remote kill switch

[RonD]

Just put in a 2nd battery for Sound system and an Isolator

 

[Angie]

the battery post has 3 leads coming off of it, the larger one feeds the main power input to the BMW fuse box that also sends the feed to the starter/alt. this is where the kill switch is going. ONLY kills the starter feed. cheers

 

[Angie]

2nd battery won't work. hence the cat 4d battery. cheers

 

[RonD]

Then get one of these: https://www.ebay.com/itm/SOLENOID-RELAY-SWITCH-CONTINUOUS-DUTY-12V-For-GOLF-CART-4-TERMINAL-12Volt-100Amp-/301853597900

use same size "battery" cables as you have now between battery----new relay------old relay

You want "continuous duty" because it will be on the whole time you are driving
Regular starter relays are not made for that, they can be used but will not last as long

These 4 terminal models can be used as Ground activation if you want a switch in the cab
The smaller terminals are for 12v and Ground to activate the relay, the metal case if often isolated, not a ground for relay, i.e. made to be mounted on fiberglass or wood not metal

So full time 12v larger post, small gauge jumper wire to one of the small posts, so it has 12v all the time
Then a small gauge ground wire on other small post run thru firewall into the cab, to a switch, ground other terminal on switch to Cab ground point
Can't short, worst that could happen is relay turns on if the wire into cab shorts to ground

 

[Angie]

I can't believe how a simple question of, "has anyone used one of these before" has turned into an exercise that wants me to rewire my complete system.

I am running a chevy style 1 wire starter system into the starter with the relay built onto the starter, then the alternator is wired to that same starter that is wired directly to the battery, and I want to place in a switch to kill the power to the starter system.

Pretty simple really.

thanks for all comments and suggestions. cheers

 

[RonD]

One wire would mean there is no relay on the starter motor, you would need larger cable to starter then smaller wire to relay on starter, so 2 wires

Older style, 1980's, did have just the one larger cable to starter and then a starter relay(solenoid) on the fender

Single wire is
Battery positive---(75Amp)-----/fender relay-------(75amp)----------starter motor
Alternator---------(75amp)----/

From your description you want this
Battery positive--(75amps)--(cut off)--(75Amp)-----/fender relay-------(75amp)----------starter motor
Alternator-------------------------------(75amp)----/

So you need a relay/switch rated for 100amps, to be on the safe side, at the Cut Off point, because it needs to pass 75 amps when in use

That's why the 100amp golf cart relay

My understanding is that you want to separate Battery Voltage from starter and alternator, these are the two biggest amp users there are in a vehicle

If you just wanted a "kill switch" to prevent theft by disabling starter motor, then you do that under the dash by intercepting the START wire from ignition switch to starter relay

 

[Angie]

hi ron, no what is happening is this...

battery to the BMW fused 200 amp fuse (see pic 2) - then a short cable to a kill switch - then to the starter - then to the alt. - the kill switch will kill power to the starter when the truck is parked or not running, this will eliminate all power drain and be a dead wire to the starter. When the truck is running, the alt power goes through the starter terminal and then back to the battery system giving the 140+ amp charge.

The other wire that is from the ignition switch to the relay that is built on the starter, this starts the truck as would a (on the fender/firewall) ford relay system.

 

[RonD]

So

Battery-----------------------------------------/starter motor
Alternator------------------------------------/

This is what you have

You want
Battery-------(Cut off)----------------------------------/starter motor
Alternator---------------------------------------------/

I would do this

Battery-----(100amp relay)/---------------------------------starter motor
Alternator-----------------/

Shorten up that alternator cable, and if its 140amp model it better be a cable, lol
But I doubt it would ever get above 80amps


Starter or alternator can't drain battery when they are off, even the one wire alternators have an RPM switch so are OFF when engine is under say 500rpms

 

[Angie]

Battery-----(100amp relay)/---------------------------------starter motor
Alternator-----------------/

or battery----- kill switch -------- starter motor --- alt ....... same thing and less wires to re-run

the relay would also need a secondary power switch to turn it off and on, ie ignition source. why not just a throw switch in cab that kills all connections?

how do I shorten the alt cable? it goes from the alt straight down 2.5 feet to the starter. the engine has a moroso under hung bracket system for the alt. very low hung.

 

[Angie]

Starter or alternator can't drain battery when they are off, even the one wire alternators have an RPM switch so are OFF when engine is under say 500rpms


i must have a faulty alt then, when the engine is off and the battery is connected to the starter wire, i have 12volt power at the alt terminal, and the starter terminal. it is 100% direct wired as stated. bat start alt.

cheers and thanks for the help

 

[Bgunner]

Please forgive my ignorance here but why not do a Voltage/Amp leak down test to find the circuit that is drawing the battery down? Doing so would find what is causing the battery drain so you can fix it.

 

[RonD]

Yes, starter cable and Alternator B+ cable will always have 12volts, but no ground so no current can flow

On starter motor the Relay connects the 12v to ground to activate starter motor

On alternator the voltage regulator gets 12v key on and that turns on alternator

In order for current to flow there needs to be a Path to Ground

When you turn something on you are not giving it 12volts, you have to give it 12v AND a ground, so current(electrons) can flow THRU the devices components

i.e. disconnect the Negative terminal on the battery, the ground
Now see how many electrical things work, it will be 0 of course, even though they ALL still have access to 12volts from positive cable

So for anything electrical to work it needs to have a PATHWAY for current to flow, in this case from Positive to Negative on the battery, remove either and no current can flow

And actually current flows from Negative to Positive, lol, but we didn't have instruments capable of seeing that until the 1940's and the whole positive/negative thing was entrenched, and doesn't matter really