What is the alternator "field wire"? Kill switch install

[altitude7200]

211716

I'm trying to install a 4 terminal kill switch and I need to run the alternator field wire through it. I have been on every forum and I'm 4 hours of searching and I still don't know if it's the white/black or the red/blue that I need to interrupt with the switch. 211717

This seems so easy but I'm losing my mind trying to figure it out. Thanks for any help.

[egchewy79]

Good luck. I just put mine inline with my fuel pump relay

[BornWestUSA]

Why would a Kill switch need to be connected to the alternator in any way? It would not stop the car from starting, and the car could run off the battery until it was dead.

Kill the start signal to the starter, kill the fuel pump (if EFI), kill the ignition power or signal... but the alternator?

[RPGs818SNA]

Short version: the Black White wire.

Long version: Here’s how I understand it. You want the Kill Switch to kill the running engine in the event of a fire, rollover, crash, stuck open throttle, etc. Disconnecting the battery will prevent engine from starting, but won’t stop the engine as the alternator will continue supplying current to the electrical system. To disable the alternator so as to cut all electrical power to the electrical system when the engine is running, one can switch off the alternator’s field current. A 4 terminal kill switch interrupts both the battery current and the alternator’s field current to kill a running engine. Here’s some background on alternators and how to wire up your kill switch. For a more detailed explanation, see: https://theengineeringmindset.com/car-alternator-how-alternators-work/

Alternators contain stationary wire coils in which charging and running current is produced by a rotating electromagnet (magnetic field). This rotating magnet is initially magnetized by passing battery current through the magnet. Once the engine and alternator are running, the alternator makes enough current itself to keep the spinning magnet magnetized. The best way to make the alternator stop making current is to switch off the magnetizing (field) current. Interrupting the battery and alternator field current will cause the engine to stop.

Incidentally, a refinement in alternator technology is the inclusion of a sensing wire that goes directly to the battery. That lets the alternator adjust its output so that the battery voltage is kept around 13.5 volts.

The early 2000s Subaru alternator has a stud with heavy white wires that carry the charging and running current back to the fusebox and battery. It also has a small gauge Blue Red sensing wire that goes to the fusebox, through a fuse, and on to the battery. The small gauge Black White wire is the field current wire that goes to the combination meter and on to the ignition switch and battery. This is the other wire that the kill switch needs to interrupt.

Hope this helps you out.
RPG

[Dgc333]

Not sure about the 818 but the Ron Francis wiring harness that came with my 33 the field wind wire is brown and goes from the field winding connection on the alternator back to the accessory connection on the ignition switch.

[driveslikejehu]

I'm researching a kill switch as well. Been seeing the recommendation of a resister to dump the alternator output for this kind of shutdown. This would be for a street/track day car so need to keep all the electronics
Don't know how you do it with a 4-pole switch tho.

211741

[RPGs818SNA]

That’s an interesting idea to dump any remaining capacitive charge or any current from residual magnetism in the alternator rotor. I tested mine today and verified that cutting the battery off while the engine was idling did not slow down the engine at all, even if I tried revving it. I then tried cutting off the ignition switch with the battery switched off at idle and at 2000 rpm. In both cases, the engine immediately lost power and coasted to a rapid stop.

I did keep my daytime running lights, so that’s kind of like having that 3 ohm resistor connected all the time. If you don’t have DRL connected, you may want to consider simply connecting that 3 ohm resistor to the ignition switch ON wire all the time. It would only cost you 4.5 amps, or 60 watts, or 0.08 horsepower. If you do that, I’d recommend a 75 to 100 watt resistor mounted somewhere it can get hot without damaging anything.

Thanks for sharing the concept,
RPG