Battery Cut-off switch...again...

fastsc92

Registered User
I know this topic has been beaten to death but since my member account still remains inactive for two months for who knows why:mad:..... I can't get into the members only section in order to get the info I need.

I put my battery in the trunk, made a nice sealed box, got a STDP switch and some #2 welding cable. I thought I had everything wired up to the switch right so that it would kill the car, it does not. I was away for months so I haven't been able to try it out until today.

Here is a run-down of how it's wired up. The battery in the trunk is grounded to the frame. The pos side of the battery goes to one of the heavy poles on the switch. The other heavy pole of the switch is run through the car via #2 cable and goes to the starter solenoid. The LG/R wire from the alternator is cut and is run back to the smaller leads on the switch.

I assumed that this would break the excite wire on the alternator and kill the car. Again, it doesn't kill it. The car remains to run but I quickly killed it myself so that I wouldn't do any damage to any electronic equipment.

I've heard mixed reviews of what to do and everyone goes back and forth on the right and wrong way to do things. I DO NOT want to run the charge wire (black/orange) from the alternator back to the switch. This just means more heavy gauge wire, more $$ and more holes in the firewall. I already have some 16awg running to the switch so I'd like to use those.

Now I guess I could break the power to the ignition coil, but I dont think this is the proper way to do it. This would be the red/green tracer at the power distribution box. I really want to do it via the excite wire but what am I doing wrong??
 
The alternator charge cable goes to the same part of the switch that the positive battery cable goes to. As in, the opposing post that the switch to starter solenoid cable is bolted to.
 
even if you kill the exciter wire for the alt, your car will run till the battery dies,
all the excite wire does is make a field current for the stator to the electromagnet (field current) can let the rotor cut flux lines to produce voltage
 
even if you kill the exciter wire for the alt, your car will run till the battery dies,
all the excite wire does is make a field current for the stator to the electromagnet (field current) can let the rotor cut flux lines to produce voltage

yes but in my case, the switch is cutting both the main power AND the excite wire. Which i figured would stop the alternator from producing power. As I said earlier, I'd rather not run the charge cable back to the switch. I know people have done it the excite wire route, I'm just not sure where I'm going wrong... I'm a mechanical engineer, not an electrical:rolleyes:
 
Jay,

On the starter solenoid, do you have the charge wire on a different post than the solenoid-to-switch cable?

If not, then try it.
 
you need a Single pull, Double Throw switch. this would be a cut out switch that has two sets of terminals.

One is for the battery. One for the Alt or other method.

If you gang those together, you essentially just closed the circuit

When the alternator is spinning it creates 12 or 13 volts which if you gang that to the positive wire that you are cutting, it will then push 12v down that wire you thought you broke from the batteries 12v.

If your switch only has 1 terminal (single pull... single throw) you could add a high voltage relay that is normally open. Wire up the relay so that when the main switch is closed, the relay closes. The down side with relays is they simply are not overly reliable.
 
Jay,

On the starter solenoid, do you have the charge wire on a different post than the solenoid-to-switch cable?

If not, then try it.

Just to clarify to all......I have a switch with two sets of terminals, two heavy, and two smaller. The heavier terminals break the main power feed and the two smaller terminals break the LG/R wire from the alt.


The charge wire from the alt is attatched on the solenoid to the post that is closest to the firewall. This post has 3 wires connected to it. One the charge wire, one the main battery positive, and the other i'm not sure of. The other post of the solenoid, the one towards the front of the car has only one connection and I assume that it goes to the starter. Are you saying to bring the charging wire to the other post? But isn't this post only active when I start the car?
 
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The solenoid on the fender works like this.

Post towards the fire wall is hot 24/7. You can verify this with a voltage meter.

The wire on the Alt you need to switch is BK/O (black with orange stripe)

The LG/R wire is the ouput that goes to the batt light in the dash.

The LG/R wire gets it's power from the ignition switch. The ignition switch gets it's power from the battery.

If you cut the battery, you automatically also cut the LG/R wire. (you can verify this with a meter)

The BK/O wire is the one you want to interrupt on the alt.
 
so what's all this talk about cutting the excite wire in order to kill the car? I assumed that the LG/R wire stopped the alt from getting its 12v source and in turn stopped the stator from making power.
 
The 12v source energizes the alternator so that it can start generating power at low rpm. The other wire is what actually sends 12v out of the alt to charge the battery. This 12v out is common with the + side of the battery post at the solenoid on the fender. Thus everything connected there will get 12v from the alt if the battery is taken out of the loop.

Thus you have to break that 12v chain on the alt side.

There are other ways to do this. You could break the 12v supply to the coil pack, which will kill the ignition and the engine will stop as well. But your alt wiring mod should take care of it as well.
 
i understand that the charge wire is what sends the power to the battery, but i was assuming that if the alt. is not energized, then it would stop making power....???
 
i understand that the charge wire is what sends the power to the battery, but i was assuming that if the alt. is not energized, then it would stop making power....???

Alt needs to be energized to start making power. This is the halmark of a 3 wire alt. The single wire Alts work without the energizing wire, but require RPM's to be brought high for a few seconds to get enough speed into the armature to start making power.

Once the alt is making power, there is essentially no need to have 12v on the energize wire anymore.
 
so you're saying once the alt is excited, if i break the excite wire, it wont matter anymore and it'll to continue to make power?
 
No, what I'm saying is that if the alt is generating power, the excite line is no longer necessary, as the alternator no longer needs to be excited.

The ignition switch in the car doesn't cut power to the alt. It shuts down the ignition which shuts down the engine. When the alternator isn't spinning any more, it's field drops and no longer produces energy.

You need to interrupt the 12v output of the alternator.
 
Alright...here is what I did, i figured it'll be easier to explain....
 

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Now cut your Green wire in the drawing as well as the welding cable to the battery simulating your switch off. (open)

See how you still have a circuit coming from the alternator to the solenoid? That circuit will have 13v being generated by the alternator. It won't care that the excite wire no longer has 12v.

The alt will feed that power wire on that solenoid terminal. thus that terminal is hot, which is ganged with many other connectors including those power connectors for the power distribution box. That's where items get the stuff from the battery, but with the battery gone, it'll get the power from the alternator. The alternator keeps having power because the engine keeps turning it.

So instead of the green wire you need to run a 8awg wire from the alt charge wire back to the switch, then back up to the solenoid block. Thus such that if the switch is pulled, both the battery is cut, and the power from the alt is cut. That will shut your motor off. It does this because it removes all voltage from the fender solenoid which effectively cuts eec and ignition power.
 
this is what I was trying to avoid. I didn't want to run that #8 back there. I was told you could cut the excite wire and kill the field and the alt would stop making power. I understand that the alt is still hooked up to the terminal block, i just thought that the green wire would do it.

This is a 110amp alt, dont you think that I should go a lot heavier on the wire? Like 4# or #6? I think a #* wire is only rated for 60 amps
 
Look at what the wire is now. I believe it is #10. If you want to do #6, go for it.

Breaking the excite wire is not sufficient with our alternator design.
 
I used 2 conductor cable to the small set of contacts on the battery switch. Then I cut the 12 volt connection at the coil pack and connected the other end of 2 conductor cable to them. When I turn off the battery switch it interrupts both the battery and the coil pack voltage which kills the ignition. I never could get the alternator to shut off, either, and gave up on that approach.
 
like i said, i read about how to cut the excite wire which is LG/R, but others have cut the positive coil wire which is R/LG. I can see where the confusion comes from. I'd rather avoid running larger cable from the alt. charge wire(black/orange) back to the switch. I know this will work but this is what I was trying to avoid.
 
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