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Battery Relocation, Remote Cutoff Switch, and Alternator Solenoid

Practical Motorsport Wiring - Club Level

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Discussion and questions related to the course Practical Motorsport Wiring - Club Level

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I am redoing my battery wiring at the same time as building my club level engine harness, so I was hoping to get some input on the correct way to wire the system. I've moved the battery to the back of the car, installed in a battery box. I have a Moroso cutoff switch mounted in the back, it's the 4 pole type with 2 large poles and 2 small. I also have a large solenoid that I was planning to use to cut off the vehicle systems from the alternator.

It's my understanding that even if you flip the cutoff switch to disconnect the battery, there is still a chance that the alternator could continue to power the vehicle and leave it running. So how would I go about wiring in the solenoid to cut this off as well? My thinking was wiring one pole of the solenoid to the alternator, and connect the other pole to the existing front battery post (and all the vehicle systems) as well as a cable running back to the cutoff switch and battery. And then activating the solenoid with the ignition, or could I also wire that to the cutoff switch?

Is that correct, or can you give me a proper diagram showing how I should wire it?

Or should I change out to a battery isolator like the Cartek GT?

the smaller of the 2 poles is meant for the alternator field wire. this will kill the alternator putting out voltage and the other pole cuts off the battery. I don't think you need the solenoid but if you wanted a way to kill the car from the drivers seat that isn't a cable to the Moroso one thats a way to do.

Do you have pics or part numbers of the switch in question?

It's Moroso 74102.

As I understand it, even if you kill the field of the alternator, it can still potentially drive itself, self exciting. Plus I may be upgrading to a 1-wire alternator in the future. So that's why I was looking at the solenoid to completely cut off the system.

You're over-thinking it - while connecting the alternator output directly to the engine side of the cut-off switch would continue to power the vehicle, that isn't how it is wired in the vehicle.

The normal type of master cut-off has an isolated return for the alternator output, with a diode to prevent the potential for a short because of power coming from the battery side, because simply open circuiting a running alternator can result in a voltage spike that will destroy the diodes and/or voltage regulator. It doesn't matter if the alternator is still turned by the engine, and self exciting, as may happen when a manual is in gear on the over-run, or during the brief moment while the clutch is dipped, or whatever may be doing it, it has a return path the battery that is independent of the rest of the wiring. Of course, when the engine stops turning the alternator there's no output, anyway.

You'll find the wiring diagram and other information here - https://www.moroso.com/pub/media/instructions/74102_instweb.pdf

I don't know if that is correct though. I've got a solenoid/relay similar to Painless 50105, which shows it wired in line with the alternator main power line, see attached.

My concern with this wiring is that when the car is parked but the kill switch is not engaged, it is powering the relay, which is drawing power. So rather than running the "RD/WT 18GA" they connect to the switch, change that to the 12V switched ignition?

Attached Files

One way I can see to mitigate your concern about the solenoid remaining powered when the car is off, but the kill switch is still on, is to add an RPM activated relay (whether you use an MSD RPM activated switch or an output from your ECU, assuming EFI of course), to add another control point for the solenoid. You can insert this relay inline with the positive control of the solenoid or the ground side. Set the relay to turn on at around 100 RPM. This way would also disconnect the alternator output in the event that the engine stalls, which may be a benefit.

Another way is to control the relay with the ignition switch, so that the solenoid is also powered off when the ignition is turned off. Again this relay can be inserted in line with the positive or negative control of the solenoid.

Also don't go 1-wire alternator, that tech belongs left in the '70's when it made sense to have that option for hot rodders that didn't have the access to information we have today, and have lower current needs. Sure 1-wire alternators do work a bit better than they did back then, but I have yet to find any reason to ever use a 1-wire alternator in a modern build.

I may be mis-reading the diagram, Toby, or you may have drawn it wrong, but the relay plays no real function as drawn - worse, if the kill switch is turned off with the engine running, especially at speed, there is a good chance of blowing the alt'rs internal electronics.

The kill switch should have a specific terminal for the alternator, and if it's a simple battery isolation/disconnect switch just run the alt' output to the starter terminal, or other convenient place.

Gord, you're mis-reading the diagram.

The solenoid will indeed disconnect the alternator output from the car when the main switch is turned off, and isolate that lead from the car.

In some (most?) racing organizations that require a battery shut off switch requires the vehicle to also stop running when the switch is shut off, when it's running. Connecting the alternator to the starter post or anywhere "convenient" that doesn't disconnect it from the ignition switch, won't accomplish this need.

Typical way to connect the alternator has been to run the charge lead to the battery side of the switch. This accomplishes the need of shutting the car off when it's running but does leave a still live alternator charge lead, with the switch off. This is where that solenoid comes into play, to also kill the alternator output.

Yes, there's small a chance of the alternator being damaged when disconnecting the charge lead in this situation, but in the event that this is needed, then a damaged alternator will be of little concern, because this typically means the car is into the wall, or otherwise damaged.

So what do people think about this setup that I've drawn?

And when I mentioned 1 wire alternator, I guess I really mean a Ford 3G, so not really a 1 wire. They are the most affordable way to mount an upgraded higher output alternator since I'm custom building mounts.

Attached Files

hmm, in my opinion that diagram still leaves the solenoid redundant and doesn't prevent alternator load dumping. When the kill switch is turned off the solenoid will drop out milliseconds later and the engine will still likely be turning the alt.

I've done systems like this a few ways, killing the alt field has worked fine and never had an alt self excite that wasn't a self exciting to begin with.

I have also ran a big cable from the alt output to the battery side of the kill switch. This prevents the alt from load dumping as its always connected to the battery, and the starter and vehicle electronics are isolated by the kill switch. More cable (weight and cost) then also cutting the field but works. I was more comfortable with that method before I understood how alternators generate voltage and tried it another way.

I'd also agree with Chris, just use a modern alternator. I like the AD230 or 244 (gm LS style) as they are super common and there's tons of rebuilders and high output versions. OEM GM ones do 100 amps, usually plenty.

I'm not mis-reading it, just being polite.

Toby, exactly what are you trying to achieve, and what are the requirments for the specific series' you are going to be competing in?

For motorsport, the primary requirement is for an engine shut-off, that is required to shut off the electical circuit(s) that allow the engine to run, including (especially!) fuel pumps, etc. The requirement is normally phrased in such a way as to allow lights, wipers, etc. to still operate because, depending on the series and use, windscreen wipers and headlights may be needed for safety and allowing the driver to see where (s)he is going - if there's an engine/fuel problem you don't want the complication of being struck blind as well, which can turn a minor fire/incident into a major crash.

I suspect there are two different functions being conflated. An engine kill switch/device, and a master battery switch for the full electrical system? Either way, the alt' output will normally either be run to the kill switch or the battery.

I assume what you're concerned about it is the potential for the alternator output being live when the engine kill is used, rather than the concern about it powering the engine electrics? Again, I'd suggest using a proper kill switch - there are several varieties with differnt operation options, and a proper battery disconnect switch which, again, are available with a range of operation options.

No specific series, just a street car with the ability to enjoy autox, track, and drag. I'm trying to achieve a safe setup that will be allowed by the tech inspectors.

I don't need a master battery switch (inside the car), just the cutoff switch at the rear.

Yes, my main concern is that the alternator will continue to power the car even after the cutoff is switched off. The car is a 1988 Toyota Supra Turbo with the 7MGTE, stock alternator for the moment. I'm going to upgrade the alternator and move is to the other side of the engine, as I'm planning on adding a bunch of loads that the stock alt won't keep up with (efans, electric water pump for an air to water intercooler, electrohydraulic power steering).

Regarding activating the solenoid, I just want to make sure it isn't constantly powered while the car is parked, as that will just drain the battery.

Honestly I'm totally open to suggestions for this, as the more I look at it the more I seem to confuse myself. Moroso, Painless, etc all seem to suggest different things, and a lot of the time the things I'm reading are conflicting.

To add to my confusion, I just watched this from HPA: https://www.youtube.com/watch?v=lMpTT-f72g0

Battery Isolators are the next level up from kill switches in price and features. Especially the CAN enabled ones like MSEL and Ecumaster. I'm going to ignore the existence of the Cartek isolator to avoid more confusion.

Attached are 2 diagrams to help illustrate;

V1 is how I do battery isolators, you can have a delay that prevents load dumping voltage build up.

V2 is the simpler way, uses more cable though.

Gord's suggestion is on point too, depending on rule book they may just want the engine killed as opposed to all electrical power.

Attached Files

Having spent some more time, checking the requirements and options, there still seems to some question about what EXACTLY is meant by "If the battery is not in the stock location, and relocated to the trunk, for example, you must have an external cut-off switch on the back of the car which cuts off the electrical system and must also stop the car from running." The latter part may be aimed at magneto ignitions that may require a ground to turn them 'off'.

My, personal, preference would be the first for a master battery disconnect*, but the latter seems to be the way many people are doing it, using what are known as 4 pole and 2 pole switches, respectively.

However, sometimes "tech" folks have their own strange ideas on what things should do - it wouldn't hurt to run through your vehicle's setup during the build as some sanctioning bodies may have other specific requirements. For example, if you need to carry a fire extinguisher the size/type may be different as may the minimum mounting fixtures; here engine and transmission drain plugs used to need to be lock-wired (don't know if they do) for some types of competition; others require laminated windscreens; some require at least basic 'fire-suits' for any sort of speed event; back in the day (still?), head-lights had to be taped in case of breakage for some classes of events; etc.

*That would seem to cover the two main types of cut-offs, and the first seems to be aceptable as this is NHRA rated - https://www.jegs.com/i/Taylor/895/1033/10002/-1

It seems like even amongst people here, there are varying opinions on the proper way to do this.

From the NHRA rulebook:

8:4 MASTER CUTOFF Mandatory when battery is relocated, or as outlined in Class Requirements. An electrical power cutoff switch (one only) must be installed on the rearmost part of each vehicle and be easily accessible from outside the car body. This cutoff switch must be connected to the positive side of the electrical system and must stop all electrical functions including magneto ignition. The off position must be clearly indicated with the word “OFF.” If switch is “push/pull” type, “push” must be the action for shutting off the electrical system, “pull” to turn it on. Any rods or cables used to activate the switch must be minimum 1/8-inch diameter. Plastic or keyed switches prohibited. Switches and/or controls must be located behind rear wheels on rear-engine dragsters.

Here is the wiring diagram from the TSRM for my stock alternator: