[max46]

Should I connect my battery cut off switch to the positive or negative cable

UPDATE

Moroso 74108 - Moroso Battery Disconnect

This switch shuts power off to the battery and alternator using the positive battery cable and alternator wiring.

 

[hotrodg726]

Positive cable only. I learned this lesson the hard way.

 

[Pops @ Tri-5.com]

Positive cable only. I learned this lesson the hard way.

John...I'm sure you have good reason...However, would you please amplify.

 

[Carson Blocks]

I'm interested as well, as it shouldn't matter. Breaking the circuit is breaking the circuit.

 

[hotrodg726]

when you break the ground every thing that has +12v is searching for a ground, things will find that path back one way or another. in my case the stereo amps were connected to both pos. and neg on the battery and the main neg bat cable went to the switch then to the chassis. first I noticed when working on the car with the switch off, if I pressed the brake pedal there was a light thump in the speakers every time I bumped it , next I noticed my LED dome light was staying on, then I developed a small drain on the battery that killed it in a few days. turns out everything was trying to ground back thru the stereo amp and fried the internal relay into the on position. I re-wired the battery switch to the positive cable with a solid unbroken ground cable and all the dray went away.

 

[Rick_L]

What tempts us to put the switch on the negative is that we learn early when working on cars that if you disconnect a battery terminal there won't be a spark if you remove the negative side. But using the switch there won't be a spark anyway.

The other half of this is that if you want an NHRA approved installation, you must switch the positive side and wire the switch in such a way that the alternator output will kill the ignition when the switch is opened. This means the alternator output must go through the switch too.

 

[kbuhagiar]

If you isolate (disconnect) the negative battery terminal from the rest of the wiring there is no way that any electrical component can form a complete circuit. The body or frame cannot complete a ground path unless it is connected to the battery. No connection, no ground, no complete circuit.

In much the same way, if you isolate the positive battery terminal from the rest of the wiring there is no way for any electrical component to get 12V power.

Either way achieves the same outcome; I prefer to mount the disconnect at the negative terminal.

 

[hotrodg726]

if you start a car with a neg ground switch and shut the switch off after it is running the c r will continue to run as the electrical circuit is complete. the alternator is the source of power and is grounded.

 

[Pops @ Tri-5.com]

Thanks John...Good to know....Mine is in the trunk and I just use it as a theft deterrent...However, I can see where in an emergency engine shut-off that it wouldn't do me much good.

 

[kbuhagiar]

if you start a car with a neg ground switch and shut the switch off after it is running the c r will continue to run as the electrical circuit is complete. the alternator is the source of power and is grounded.

Thanks John...Good to know....Mine is in the trunk and I just use it as a theft deterrent...However, I can see where in an emergency engine shut-off that it wouldn't do me much good.

I was also thinking in terms of a theft deterrent, and as a way to avoid battery drain.

 

[Old Buzzard]

There's are ways to wire the system, so the alt won't continue to power the car.
Some reading matl I've come across.
Drag Racing with Jim Hand – Part 19: Tech tips | Dallas Area Pontiac Association (dapa.org)

 

[DHR Darren]

if you start a car with a neg ground switch and shut the switch off after it is running the c r will continue to run as the electrical circuit is complete. the alternator is the source of power and is grounded.

Generally true.
Mine is wired thru the ground side and it dies when switch is thrown.
I did it this way as it is how all our equipment & class 8 trucks are done from the factory for one and two, the switch is not carrying any amp load like on the positive side.
I will monitor for the gremlins you reported though!

 

[hotrodg726]

Generally true.
Mine is wired thru the ground side and it dies when switch is thrown.
I did it this way as it is how all our equipment & class 8 trucks are done from the factory for one and two, the switch is not carrying any amp load like on the positive side.
I will monitor for the gremlins you reported though!

how do you get it thru tech at the track??

 

[55 Rescue Dog]

I put my disconnect on the negative side inside a plastic battery tool box within reach behind the drivers seat, along with the fuel pump relay ,etc. I do not intend to shut the engine off with it anyway since I have an ignition off emergency switch on the console. I've always disconnected the negative cable first on everything, plus there is less chance of accidently contacting ground working around positive side of the battery.

 

[chasracer]

when you break the ground every thing that has +12v is searching for a ground, things will find that path back one way or another. in my case the stereo amps were connected to both pos. and neg on the battery and the main neg bat cable went to the switch then to the chassis. first I noticed when working on the car with the switch off, if I pressed the brake pedal there was a light thump in the speakers every time I bumped it , next I noticed my LED dome light was staying on, then I developed a small drain on the battery that killed it in a few days. turns out everything was trying to ground back thru the stereo amp and fried the internal relay into the on position. I re-wired the battery switch to the positive cable with a solid unbroken ground cable and all the dray went away.

So basically you didn't install the cutoff switch correctly. It doesn't matter a bit which side it is connected to but that means everything has to go through the switch. If you left the amp ground connector going to the battery ground side and not through the cutoff switch, you provided a current path.
As to the alternator, again if all ground cables that connect to the ground side of the battery run thru the cutoff switch it is not going to be "grounded" and provide power to operate the ignition.

 

[acardon]

As to the alternator, again if all ground cables that connect to the ground side of the battery run thru the cutoff switch it is not going to be "grounded" and provide power to operate the ignition.

It would be very difficult to disconnect the alternator ground from the engine. The battery ground cable is only disconnecting the battery, not the alternator. The alternator has a solid ground and a constant 12 volt source as long as the engine is running.

 

[DHR Darren]

how do you get it thru tech at the track??

They don't even know what they are looking at. The car is running, they hit the switch & it kills it. That's all they do.

 

[DHR Darren]

It would be very difficult to disconnect the alternator ground from the engine. The battery ground cable is only disconnecting the battery, not the alternator. The alternator has a solid ground and a constant 12 volt source as long as the engine is running.

Not on my setup.
Kill switch is on ground & it kills everything upon hitting switch.

 

[Pops @ Tri-5.com]

Not on my setup. Kill switch is on ground & it kills everything upon hitting switch.

I'll have to give mine a try sometime.

 

[nad427]

Not on my setup.
Kill switch is on ground & it kills everything upon hitting switch.

I'm putting mine to cut the ground, but the alt power wire is run through a relay in the system so that zero power can go anywhere once the kill switch is opened.