I hope I'm on the correct list--I need some help with a "drag racing" application wiring problem. I've got one of those external rear mounted "off/on" electrical switches at the back of my 58F100 prostreet and it doesn't "shut off the motor" when I turn it to the "off" position. Obviously I've wired this incorrectly, and I want to make the necessary correction, but I'm not sure what to do. But the switch does work. If one attempts to start the truck or turn on the headlights etc--nothing works until that rear mounted switch is turned to the "on" postion.

I wired the truck and all seems to work good. The truck quickly responds to being turned off at the "keyed" ignition switch etc, but when running one can't stop it by turning off the external rear safety "off/on" switch. I've asked around and gotten various advice.

Someone last night told me that my problem is because I've wired the external switch to "interrupt" the positive as it leaves the battery and that I should have wire it to instead interrupt the negative. Is that right? I may be misunderstanding them.
I've got grounds all over the place. Will that really solve it or what? I know its just me because other racers don't have this problem and I just discovered it the other day and know I don't have it right,etc. I understand its a common mistake, and I've made it, but not sure how to correct as easily as possible. I've got some tight quarters back under the bed and all is tidy with big wire lugs and all, and I hope I don't wander around making this fix too much because its not easy working and moving that big cable+.

Anyone got some help for me. I've got a rear mounted battery under the floor with big welding cables that run up forward to the starter/one wire alternator etc.

Appreciate any help.

Goodluck Tom

 

The truck is continuing to run off the alternator. The switch is just cutting off the battery. The battery is only there to start the engine. Once the engine is running, the battery is only there for the ride. You need to somehow kill the power coming from the alternator.

 

Could you break the positive side of the coil to shut the engine down?

 

I would try to find a double pole kill switch, with one pole for positive battery wire and the other for the charge wire coming from the alternator to the battery. You could run the charge wire from the alternator directly to the battery, but that would be a big amperage circuit with no control (aside from a fuse), so I don't recommend doing that.

 

Thanks for your help guys. I think I've got it figured. I went out and layed under the car and reminded myself of how I did the wiring, and I went the direction that Eric was mentioning. I didn't understand him at first--me not him--but once I looked and drew a diagram of what I have it all seems to make since. In fact I have a 2pole switch and the positive bat cable goes to one pole and on the other pole the "closed switch" position I have the main cable that runs up front to all the electrics. So what I need to do is simply disconnect the one wire alternator from its current connection from the power on the starter solenoid, and instead connect the alternator all the way back to the switch pole at the rear of the truck where that short connection from the Positive Bat is attached which I could call the open switch position. And I'm thinking that the alternator will continue to charge the BAt but will now be disconnected when the elect shut off switch is open.

I just tested my theory be removing the alternator wire and had no problem turning everything off at the rear exterior emergency cut off switch. So now all I need to do --I guess--is replace the beforementioned 8gauge 2 foot wire with a wire 4? or bigger-not sure how big a lug I can attach to the back of that little alternator post--I think its 80Amp alternator--maybe I'll need to go with a small cable because of the 8ft? run to the rear switch?

I'll sleep on this and give it a try tomorrow.

Let me know if you think this sounds crazy- I think its OK. And I'm wondering whether I need to include a self resetting circuit breaker etc--I'm guessing I dont', but its just a guess. I don't want to interfere with the alternator charging at whatever level it needs. The bat cables are carrying the load at the starter etc--of course they are big 9/16? welder cables etc. Do I need to go that big again. I guess I could if I could get it attached to the alternator but I wouldnt guess it would be necessary.

Thanks for your help.

Good luck Tom

 

I've accumulated the cable/fittings etc, but I haven't gone ahead with the mod because I'm still thinking about the suggestion from someone who told me I should consider just running the BATnegative cable to the 2pole emergency cut off switch. Run from BAT negative terminal on battery to the 1st pole on the emergency electric cutoff switch and then out from the switches' 2nd pole to the existing ground on engine block. And according to them that's all I need and that even with the motor running and the alternator generating electricity all will stop when this BAT ground circuit is opened at the emegency cut off switch. This sounds better if it works. But I'm no electrician and with all the in place and required cab to block and frame to block grounds etc will this work? Part of me thinks he's right it all has to connect with that negative post on the Battery somhow. I guess maybe it can and this would be a "safer" way to go I'm sure. I'm kinda busy right now but I'll probably reattach the alternator et al and run a test on his idea as best possible.

Goodluck Tom

 

The current flows in a loop, so I don't see why it would make any difference if you switched the positive or negative side. The 2-pole switch should have four posts for wiring. See linked diagram on switching/disconnecting the battery supply and alternator charge wire at the same time:
http://www.formulastudent.de/uploads..._switch_01.pdf

 

A quick test is to pull the negative cable off while it's running. I can tell you it will not shut off.
The way I inderstand it, you want the alternator to run directly to the battery. Then the power cable from the battery goes through the disconnect switch, then to the rest of the system.

For up to a 100 amp alternator, 4 gauge wire is good for up to 20 feet.

I found this wiring diagram on the LS1 site.

 

Thanks Shaun I was gonna try removing the negative ground at the bat and you're saying it will keep running. That's too bad.

The diagram you show is exactly what I was planning to do. I've got a spare 1awg battery cable that I was going to use to run from the alternator back to the emergency switch and join it to the same post as is used to attach the Positive wire from the battery(the open switch position). I'm sure that will work, but I was hoping this idea about working with just the ground side was useable. Too bad it won't work that way.


Thanks for your help

Good luck Tom

 

The only down side to doing it that way is that there will always be a hot wire between the battery and the engine (alternator). The master disconnect doesn't truly disconnect everything.

 

Ya, that's what I'm not really sure about. From what I read on a couple sites, the NHRA requirement is for shut down of engine, fuel pumps and anything that would pose a danger after a wreck. But the power will still be coming from the battery, up to the alt. and still have the chance of sparks after a wreck. One guy had a dual switch, one side to kill the battery and one to separate the alt from the battery. That way there is only a short cable between the rear mount battery and the switch.

 

Eric

thanks for the link to the page on the 2 pole switch that has 4 connections etc I didn't understand before. Now I have a better understanding. I see these switches on "road" cars but never had any experience with them, but they are clearly superior.

Your point about still having the "hot wire" to the alternator with the master disconnect turned off is valid. I wonder if all the other drag racers required to have this switch are running with a hot alternator wire? I don't know for a fact. But I'm guessing they are. I'll ask around and try to get more data

Maybe I need to replace the switch I'm using. I'm new to this "issue" but I see this switch commonly in drag racing. I've just never myself had to have the switch until now. I don't know how other drag racers are configuring the wiring for this switch, but I'm guessing they're using it with the hot wire to the alternator. Why drag racing allows this; I don't know. Is drag racing versus road racing only concerned with getting the motor turned off? Maybe. I don't know. I know drag racing is super serious about "safety". They're not casual. They've got tons of rules. They don't "seem" to address this. Maybe I'm not reading the rule book enough? But I don't see it. I'll go back and read the rule book again.

Thanks for your help.

Goodluck Tom

 

Shawn

Thanks for your checking out the NHRA side of things. I too have been looking for info. And if I was at the track I'd just ask a Tech inspector==those guys are pretty sharp. I won't be at the track for a couple weeks. Next time I go. I'll ask one and see what I can learn.

Goodluck Tom

 

At the moment I'm installing an in line 200amp circuit breaker on that short cable from the battery positive to the Emergency Cut Off Switch, and I'm expecting this can help in the case of a short to ground in any of these big cables running to the front of the truck. I'm thinking I might order a dual pole FIA switch and look at it, and maybe just remove the existing commonly used NHRA switch. It seems like the FIA type switch would be better in a street/strip car like mine. I'm surprised NHRA doesn't just go this route, I'm sure they know about it. And I'm sure they know better than me. Its me that doesn't know the whole story, but anyway I'm planning to get the 2pole switch and investigate going that way. I'll use the current switch for a period of time and will make what appears to be the "standard" mod of running the #4 wire between alternator and open switch connection on emergency cutoff so it operates as intended etc.

At least that's my plan at the moment. If I learn anything new that I think might be helpful to others I'll add to this thread.

If you get any more ideas let me know.

Thanks for all your help

Goodluck Tom

 

Hi all

Just as a followup I thought I'd mention that i did go ahead and install the 200amp in line buss type fuse on the 2foot positive battery cable to the open side of the elect off/on switch which is mounted to the exterior at the back of the truck, and removed the existing alternator wire that was attached to the starter solenoid and replaced it with a larger wire--a #4 wire 15ft total--and connected it also to the open side of the same electric off/on switch.

I've tested turning off the truck from the switch and all goes well. I've driven around with the lights on etc and I don't see a problem, and the
dash mounted alternator gauge reads 14volts.

I'm not saying this is the best way to go, but its working. The road racing dual pole switch sounds more complete--it was mentioned earlier in the thread. And just today a fellow local drag racer left a copy of instructions and wiring diagram from one time aftermarket hot rod wiring supplier--M.A.D. Enterprises--I think they're no longer in business? But not positive.

Anyway M.A.D. recommends using a second Insulated Terminal Block--one of their items --and relays in such a manner as to turn off the truck without having to use a 15ft wire from the rear of the truck as I'm doing. I glanced at their diagram, but I don't totally understand it. They talk a lot about relays--etc--which I'm using already for headlights and fuel pump and the horn too(?) So I'm not sure if they're idea is really better than the simple thing I've done. Maybe it is. Maybe it isn't. I don't know.

I think the only down side as far as "straining" the elect system has to do with the 15 ft wire and how much if any voltage drop and or spike situation etc would occur. I'm not an expert so I wouldn't give others advice but I'm using for now and we'll see how it works. I think I'll be OK--I'm still getting 14volts on the voltmeter--even with all the electrics ON, and I only have a 80amp alternator and modest(I think) electrical load.

So that's where I'm at today.

Goodluck Tom