Getting to the wiring portion of my latest rebuild. I've come across a few different options for running a disconnect with a one wire alternator. I would like to get some feedback from others who may have used any of these types of set ups.

First one is with the main battery cable is constantly hot using a four post disconnect. I've used similar set ups in race cars but have heard recommendations for avoiding that long run of hot wire in a street car. Will require a fairly stout 4 pole disconnect switch too.

Next one is an alternative approach using two Ford solenoids to energize cables during cranking and routing alternator output.

Final one is a MAD layout I came across. Also uses a Ford solenoid but only one of them. Also has nice break down of wire gauges and fusible links in it too.

I'd use the top diagram with one modification. Put a continuous use relay in the main alternator wire. Use the second set of on/off contacts in the main switch to control it. That way your charge current isn't going thru the switch. I found with my double pole switch the 2nd set of contacts didn't have the ability to carry enough current. As far as the main wire being hot, look at a new Challenger , Charger, BMW, etc. They all have a hot main feed from the trunk.
Doug

I used a solenoid that was rated for continuous duty , but that is definitely an area you want to address.

I ran a #10 gauge wire from the 100 amp one wire alternator to the battery side of my battery disconnect switch mounted on the rear taillight panel on my 1970 S/P Cuda, works good, last a long time! I like and use the KISS principle
How many amps is your alternator? If more than 120 Amps use a #8 gauge wire from the alternator to the battery connection.

Tony -Is this for battery in trunk and kill or disconnect switch in back?

Any additional running equipment? such as electric fuel pumps, fans, efi? This makes a big difference in terms of power distribution location.

With a one wire alternator, unless I'm missing something, I think it option 2 is too complicated.

If you like the idea of a relay, put it in the engine compartment and arrange it more like alternative #1.
Advantages would be much shorter path from power source to equipment (ignition, lights, etc).
Alternator to Starter relay stud could be an 8 or 10 gage wire depending on the expected load.

It still should get some fusible links in case there is a short before the breakers and fuse box.
Probably the best location in this arrangement will be at the starter relay.

I like that configuration.

dvw and tubtar, definietly looking at some higher rated switches other than the generic $40 disconnect switch. There are some pretty high rated units up to 250 amps continouos with 1000 surge.

Cab, my alternator is 50 amps at idle 75 amps at 3000 rpm. That should be the ceiling on it.

Mattax, this a trunk mount battery with disconnect switch. As a street car, yea, I've got a full complement of other functional items in addition to the ignition; all light systems (head, tail, dome, dash, ash, glove, brake, hazard, turn signals), hvac, radio, fuel pumps. May consider O2 sensors or efi in the future as well. Can't foresee ever doing power windows or seats. As a '74, it also has all the seatbelt interlock stuff that I'm working out of the system and replacing with the clutch start circuit. So when you say relay, something capable of the full amp load of the balance of the system plus a margin or the total alt output? In that case, would I still want the power lead from the starter to the OEM relay?

OK. And I'll assume the switch must also kill the motor, and grounding out the engine is not allowed.
Fuel pump, EFI are the big ones that might benefit from a strategy different than stock. These can draw down the battery when the alternator is not running (start) or not capable of providing the power demanded.
Lets say an alternator is used that can produce 280 Watts when the engine is at 650 rpm. This means it can provide up to 20 amps at 14 Volts.
I'm not sure what the 'turn on' speed is for a 1-wire, but for sake of example, lets assume its self powering at 650rpm and the field is drawing off 2.5 amps.
A solid state ignition maybe draws another 2.5 amps.
That leaves 15 amps for everything else.
Turn on the lights and that's around 14 amps,
Step on the brakes and now the alternator can not supply the power needed. System voltage drops and when it gets around 12.8 Volts, the battery becomes the power source with highest voltage.
This was somewhat tolerable with mechanically supplied fuel, since it only happened briefly at stop lights and stop signs.
But with more equipment, this is situation will occur more frequently. The typical stock systems through '75 assumed the power would almost always come from the alternator and the battery would only need minor recharging.


Can't help with the interlock.
When I say full amp load, I mean the equipment that will be calling for power.
I don't know if it makes a difference whether an second wire is run all the way from the cut off switch to the OEM Relay stud, vs connecting it as shown in the drawing.

I'll work on a drawing but post this now, as the board may shut soon.

Here's that sketch altered a little.

A fusible link in the line going to the oem system power distribution can be the same as it always was.
All of the new equipment, (fuel pumps, EFI) and any existing foward equipment that pulls significant power (headlight hi, low) can each be put on a relay after a fuse.
I would do this using an engine side fuse/breaker and relay box on the inner fender. If you feed it with an 10 gage wire,then use a 14 gage fusible link.

I don't like the idea of hanging lots of wires off the relay stud, although that's basically what AMC and Chrysler did on the '86 to '89 Jeep SJs. And each one of those wires except the heavy cable from the battery got a fusible link. It's interesting that they put a fusible link on the alternator output wire just in case the alternator internally grounded. I'll have to look at the final years again when a Chrysler starter relay was added to see what they did.

Just using working off the factorry wire sizes as examples to see if there was possibly a way to eliminate the need to put a fusible link in every line off of the stud.
I know the later cars with larger alternators show 10 gage wires from the alternator output. And in your design the loads will be possibly higher (fuel pumps, EFI) and the distances longer.
Therefore it seems reasonable that 8 gage might be needed from the alternator and to the battery.
The inside loads are the same or less, so if your factory feeds are probably 12 gage.

That means we could protect the 8 gage wires with just one 12 gage link on the battery feed.
But that wouldn't protect the originals wires to and through the bulkhead. They're 12 gage and some will be even smaller (ignition for example). But if the ignition grounds, the engine will stop. In that way its partially protected. ( You'll still need to turn the key switch off to kill power from the battery to the short.)
However it does reduce the number of fusible links needed, so its worth considering.

There are lots of ways to do this, but for me simple is best. I design systems with short wire lengths where possible to help with voltage drop and weight.

The problem with any relay controlled by the battery disconnect switch is the relay uses power and will kill the battery if left on for long periods of time. The higher the relay current rating the more power it takes to operate normally.

Good point!
The relay would need a second switch in the driver's compartment to be a little more convenient.

Suppose could have a pair of relays, the first connected to ignition Run, that relay then opening or closing the trigger or ground for relay 2.
A little more complicated but still seems simpler and cleaner than alternative 2.

The hand drawn diagram will kill the engine but doesn't completely disconnect the battery.
Some of this comes down to what the goals are. isolate the battery? kill the engine? both?

Cool. Care to share any designs?



The goal is to kill the engine and isolate the battery to meet NHRA, SCCA, and NASA requirements. I figured there are way more NHRA drivers on Moparts who have done this than either SCCA or NASA, so I'm here in the race forum.

I have previously done this on a dedicated track car. Amp draw was low and of limited time, so I set it up to run everything off the battery and the alternator feed the battery. This is my first effort in a dual purpose car where there is more power distribution than just ignition and fuel. I figure I will need to have a distribution point near the front of the car for these street items. Running cabin items off distribution directly from the alternator is probably fine, so long as fuel and ignition are shut down with the disconnect switch.

My alternator does produce 50 amps at 800 rpm, so I've got decent coverage for draw at stop lights, even if/when EFI is factored in.

On my street car I took out all the factory stuff for the engine, the factory wiring only runs the lights and wipers.

This is how I wired mine.

Battery to 4 pole disconnect
Disconnect to Starter Relay in Trunk-Relay to starter(only hot when cranking)
Disconnect to Power Distribution-amp gauge wire goes to this and powers up the lights and wipers.

There is a continuous duty solenoid between the alternator output and my power distribution.
That Continuous duty solenoid is powered up by ignition on, however the power runs from ignition on through the small poles on the 4 pole disconnect and back up to the solenoid.

This way when the key is off, the continuous duty solenoid is off, but the master disconnect still kills everything.

That was hard to explain and probably makes no sense lol.

On your deal I would run a #8 or#10 gauge wire from the alternator output directly back the battery side of the cut off switch so the car will not run with the switch in the off position, no volts to the rest of the cars electrical from the alternator in the off position. Make all of your other needed electrical feeds from the battery cable up front to keep the rest of the wiring needed short as possible to reduce resistance and voltage drop.
Safe, simple and effective !

Cab the only thing I don't like about that is the alternator wire is still hot, the car will die but the alt wire will have power.

With loads of accessories on mine Street car stuff(lights wipers), Fuel Injection ECU, Two Fuel Pumps, two fans, electric water pump, and laptop and phone charging on drag week I like to have a sense wire for the alternator to work off of the power distribution point. It complicates the shut off no doubt but I feel it works better in the end.

Why not put the disconnect on the ground side.

That's against the rules and you don't want it that way anyhow. My buddy had a car like that and it would blow your mind. Crazy stuff like, when you turned off the disconnect, the brake lights would come on. It would drive you absolutely crazy.