Battery Kill Switch on ground side?

[PatientZero]

I've looked all over the interwebz the past couple days without finding any 100% solid info. I've seen that NHRA requires a kill switch on the positive side of the battery but other than that is seems very much up in the air and comes down to preference. From what I read in the SCCA rule book it doesn't specifically state positive or negative unless I'm reading it wrong.

So, for open track days and SCCA Time Trials; Can the battery kill switch be on the negative terminal? This would remove ground from every circuit in the car and isolate the battery.

 

[Swiss Boss]

I'm using the Cartek GT which does disconnect battery ground from the chassis.
It also has a 40amp switch to kill the electronic. I use that connection to cut the power to the PCM power relay which then kill power to the ECU, coils, etc...

Battery Isolator Switch | CARTEK Motorsport Electronics

Battery Isolator Systems have no moving parts meaning you can always rely on our kits to work in the tough environments of automotive motorsports

www.cartekmotorsport.com

 

[blacksheep-1]

I think ( I think) that if you run it on the ground side, and you flip it, everything on the car will try to find a ground, which means a lot of stuff will burn up. Back in the day, a lot of cars (including Ford) were positive ground (so were the Brits, but you really can't count them since most of the Lucas junk was only functioning about 30% of the time anyway). but those were back in the generator days. Remember alternators use diodes which allow current to travel in only 1 direction, faking them out could lead to catastrophic consequences... I think.
Most dragster guys don't run their alternators during the race since they draw power, so they have a separate alternator switch, my autocross cars did the same, I only ran the alternator around in the paddock, but modern cars use tons of voltage like the steering rack, so that is not an option.

 

[bob]

You cut the positive side for safety and isolate it So the positive does not make contact to ground. This prevents a Fire source.

 

[PatientZero]

You cut the positive side for safety and isolate it So the positive does not make contact to ground. This prevents a Fire source.

Would removing the ground not have the same effect? If everything is grounded through the chassis back to the negative battery terminal but you disconnect the negative battery terminal how can it ground?

 

[blacksheep-1]

Would removing the ground not have the same effect? If everything is grounded through the chassis back to the negative battery terminal but you disconnect the negative battery terminal how can it ground?

It would, in the fire service you remove the negative side first, so it won't spark, and also if you drop the wrench it won't weld things, however, I think bob is referring to the operation of the switch itself. Plus all the issues I referred to in my above post.

 

[Dave_W]

Does the alternator ground go directly and only to the battery ground? If you disconnect the battery ground with the engine running, would the alternator still have a current path to run the engine?

Pegasus (sometimes pricey, but it's because they carry enormous inventory) sells a cutoff switch with alternator connection - https://www.pegasusautoracing.com/productdetails.asp?RecID=1464
Wiring diagram for it (connects to battery positive) - https://www.pegasusautoracing.com/document.asp?DocID=TECH00109
From the wirng doc -"If you disconnect the battery on an alternator-equipped car while the engine is running, the alternator could be damaged by a voltage spike... A spinning alternator can still put out electricity, so just disconnecting the battery will not stop a running engine."

 

[EcoGulf1]

I'm using the Cartek GT which does disconnect battery ground from the chassis.
It also has a 40amp switch to kill the electronic. I use that connection to cut the power to the PCM power relay which then kill power to the ECU, coils, etc...

Battery Isolator Switch | CARTEK Motorsport Electronics

Battery Isolator Systems have no moving parts meaning you can always rely on our kits to work in the tough environments of automotive motorsports

www.cartekmotorsport.com

My son and I are interested in the Cartek GT for our build as well...can you give us any specifics on how you wired it to cut the power to the PCM relay? Thanks in advance for your help!

 

[Swiss Boss]

My son and I are interested in the Cartek GT for our build as well...can you give us any specifics on how you wired it to cut the power to the PCM relay? Thanks in advance for your help!

I see that you have a S550, mine is a S197 so it might be different but here's the way I did it

Here's the wiring diagram for the Cartek. It's on page 2.

https://www.cartekmotorsport.com/wp/wp-content/uploads/2019/06/BATTERY-ISOLATOR-GT-INSTRUCTIONS.pdf

.
I wired my car the way it's explained. You keep everything that's connected to the positive side of the battery the same and you run an extra wire to power up the Cartek.
The important part is that every connections that was on the negative side of the battery should be connected to the chassis of the car. Then run a heavy gauge wire from the negative side of the battery to the negative terminal of the Cartek. That should be the only wire that's connected to the battery negative and Cartek negative. Also make sure the body of the Cartek get a nice connection to the chassis (no paint)

Then remove fuse 21 from the battery junction box and you'll see 2 terminal spade where the fuse connect to. One is a constant 12V. Isolate it to prevent a short.
Next run a wire from the 40amps output of the Cartek to the second terminal spade.

Here's the wiring of fuse21

Ford Workshop Service & Repair Manuals - fordrepair.info - 2011 Mustang 2011 Mustang 024 - electronic engine controls - 5.0l

Ford Workshop Service and Repair Manuals, 2011 Mustang Wiring Diagrams 024 Electronic Engine Controls 5.0L

iihs.net

 

[EcoGulf1]

I see that you have a S550, mine is a S197 so it might be different but here's the way I did it

Here's the wiring diagram for the Cartek. It's on page 2.

https://www.cartekmotorsport.com/wp/wp-content/uploads/2019/06/BATTERY-ISOLATOR-GT-INSTRUCTIONS.pdf

.
I wired my car the way it's explained. You keep everything that's connected to the positive side of the battery the same and you run an extra wire to power up the Cartek.
The important part is that every connections that was on the negative side of the battery should be connected to the chassis of the car. Then run a heavy gauge wire from the negative side of the battery to the negative terminal of the Cartek. That should be the only wire that's connected to the battery negative and Cartek negative. Also make sure the body of the Cartek get a nice connection to the chassis (no paint)

Then remove fuse 21 from the battery junction box and you'll see 2 terminal spade where the fuse connect to. One is a constant 12V. Isolate it to prevent a short.
Next run a wire from the 40amps output of the Cartek to the second terminal spade.

Here's the wiring of fuse21

Ford Workshop Service & Repair Manuals - fordrepair.info - 2011 Mustang 2011 Mustang 024 - electronic engine controls - 5.0l

Ford Workshop Service and Repair Manuals, 2011 Mustang Wiring Diagrams 024 Electronic Engine Controls 5.0L

iihs.net

Thanks for all of the helpful information. I honestly was stuck on the idea of wiring it directly to the relay but the fuse is such an easier option! Did you have to install a fuse on the lead coming from the Cartek unit to the fuse box or is it self-regulating where you don't need one?

 

[Swiss Boss]

Thanks for all of the helpful information. I honestly was stuck on the idea of wiring it directly to the relay but the fuse is such an easier option! Did you have to install a fuse on the lead coming from the Cartek unit to the fuse box or is it self-regulating where you don't need one?

Since the fuse I removed was 30amps and the Cartek has a build in 40amps fuse so I didn't bother installing an extra fuse.