How to wire in a vehicle "kill" switch....
How to wire in a vehicle "kill" switch.... OK....so I bought and installed a battery ground termination switch, and installed it between the main battery negative terminal, and the body of my car...according to included instructions. The idea (obviously) is that when I'm at the track.....if a track official sees a problem, or I wreck, or whatever....they can kill the entire electrical system in my car by pushing in the t-handle on the back of my car. Nothing new there. But here's the problem: When I tested it, the vehicle still runs!
Now I know it's due to a backfeed to the alternator (not good at all...PCM killer!), but I'm curious as to how some of you might have solved this. I talked to a local guy today, and bought a high-amperage relay to install between my alternator main output and battery, but was told that I should just keep the relay's positive "power feed" terminal wired to "ignition positive", and the relay's ground terminal wired to a toggle...with the other side of the toggle going to ground. The guy said to leave the toggle "on" during everyday driving....but when I go to the track, turn the toggle to "off", thereby letting the car run SOLELY from battery power.
Something does not sound right about this to me. He said that during a pass, the car will run "just fine" off of only the battery. I have a hard time agreeing with this being the optimal way to go.
So, has anyone else here had this issue?? And if so, how did you solve it?
Thanks....and sorry for the long post....just trying to make it as clear as possible.
Now I know it's due to a backfeed to the alternator (not good at all...PCM killer!), but I'm curious as to how some of you might have solved this. I talked to a local guy today, and bought a high-amperage relay to install between my alternator main output and battery, but was told that I should just keep the relay's positive "power feed" terminal wired to "ignition positive", and the relay's ground terminal wired to a toggle...with the other side of the toggle going to ground. The guy said to leave the toggle "on" during everyday driving....but when I go to the track, turn the toggle to "off", thereby letting the car run SOLELY from battery power.
Something does not sound right about this to me. He said that during a pass, the car will run "just fine" off of only the battery. I have a hard time agreeing with this being the optimal way to go.
So, has anyone else here had this issue?? And if so, how did you solve it?
Thanks....and sorry for the long post....just trying to make it as clear as possible.
Last edited by salemetro; Aug 4, 2011 at 01:04 AM.
Should not need to do that if the entire vehicle ground is killed, and the alternator output can be interrupted. A local shop here has told me that the alternator can and will "self-excite" anywhere above 1500rpm....so therein lies the problem. I'm thinking maybe that isolating the alternator output "interrupt" relay power feed and ground with diodes might do the trick?
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From: Massachusetts
Er, disconnecting the battery negative does not kill vehicle ground. The alternator creates its own ground - its own casing.
I don't know if what your local guy said is the BEST way to do this, but he is otherwise correct. You could also specifically kill other key components like ignition/fuel delivery components using a relay, yes. There are multiple ways to go about tackling this issue.
I don't know if what your local guy said is the BEST way to do this, but he is otherwise correct. You could also specifically kill other key components like ignition/fuel delivery components using a relay, yes. There are multiple ways to go about tackling this issue.
Last edited by dragonrage; Aug 5, 2011 at 03:36 PM.
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Er, disconnecting the battery negative does not kill vehicle ground. The alternator creates its own ground - its own casing.
I don't know if what your local guy said is the BEST way to do this, but he is otherwise correct. You could also specifically kill other key components like ignition/fuel delivery components using a relay, yes. There are multiple ways to go about tackling this issue.
I don't know if what your local guy said is the BEST way to do this, but he is otherwise correct. You could also specifically kill other key components like ignition/fuel delivery components using a relay, yes. There are multiple ways to go about tackling this issue.
I also doubt that the car will run as well as it should with just 12 volts of power to the entire car from a battery that is under considerable load. I'd be curious to know how badly coils output would suffer as well.
Any thoughts? I'm really kinda surprised that Flaming River (the switch manufacturer) mentions nothing of this issue in their installation instructions.
Last edited by salemetro; Aug 5, 2011 at 08:07 PM.
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Verdad Gallardo TECH Addict
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From: Massachusetts
The alternator does not produce AC current. It produces 3-phase rectified AC current, which is actually fairly close to DC. Your car is not going to fry without the battery. Again, the battery negative is the source of ground from any current driven by the battery only. With the alternator in play, the ground point is the casing of the alternator.
So then....what might be my best option to ensure that the main power supply relay to my fuel pump controller de-energizes once the battery ground path is interrupted? I believe that I wired the relay power feed and ground directly to the battery....but it's late, and I can't remember for sure. Same question for using a high amp relay on the alternator output....between the battery and the alternator?
I'm just really hating the thought of having to use a toggle switch. It would be really nice if everything just shut off when the battery negative was cut off.
Thanks.
I'm just really hating the thought of having to use a toggle switch. It would be really nice if everything just shut off when the battery negative was cut off.
Thanks.
The relay for the fuel pump controller is wired as follows:
#85 is grounded directly to the neg battery terminal
#30 is 12v + (fused) directly from the battery positive terminal
#86 is 12v + (trigger) from PCM
#87 is 12 relay output wire that feeds power to the fuel pump controller
#85 is grounded directly to the neg battery terminal
#30 is 12v + (fused) directly from the battery positive terminal
#86 is 12v + (trigger) from PCM
#87 is 12 relay output wire that feeds power to the fuel pump controller
Alternator relay proposed wiring picture... crap....I just realized that the negative for the relay that feeds the fuel pump controller is not wired AFTER the neg kill switch. I'll have to fix that, and see what effect it has. Damn, I feel stupid. 
Regarding the proposed "alternator interrupt" relay, I've attached a pic of how I've been told to wire it....I just can't stand the thought of having to use a toggle switch. There has to be a better way that will allow me to keep the alternator on during a track pass.
Regarding the proposed "alternator interrupt" relay, I've attached a pic of how I've been told to wire it....I just can't stand the thought of having to use a toggle switch. There has to be a better way that will allow me to keep the alternator on during a track pass.
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Joined: Sep 2006
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From: Massachusetts
If you use your kill switch to instead switch the positive of the battery, you could then use the fact that alternator voltage is always going to be higher than the battery voltage, put the car-side of the switch to the positive input of an op amp, the battery side to the negative input of said op amp, and run the output of the op amp to one side of a relay sense coil, and the other side of the coil to ground. The relay will trigger (turn on) when there's a voltage difference between the sides of the switch, and you can use that relay to trigger your ignition and/or fuel cut.
I can also design you another method if you change your kill switch to a SPDT (3 terminal), DPST (4 terminal) or DPDT (6 terminal) one. DPST or DPDT are really ideal, if you could find them.
Also, the 'run from the battery only' idea from your local guy IS still a valid idea. As long as you're not bumping a 2000w system down the track, your battery will barely get drained, and you'll have the advantage of the alternator producing less drag. The higher the current draw from them, the more drag they produce.
I can also design you another method if you change your kill switch to a SPDT (3 terminal), DPST (4 terminal) or DPDT (6 terminal) one. DPST or DPDT are really ideal, if you could find them.
Also, the 'run from the battery only' idea from your local guy IS still a valid idea. As long as you're not bumping a 2000w system down the track, your battery will barely get drained, and you'll have the advantage of the alternator producing less drag. The higher the current draw from them, the more drag they produce.
Last edited by dragonrage; Aug 8, 2011 at 02:34 PM.