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Discussion Starter · #1 ·
aside from sending the charging wire all the way back to the lug on the cutoff switch, is there a different way to wire the cutoff to prevent run-on with a 1-wire alternator? i'm looking at the Painless wiring high-amp alternator relay cutoff, but i'm wondering if it'll work in my situation. i'm using the Painless pro-street wiring harness and i want to keep the wiring to a minimum, i was planning on just running the BAT wire from the alternator to the starter lug where the battery hookup is. if i put the relay on this wire will it disconnect the alternator output without damaging the alternator?

my confusion is how does the alternator get disconnected without a momentary spike in output when the battery is disconnected, removing the voltage regulation?
 

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“aside from sending the charging wire all the way back to the lug on the cutoff switch, is there a different way to wire the cutoff to prevent run-on with a 1-wire alternator?”

I’m not sure how you are experiencing a run-on problem by having a one-wire in there. Run-on is caused by the alternator feeding the ignition circuit after the key is off. I don’t see the connection between the alternator B+ wire and the ignition circuit. Something in there that I can’t see? Running a HEI? How’s does it get it’s power?
 

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Discussion Starter · #3 ·
Originally posted by John_Muha:
“aside from sending the charging wire all the way back to the lug on the cutoff switch, is there a different way to wire the cutoff to prevent run-on with a 1-wire alternator?”

I’m not sure how you are experiencing a run-on problem by having a one-wire in there. Run-on is caused by the alternator feeding the ignition circuit after the key is off. I don’t see the connection between the alternator B+ wire and the ignition circuit. Something in there that I can’t see? Running a HEI? How’s does it get it’s power?
see, here i'm showing my ignorance at how automotive electrical systems work! (this is my first project and i'm still learning) my current setup (this motor hasn't run yet, i'm just getting all the wiring/fuel system completed now):

using painless wiring pro-street 12ckt harness.
battery in trunk, B+ connected to cut-off switch. other side of switch runs to starter lug. feed wire from starter lug goes to terminal block on firewall. connected to this terminal block are things like B+ to fan and water pump relays, and the B+ to the MSD 6AL ignition and the B+ to my fuse block. from the fuse block/switch panel comes my ignition hot wire to another terminal block on the firewall. to this are wired thing requiring ignition hot (MSD).
now for the part i don't have yet (what i'm trying to figure out here):
B+ from 1 wire alternator goes to B+ terminal block on firewall, completing the charging ckt. if engine is running and alternator charging, flipping battery disconnect switch removes the battery from the ckt, but alternator still provides power (perhaps until it's damaged by no battery to provide field regulation).

i'm sure i'm gonna go 'doh!' when i'm put straight!
 

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Guess I don’t understand the concern. That’s pretty much the way the car works. As long as the ignition key is on, the small red MSD lead will have power. Turn the key off the MSD box will shut off.
Are you trying to get a running engine to shut off by the disconnect switch? I don’t race. Is this a requirement? Thought the disconnect was meant to prevent the car from starting.
 

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Discussion Starter · #5 ·
Originally posted by John_Muha:
Guess I don’t understand the concern. That’s pretty much the way the car works. As long as the ignition key is on, the small red MSD lead will have power. Turn the key off the MSD box will shut off.
Are you trying to get a running engine to shut off by the disconnect switch? I don’t race. Is this a requirement? Thought the disconnect was meant to prevent the car from starting.
alrighty. yes, this is a race car (no key, just an ignition-on switch in a panel on the dash) turning the switch off will shut off the car no matter what. problem is as you figured out is to shut off *all* electrical power with the disconnect switch. this is a requirement at all affiliated tracks. my home track (IHRA) techs will test the cut-off switch during inspection. the actual purpose of the switch is to turn off the car in case of an emergency (fire, rollover, etc, no matter if the driver touches the iginition switch or not)

so, you see the problem. say a fire starts under my car that i can't see, but the track official does see. he goes to the back of the car and flips the switch while the fire crew goes to the front of the car and save me ;) with the setup i described, the car will continue to run.

now, if i instead run the B+ of the alternator to the battery side of the disconnect switch at the rear of the car(with a big cable), then flipping the switch will cutoff the power feed back to the engine compartment.
adding this additional cable is something that i am trying to figure out how to avoid. for alternators with a field connection, you can get a switch that will disconnect the field wire just before cutting off the main wire. disconnecting the field wire makes the alternator shut down.

if instead of a 1 wire alternator, is an 'ignition triggered' alternator like a 3 wire alternator with internal regulation? (with the racing harness i don't have the external regulator) most wiring diagrams for internally regulated alternators just tell you to jumper the field connection to the B+ lug on the back of the alternator. while this is fine for a normal car without the master electrical disconnect because turning off the key will shut off the ignition, this isn't good enough for racing. i think what would work is to use an internally regulated alternator that needs the field wire, but i haven't gotten clear answers to my questions anywhere yet. (everything i've learned about the charging system has been in the past 2 weeks)
 

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“adding this additional cable is something that i am trying to figure out how to avoid. for alternators with a field connection, you can get a switch that will disconnect the field wire just before cutting off the main wire. disconnecting the field wire makes the alternator shut down.”

So why not get this type of switch? Use it to run and open/close a decent relay. Use the relay to open/close the MSD small red wire to the ignition.
The only way I see around running the B+ lead back to the battery is to shut down the MSD box.
 

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Discussion Starter · #7 ·
hmm. that might be an idea to think about. a concern i have is avoiding burning out the diodes in the alternator by removing battery load. (mini racing alternators are expensive) those special disconnect switches aren't cheap either.
 

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You should really just run the alternator wire to the switch at the back. That's the correct way to do it in your application.

Depending on what you want to run depends on the wire size. If we're talking nothing more than stock car loads then #8 will work but if you run a bunch of high power accessories then you may need a #4 or so.

Peter
 

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Oh yeah, if you run a 3-wire and disconnect the 2 wires going to the plug-in then it will quit charging.

Alternatively, you can run the plug in wire, that is usually shown connected to the B+ post, to the junction block of the car and still run the B+ to the trunk. This connection will let the alternator compensate for the long wire run. The plug-in wire that is usually connected to the B+ post is brought out to the connector like that because it is also the voltage sensing terminal. This means the alternator always regulates the voltage according to this terminal, not the B+ terminal. This is also called remote voltage sensing.

Peter
 

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Discussion Starter · #10 ·
i'm going to have to look at the wiring harness again to see what is connected to what. if i remember correctly, there's a wire from the fuse block to the B+ post of the alternator, and another wire from the fuse block that i think goes to the battery. maybe what i do is connect the alternator to the fuse block with the supplied wire, then connect the other wire (which i believe is for battery charging) back to the battery side of the switch. when the engine is running, the alternator is supplying power via the B+ terminal, while the iginition B+ is connected to the starter lug from the switch. when the switch is flipped, power from B+ is gone, shutting down the car.
 
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