Master shut off switch charging problem

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gm1236

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Im not sure if this is the best section for this question, but it’s race car related. I have a ‘71 Duster D/SA Nhra legal stocker. I just install a second battery in the trunk, per Nhra rules we must run a battery in the stock location also. So I have one up front and one in the trunk. Both are Group 26, nice they only weigh 28#.
So to get the car to shut down with the master switch you need to kill both batteries and the alternator. I found that if I interrupt the wire that runs from the amp gauge to the starter relay and disconnect the positive terminals the car will shut off. Wiring is mostly stock, I still use a key to start it. I ran the two positive battery cables to one side of the master switch, and the wire from the amp gauge also to the same side of the switch. Then one battery cable back to the starter and a jumper to the starter relay. I used #1 stereo amplifier cable for the batteries and #8 amplifier cable for the alternator wire. It was free seems like good stuff? The car runs and does shut off with the master switch, but still has some power, I know I can fix that by going to a 4 pole switch, I’m messing with an old 2 pole now.
The problem I’m having is the alternator is putting out 14.6 volts but I am only seeing 12.6 at the batteries when running and 12.4 or so when not running. It’s a power master 50 amp alt. The alt wire has 14.6 at the master switch.
What am I missing here?
 
Why in the world would NHRA, require 2 batteries. Battery in the trunk, I believe, has a safety reason for doing it. Having the battery in the engine compartment, could spray acid all over the place in a crash.
 
They don't require 2 batteries, he probably put the second battery in to reduce charging time between rounds. In Stock Eliminator you are required to have the battery in the stock location, hence under the hood.
 
Yes, the battery under the hood has to be the primary battery, & yes the do check it from time to time to make sure it's not a dummy case.
 
First question IS THIS a "one wire" alternator? If so the voltage sensing IS the charge line and it must be oversized, likely in your case no6

Ditch the 2 post disconnect. Buy a 4 post. There are several ways of killing the car with the second set of posts. You can interrupt a relay supplying the ignition / alternator field VR wiring either or or both.

I don't understand where your disconnect is located. Doesn't NHRA require it to be near the rear of the car? If so this means ?? you must be running the front battery clear back to the switch and then clear up front again?
 
I think we need pictures of the setup & a diagram of how you wired it so we can answer your question intelligently.
 
First question IS THIS a "one wire" alternator? If so the voltage sensing IS the charge line and it must be oversized, likely in your case no6

This is a one wire alternator, I removed the voltage regulator when I installed it. It has one small wire going into it and one hot, big wire coming out. I have 14.6 there, #8 wire. Alt. Has been in the car working fine fo several years.


Ditch the 2 post disconnect. Buy a 4 post. There are several ways of killing the car with the second set of posts. You can interrupt a relay supplying the ignition / alternator field VR wiring either or or both.

I plan ongetting one this week, looking for info on which ones are good and what continuous rating I need.

I don't understand where your disconnect is located. Doesn't NHRA require it to be near the rear of the car? If so this means ?? you must be running the front battery clear back to the switch and then clear up front again?

Yes the front battery cable goes to the back of the car then back to the front. Switch is in the rear, 1/0 cable.
 
You have to isolate the battery and alternator charge from the rest of the car.

If you have any wires attached to the alternator, like the normal black wire, it will not kill the car as the alternator will continue to provide power within that loop.

No way is a #8 cable large enough for the run from alternator to rear of car. You need a 4ga minimum, IMO. You are getting big voltage drop from the length of run and wire sizing. If you have some extra 8ga, make a jumper wire from alternator to switch at rear and see if the voltage increases. If so, you have a wire sizing issue.
 
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Here’s a schematic. The starter relay lug powers the fuel pump, m s d, water pump, starter celinoid. I’m sure a few other things. The wire with the fuseable link used to go to the starter relay. By breaking that connection and the pos. Battery connection the car would shut off.
What is between the alt. And the amp gauge and the amp gauge and fuseable link wire in the factory wiring?
 
There should be a welded splice that feeds the fuse block if I remember right.
 
Sometime back when I was wiring up my 4 post master cut off switch someone posted a very good wiring diagram. I can't find it in my file at the moment. Maybe someone here can find it quicker than I can.
 
IMO the charge wire needs to go to a hd relay or continuous duty solenoid then to the battery. The relay or solenoid will be made when the kill switch is on, no power to the alternator when the switch is off. The wire to the ammeter needs to be moved to a source that is also killed when the kill switch is off.
 
have you seen this thread? there's quite a few others on wiring a battery disconnect. i would think doing it this way, and just paralleling the two batteries with 1/0cu cable would do it

batt-cuttoff-jpg.jpg
 
If that diagram is accurate it is pretty screwed up. The only thing you seem to be disconnecting is the starter. That is, it appears the amp gauge wiring is tied to the battery full time.

You NEED to get a 4 post switch. This is the only real solution. Many guys "get by" they run the alternator direct to the battery, and use the disconnect to break everything else from the battery alternator. The problem with this is that IT LEAVES THE ALTERNATOR CHARGE WIRE and the ALTERNATOR always hot, with some fairly large wiring. "When you" are "that guy" upside down in that wreck, with fuel leaking everywhere, and the alternator / wiring/ welder is still "hot" do you think that was a good idea?

As crackedback once pointed out, "there's more than one way" to skin this cat
 
How to fix this?

I would start with some research and by asking myself some questions

That is things like

1....Do I really need a battery at both ends of this car?

2...Do I actually need to disconnect both batteries?

3...Do I need both batteries online for "running" or is one or the other just a booster for STARTING?

If you just want (example) a starting boost, I would solve that by rewiring the front battery back to normal, and feed the rear battery into the starter with a Ford type solenoid activated by the starting circuit, so that when you twist the key, and the OEM starter relay activates the starter---off the front battery, it also brings up the rear battery through the Ford solenoid.

In that case "I would guess" you would need NO disconnect..........just an example
 
They don't require 2 batteries, he probably put the second battery in to reduce charging time between rounds. In Stock Eliminator you are required to have the battery in the stock location, hence under the hood.

Correct, the battery iun the trunk thing was originally for preload over the right rear wheel =weight .
 
What is between the alt. And the amp gauge and the amp gauge and fuseable link wire in the factory wiring?
What Justin wrote.
Main splice with branches to fusebox hot, key switch, and another to headlight switch.
upload_2020-6-1_22-15-45.png
 
Sounds like some of the OEM wiring is being used and the welded splice is not being isolated from the battery/charging system.

That ENTIRE black wire needs to be separate from ANY OEM paths. Straight shot to battery, or with a CD relay in it for a cold line when car is off or main cut off switch.
 
Thanks guys for all the replies.
What I’m going to do is put a painless wiring 250 amp relay in the hot line coming from the alternator. Put a 4 pole shut off switch in and use that to shut the relay and batteries down. I can put the relay close to the terminal block post that’s between the two batteries (left top of the schematic). This will shorten the wire length, minimize the length of wire that’s still live when the shutoff switch is activated and hopefully have minimal voltage drop. I will try a #8 wire and if that doesn’t work I will go to a #6. I read where #8 is good for 100 amps at 6’, I should be within that lenght. At that point it will tie into the 1/0 battery cables.

Reattach the fuseable link wire back to its original location on the starter relay.

Power the shutoff relay from the ignition wire. Put a switch and fuse in that line.

Couple of questions?
By disconnecting the hot Wire from the alternator to ammeter will I lose power to the fuse box or will it still get power through the ammeter from the other side?

How much draw does this relay have? I have used this one before, powered by a battery that was not tied to the charging system and it would kill it rather quickly.
This set up is like the one in post #15, but the shutoff switch will get powered by a different wiring configuration.
 
There's a few wire size tables and calculators on-line.
Engine compartment runs at higher temperature, so voltage drops increase.
SAE gages have lower carrying capacity than AWG (Marine wire uses AWG sizing)
WireSizer User Guide
Wire Size Calculator

By disconnecting the hot Wire from the alternator to ammeter will I lose power to the fuse box or will it still get power through the ammeter from the other side?
it reads like it will be getting power from the 'battery' side of the ammeter.

Another way to kill the engine with a 1 wire alt, kill power to the msd or the msd on/off wire. Then you don't need to use such a heavy duty continous relay.

Fusible links IMO should be between the battery and anything that is running without a fuse. I've been making an exception for an MSD 6 power, but am now in the process of changing that. I assume your electric pump is power is fused.
 
I'm not clear why its important to be hooked to the ammeter in this situation.

If you want to use the ammeter then my suggestion is use it only for battery charging. Then it will read correctly, and not be carrying extra loads.
And if you want to do it with just one fusible, then something like this should work. Use a grommet through the firewall (or some other substantial firewall feed).
The MSD heavy red wire can connect at the terminal as can the fuel pump.
upload_2020-6-2_23-42-46.png


the other guys may see faults or drawbacks here.
 
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