Properly Installing External Master Disconnect Switch

[af_1132]

Greetings all electrical wizzes and drag racers. I'm not 100% sure this is the best place for this thread, but since I have questions on an electrical install, I figure I should try here first.

I race at an NHRA sanctioned drag strip, and I have relocated my battery to the trunk. So by NHRA regulations, I have to install a battery disconnect switch in series with the positive cable to the starter, in order to kill the motor in case of an accident. I have done so, with a four post switch: two posts for the battery and ignition, and two smaller posts for the charging circuit (alternator) which are currently not being used.





I originally purchased a 4 post switch over a 2 post switch, because one product review stated that you must use the smaller two posts in series with the alternator in order to kill the motor. Now, I am trying to figure out if I must interrupt the charging circuit with this switch as well, in order to kill the motor.

I have talked to a few mechanics and a tech official from the track since purchasing my switch, and I am getting mixed opinions. Some say, "why would you need to splice into the charging circuit when all the electric power comes from the battery?", while someone else says "yes, interrupting the charging circuit is the right way to do it, you need to create a loop of 14 gauge cable from the switch to the alternator, splicing into the exciter wire ." I also got a simplified diagram with my product, which gives yet another way to wire up all four posts:



The only testing I currently feel comfortable doing is flipping the switch when the car is off. In the "off" position, the car will not start, but the interior electrical components work (as they should, since I am not using a distribution box but sending a 4 gauge cable all the way from the positive terminal to the fuse box). In the "on" position, the car starts and everything electrical works.

Knowing all of this (sorry, I know I write a lot), do you all have any suggestions in favor or against hooking up the alternator/charging circuit to this switch? Your help is very much appreciated !

[sonikaccord]

Ok, I would use the Quickcar instructions.

You want the switch to disconnect ALL power from the battery so that your car is completely dead *when the motor is not running.* This is key here. So you would use 4, 2 or even 1/0 gauge wire for the big posts. In series of course.

Now if you were to hit the switch and *the motor is running,* you have only disconnected the battery, but your alternator is still spinning and producing electricity. So that's where the small posts come in to play. On your OEM charging harness, there is a wire that supplies voltage to the exciter ring on the rotor. You want to connect the switch in series with that wire to kill the alternator using the small posts.

If you had gotten the 2 post switch, you could cut power going to the fuse box and I believe that would stop everything as well including the alternator. You could still do this with your 4 post switch, too.

EDIT:

I race at an NHRA sanctioned drag strip, and I have relocated my battery to the trunk. So by NHRA regulations, I have to install a battery disconnect switch in series with the positive cable to the starter, in order to kill the motor in case of an accident.

Is your only objective to kill the motor with switch?

[af_1132]

Is your only objective to kill the motor with switch?

That is my main objective, while the motor is running, yes. When I say "my objective", I mean that the officials must, according to NHRA rules, have an external means of turning off the car with a switch in the event of an accident on the 1/4 mile.

While installing what I have so far, it is good to know that I can also use this as a theft deterrent even in its current state. I just turn the switch to the "off" position in as inconspicuous a manner as possible as I leave my parked car in a public place .

Anyway, wow, thank you so much for clarifying these things for me. I was deathly afraid that the tech inspectors might require a full test of the switch with the car running during inspections, and my alternator would fry.

I have the recommended 14 gauge wire for the "to alternator" path on the diagram. I just have two more questions. The first is about the jumper wire. In the box on the diagram, it says "55-120". Do you think they are referring to the recommended gauge range for that jumper wire? The second is about the exciter ring and rotor you referred to. Are you talking about the starter rotor and the ring that was attached to that?

[cp[mike]]

the main alternator power/excitor/charging wire comes out from the alternator and back into the battery, by way of the 40amp fuse in the fusebox. if the engine were running with the alternator charging, the switch won't kill the car.

QuickCar is the correct instructions - basically they are saying instead of hooking the alternator charging wire to the fusebox, bypass that fuse and connect it straight to the battery, through the switch. If you wanted to add a 40amp Alternator fuse (just for safety), it would go inline in the place of that "55-120 Jumper Wire" or anywhere along the charging wire itself.

[af_1132]

the main alternator power/excitor/charging wire comes out from the alternator and back into the battery, by way of the 40amp fuse in the fusebox. if the engine were running with the alternator charging, the switch won't kill the car.

QuickCar is the correct instructions - basically they are saying instead of hooking the alternator charging wire to the fusebox, bypass that fuse and connect it straight to the battery, through the switch. If you wanted to add a 40amp Alternator fuse (just for safety), it would go inline in the place of that "55-120 Jumper Wire" or anywhere along the charging wire itself.

OH, okay, things are even more clear now. Thank you so very much. Knowing that I'm going to bypass the fuse box, is 14 gauge really the right gauge for this charging wire? And then, lastly, I need to know what gauge to use for the jumper wire.

You guys are the best !

[cp[mike]]

OH, okay, things are even more clear now. Thank you so very much. Knowing that I'm going to bypass the fuse box, is 14 gauge really the right gauge for this charging wire? And then, lastly, I need to know what gauge to use for the jumper wire.

You guys are the best !

Just to put your Second mechanic friend's suggestion into perspective, he was saying instead of wiring the excitor wire directly to the battery via the jumper wire on the switch, to run it all the way to the switch like normal, and then run the second small post all the way back up to the fusebox (no jumper wire). This interrupts the charging wire, but leavings the starting/ending connection points in their OEM locations - that way itll connect back to the 40amp fuse, and from there, allllll the way back to the battery. That is an excessive amount of wiring! Especially since you have the battery all the way in the trunk. Skip the loop back and forth and back and hook it up the Quickcar way. Nice and clean.

In terms of correct wire size, depends on a few factors. Easiest way is to use a calculator:
http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

I don't remember the maximum amperage of the stock alternator off the top of my head, but when you find out, make sure you round UP before using that number. Charging voltage is 14v. You'll find that you need pretty beefy cable - definitely bigger than 14 gauge at that distance.





If you don't want to run a SECOND beefy high amp cable, another alternative would be to use a special heavy duty solenoid/relay to cut the stock alternator charging wire. That way, it can remain in its original location, in its original gauge, just passing through the solenoid. The solenoid runs on a low current signal, a normal smaller wire. When the switch is on and the ignition is turned on, the alternator will be connected. This site has some nice information of how this method would work:

http://www.timskelton.com/lightning/...y_relocate.htm

Here's his simple diagram. Note that he is connecting his alternator to the starter instead of the fusebox. Either one leads back to the battery.



And this is the product he refers to, made for this exact purpose:

"Painless Performance" High Amp Shut Down Relay Kit - part number: 50105
http://www.summitracing.com/parts/prf-50105

There may be other alternatives, like you may be able to get away with rigging some sort of Starter solenoid up. Something that uses a 12v source to turn on a high amperage connection.


.

[sonikaccord]

As far as running wiring, I agree with Mike. You don't want to do all of that back and forward mess. Too many failure points. For the exciter ring wire gauge, I would go for 10-12...maybe even 8 because of the distance.

So in short, you want the switch to:
A. Cut fusebox power
B. Cut Alternator exciter power. Blk/Yel wire should be the exciter wire.

Simple, right?

[af_1132]

I ran the calculator today with 2% acceptable loss, 80 amps, 14 V, and 3 meters, and got 1 AWG. Oh goodness! Should I use 3% acceptable loss and a different amperage? I was under the impression that 80 amps was the max load from our stock alternators.

Yes, I want to go the jumper wire route. Based on all I know now, combined with my present abilities, I think I have two viable options to hook this up successfully:

1. Install the jumper wire as in the diagram (what gauge, between 12 and 8?), install the Painless Performance relay in series with the alternator and fuse box, between the two, and connect the relay to the other small post on the switch with something between 1 and 8 gauge wire. I will have to go back to the calculator with the correct numbers to figure out what gauge to use here.

2. Based on the timskelton article that cp[mike] posted, I need to have the switch disable all electrical functions to be up to tech specifications. So, I would ditch my stupid idea of running 4 gauge all the way from the battery to the fuse box (sharing the positive terminal connection with the 2 gauge ignition circuit, currently), I would purchase a distribution box to mount in series with the 2 gauge positive cable, splice that into my current 2 gauge cable towards the front of the car (leaving the disconnect switch where it currently is) and distribute 2 gauge to the starter and 4 gauge to the fuse box, as in the battery relocation diagram DIY on this site. If I did not use the two smaller posts to separately interrupt the charging circuit, would this setup safely disable the alternator when switched off? Would I need to order a 2 post model instead?

[af_1132]

Just got off the phone with another official and I have a third and fourth option to ponder:

3. Remove the excitor wire altogether, replace the four post switch with a two post switch, and wire one 8 gauge (or something around that level) all the way back to the switch to the terminal on the battery. Maybe this should be coupled with a 40 amp fuse in series, not sure.

4. Start from scratch with an Odyssey battery, re-relocated to the engine bay. No more battery disconnect switches or splicing or fancy wiring or anything! It would also be better for more weight distributed over the front wheels (which is why people were asking me about the reasons for relocating in the first place!). I would just have to get a mount like jdm_92accorn has and follow his battery setup, or anyone else who is using Odyssey batteris. Too bad I have the holes in the bumper now and a sharpie arrow pointing to the current switch (grrr).

Suggestions on which option is the best one, considering that a track day drag car is the ultimate goal?

[sonikaccord]

I ran the calculator today with 2% acceptable loss, 80 amps, 14 V, and 3 meters, and got 1 AWG. Oh goodness! Should I use 3% acceptable loss and a different amperage? I was under the impression that 80 amps was the max load from our stock alternators.

Yes, I want to go the jumper wire route. Based on all I know now, combined with my present abilities, I think I have two viable options to hook this up successfully:

1. Install the jumper wire as in the diagram (what gauge, between 12 and 8?), install the Painless Performance relay in series with the alternator and fuse box, between the two, and connect the relay to the other small post on the switch with something between 1 and 8 gauge wire. I will have to go back to the calculator with the correct numbers to figure out what gauge to use here.

2. Based on the timskelton article that cp[mike] posted, I need to have the switch disable all electrical functions to be up to tech specifications. So, I would ditch my stupid idea of running 4 gauge all the way from the battery to the fuse box (sharing the positive terminal connection with the 2 gauge ignition circuit, currently), I would purchase a distribution box to mount in series with the 2 gauge positive cable, splice that into my current 2 gauge cable towards the front of the car (leaving the disconnect switch where it currently is) and distribute 2 gauge to the starter and 4 gauge to the fuse box, as in the battery relocation diagram DIY on this site. If I did not use the two smaller posts to separately interrupt the charging circuit, would this setup safely disable the alternator when switched off? Would I need to order a 2 post model instead?

It sounds like you're calculating if you you were going to connect the switch in between the alt output wire. The point of cutting it off from the exciter wire is so you don't have to run all of that big cable.

Just got off the phone with another official and I have a third and fourth option to ponder:

3. Remove the excitor wire altogether, replace the four post switch with a two post switch, and wire one 8 gauge (or something around that level) all the way back to the switch to the terminal on the battery. Maybe this should be coupled with a 40 amp fuse in series, not sure.

4. Start from scratch with an Odyssey battery, re-relocated to the engine bay. No more battery disconnect switches or splicing or fancy wiring or anything! It would also be better for more weight distributed over the front wheels (which is why people were asking me about the reasons for relocating in the first place!). I would just have to get a mount like jdm_92accorn has and follow his battery setup, or anyone else who is using Odyssey batteris. Too bad I have the holes in the bumper now and a sharpie arrow pointing to the current switch (grrr).

Suggestions on which option is the best one, considering that a track day drag car is the ultimate goal?

Lots of stuff here, all of those are possible methods.
The relay method is too many parts, imo. It can get the job done as long as your wiring is correct. The relay should interrupt the alternator power and the battery power.

The Distribution block method can also work, but you have to be certain that the alternator is not being excited by the battery or exciting itself! You have to cut the alternator output completely and again proper wiring is key here. You can ignore the small post and you'll have a "2 post".

3) How are you disconnecting the battery from the car?

4) Fabbing up a mount is extra work, removing sharpie is extra work. No to this option. You're almost there!

The absolute easiest way to do this is (drum roll):

From your battery place the swtich, (big posts) between your battery and fuse box.

Cut your exciter wire and run 2 whopping 10 gauge wires to each of the small posts.

The reason I don't fully like the quickcar method is the alternator can be self-exciting. It's the same reason you can remove the battery and your car still runs.

[cb7 calling]

Exciter wire? I have not heard this term, but I don't believe that means charge wire. The charge wire would need at least 0 gauge IMO. It sounds like the exciter wire is one of them in the clip which tells the alt to charge. It seems like you did your wire calculations for the charge wire at an 80 amp rating.

[af_1132]

3) How are you disconnecting the battery from the car?

Sorry, I didn't state that right, I meant for option 3 to read: "Remove the exciter wire altogether, replace the four post switch with a two post switch, and wire one 8 gauge (or something around that level) from the alternator all the way back to the switch post to which I also hooked up the battery terminal. Maybe this cable should be coupled with a 40 amp fuse in series, not sure. Everything else already hooked up remains the same."

From your battery place the swtich, (big posts) between your battery and fuse box.

Cut your exciter wire and run 2 whopping 10 gauge wires to each of the small posts.

The reason I don't fully like the quickcar method is the alternator can be self-exciting. It's the same reason you can remove the battery and your car still runs.

Hey, that seems reasonable, and yes, I had a feeling I had the wrong amp rating when doing my first set of calculations. So, concerning the other cable gauges in this setup, would you recommend 4 gauge for the battery - switch - fusebox line? Would you still go with 2 gauge or lower from the positive terminal to the starter?

I don't mean to sound like I'm giving up by throwing option 4 out there. I'm just frustrated because my car would not start this morning unless I jumped it from the starter (stupid, and dangerous, I know), and I had a bunch of people lined up to caravan the two hours with me to the track that I let down. In key position II, the instrument panel lights up as normal, and all electrical systems work. I don't hear the relay clicking. I decided to take the bumper off to check the existing connections to the switch, and everything was as I had installed it on Sunday (which I hoped was not the case, so I could reconnect what came undone and all would be well). To me, it just looks like I screwed up major with what I already rigged up on my own, before I started this thread. Then I wonder why I even saw the need to relocate the battery to the trunk in the first place uuuugh!

Ok I'll stop now. I am almost there; I just have to start by correctly diagnosing why the starter is not being activated from the key.

[af_1132]

Exciter wire? I have not heard this term, but I don't believe that means charge wire. The charge wire would need at least 0 gauge IMO. It sounds like the exciter wire is one of them in the clip which tells the alt to charge. It seems like you did your wire calculations for the charge wire at an 80 amp rating.

Yes, you and sonikaccord nailed it on the head. I'll just go with 10 gauge if I do the sonikaccord method, like he recommended.

[sonikaccord]

Hey, that seems reasonable, and yes, I had a feeling I had the wrong amp rating when doing my first set of calculations. So, concerning the other cable gauges in this setup, would you recommend 4 gauge for the battery - switch - fusebox line? Would you still go with 2 gauge or lower from the positive terminal to the starter?

I don't mean to sound like I'm giving up by throwing option 4 out there. I'm just frustrated because my car would not start this morning unless I jumped it from the starter (stupid, and dangerous, I know), and I had a bunch of people lined up to caravan the two hours with me to the track that I let down. In key position II, the instrument panel lights up as normal, and all electrical systems work. I don't hear the relay clicking. I decided to take the bumper off to check the existing connections to the switch, and everything was as I had installed it on Sunday (which I hoped was not the case, so I could reconnect what came undone and all would be well). To me, it just looks like I screwed up major with what I already rigged up on my own, before I started this thread. Then I wonder why I even saw the need to relocate the battery to the trunk in the first place uuuugh!

Ok I'll stop now. I am almost there; I just have to start by correctly diagnosing why the starter is not being activated from the key.

That's the mentality to have...piece by piece.

You just brought up a good point about the starter. I forgot that it needs to be connected to the battery.

A distribution block here would be nice or you could run 2 gauge from the trunk to the starter post and from the post to the alternator and fuse box, respectively. That way you are only running 1 positive wire from the trunk to the engine bay. I actually have some scrap wire that I'll let go for cheap or trade for random stuff. PM me if you are interested. I have 1/0, 4 and 8 and fuse holders. Some wire pieces have ring terminals as well. Crimped and soldered.

[af_1132]

I wrote up a wiring diagram that I am hoping matches the distribution box version of the setup you are suggesting. I do need some of those gauges in the diagram confirmed.



Also, what distribution boxes do you recommend, by which manufacturer? Would I need to be looking for something like this?

StreetWires FBXS0024