside the alternator to collapse which stops the alternator from making current. In most alternators with internal regulators, the sense voltage is bridged from the output (B+) terminal, which is why others are recommending grounding the output side of the alternator.
You need to GROUND the alternator. If you just open it you can damage the alternator AND any other electronic components in the computer. An alternator can easily generate 100v with no load connected to it.
That is what takes out the diodes.
The right kill switch comes with a resister to quench the alternator when you flip the switch off.
Yep, and a 2 pole doesn't work for that.
+1 to all of the above. Additionally, if you interrupt the battery and alternator positive poles, there aren't any other sources of power (unless you've got your pacemaker or walkie-talkie hooked up to the car's electrical), but if you interrupt the ground, you can create all sorts of new grounds if you wreck the car well enough.
I don't know how long or large of a wire you are talking about but with a 1 wire alternator spinning fast grounding it will produce a lot of melted copper and might even let the smoke out. Just open circuit it, the regulator is built in and it will see the voltage rise and kill the field. If you want to get fancy, connect a resistive load from the alt output to ground ( like your headlights ) and use that to gently let down the voltage...
If you run a factory type alternator with external 12v supply to the regulator ( ford style but probably many others ) then disconnecting that will cause the field voltage to go away killing the output.
You could also take the 2nd pole set and disconnect the PCM from the battery harness ( put it in series with the big main fuse ) then the alt only runs dumb loads when you hit the big red switch.
Thanks for all the help. My thinking was that +12V is only +12V with respect to Negative batt post and anything connected to that (chassis, engine block), so if that connection was interrupted, you'd have a floating 12V with no ground. I'll look at my other options.
So of course I had to prove it to myself, went out and pulled neg off a running car, and yes, it kept running. Shoulda done that first...
I think the alsternator would still be grounded to the engine through the bracket.
I think the alternator would still be grounded to the engine through the bracket.
He means ground the positive output of the alt when the connection through the kill-switch breaks the connection to the battery. I'd use the biggest ceramic 100ohm resistor you could find or just find a switch with alternator blades. Alternator spikes are nasty things and I think the first time I did it I blew out my Harbor Freight water temp gauge. Not a good thing to have dead for an entire race 
I actually used a relay to switch the alternator output to ground and I've never had issue operating the switch as the ignition/ECU/fuel power control.
The easiest thing is to do is get this:
http://www.ogracing.com/catalog/2-Car/3 … KILLSWITCH
Comes with the right switch and the resister. We have one on our lemon now and it works perfectly.
I think the reason the car kept running is that the alternator stays grounded to the engine when he pulled the battery cable.
BTW, is that a screw that will hold the switch handle in? If not, how do you keep it from falling out?
The handle is captive when in the "ON" position. You can only remove it if its "OFF". We have a wire to hold it nearby if you decide to remove it from the switch.
thanks
The 24 Hours of Lemons Forums → Lemons Tech → Kill Switch in Neg circuit