unknown_host

iTrader: (1)

I have always tested my kill switch by just flipping it off, when I tech'd in at the track last week, they had me rev my car to make sure the alternator was running, then kill the switch. Doing that, my car ran for about 3 seconds before it shut off (rev to 2,000 rpm, then shut off from idle, instead of just shutting off from idle).

My alternator wire is a #4 that runs from the 1 wire alternator back to the battery side of my switch. Any ideas why this is occuring?

AlkyIROC

So he got you to rev up the engine to make sure the alternator was providing a charge then allowed it to come back to a stable low idle before flipping the switch? Just as long as he isn't flipping the switch at a high idle.

I prefer to use an alternator that I can control the field circuit on. If a single wire alternator is connected directly to the battery side of the switch, when the switch is turned off, the battery and alternator power are disconnected from the rest of the circuits. With no power, ignition, fuel pump etc should all die.

If it takes a short time before the engine dies then something is providing power back into the system. A capacitor would discharge too quickly so my guess would be an electric motor like an engine fan that's spinning down is still providing power back into the system enough to keep the ignition powered. A spinning motor becomes a generator when power isn't being put into it.

I use relays for all my high load and electric motor circuits. When power is disconnected, relay circuits open which disconnect from anything like a fan motor. I also use a relay to power my ignition circuit. Kill the main power and the relay opens which kills the ignition.

If electric motors are causing a back feed while they spin down, a diode in the circuit could also work.

DIGGLER

iTrader: (1)

Do you have an msd noise reduction capacitor by chance?

FRMULA88

The whole purpose of the "kill switch" is so that in an emergency (a wreck) the pit crew can kill the engine (and electrical system) from outside the vehicle.

The switch has to operate properly at idle, WOT and all points in between.. in a emergency 3 seconds can turn a into an electrical fire..it happens that fast.

So the big question is: did you fail tech?

unknown_host

iTrader: (1)

No I do not, I actually don't even have an MSD box yet.

I probably should have but I did not, basically was told that next time I come out it has to shut off immediately, but I was still able to race. I wasn't trying to sneak anything by them, I actually pointed out my kill switch is new and wanted to make sure it was good to go. I showed him it shuts off immediately, then he asked me to rev to 2,000 and then shut it off, that's when I discovered my problem.

Yes rev it up to engage the alternator, let it idle, then turn it off. If I don't do the former, it shuts off instantly.

That is interesting about the electric fans, I have the factor 350TPI dual fan setup, I wonder if somehow that is backfeeding?

Do I need a diode on my alternator wire or something? This makes no sense to me

AlkyIROC

A diode to the fans would work of just a relay to power them. When the main power is killed, the relay should open and disconnect the fan circuit.

That's only a guess as to what's keeping the ignition powered up for those few seconds. According to any single wire alternator schematic with a master switch, if it's wired up properly, the engine should die when the switch is turned off. I'm only guessing that it's fans backfeeding into the system. You'll have to do some testing to find out. If you can't manually turn them off, just unplug them.

The single wire alternators are easier than using one with a field circuit but I don't like a single wire for a few other reasons.

I run a newer CS style alternator. Weighs 4 pounds less than an older 10SI and puts out a lot of amps even at an idle. Instead of a field circuit, it uses a load sensing circuit. I had to wire a light into the circuit (can also use a resister) to make it work. Internal fan and I mounted it backwards because I had no other way to mount it to my engine. The alternator doesn't care which way it spins.