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  #1  
Old 11-24-2015, 12:43 AM
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TheBandit TheBandit is offline
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Wiring a battery disconnect

I have a trunk mounted battery and DPST (4-terminal) battery disconnect switch. The conventional wiring method is to run one line to power the car including the starter and run an isolated line to the alternator. When shut off, power from the battery is removed and the alternator is separately shut off so it can't power the engine. That's all well and good but it takes pretty heavy gauge wire from the back of the car to the front and I would end up with likely a 1/0 main power feed and a 2awg alternator feed running the full length of the car.

I'm thinking about alternates options which keep the 1/0 power feed but don't isolate the alternator.

Option 1 is to isolate the fuel pump so that actuating the switch would disconnect the battery and stop the fuel pump. The alternator could continue to run the engine, including ignition, but without fuel the engine should stop. This would be very easy to wire because the fuel pump relay will be in the trunk next to the battery. I wouldn't need any additional wires going to the front of the car. Would shutting off the fuel pump without stopping ignition have any ill consequences?

Option 2 is to run the fuel pump, ignition coils and fuel injectors from the isolated terminal. These are each enabled upon ignition through the ecm by a relay. Fuel injectors and coils are fused at 40amps total so I could probably run this from a 6awg wire from the back to the front of the car.

Option 3 is the same as option 2 except I would run the ignition and fuel injector relay power from the isolated terminal instead of the actual component power. The ecm would continue to run the relay because it controls the negative input. This would kill the relay for the ignition coils and fuel injectors, effectively shutting them down. The difference is I only need to supply the small current required by the relay, which would allow me to run a very small signal wire from the back to the front of the car. Another advantage is the ignition would be running off short lengths of wire from the regulated part of the circuit, ensuring a strong voltage for those components.

If I'm thinking correctly, any of the above options would stop the engine and remove all power from the vehicle. I really like option 1 for the simplicity followed by option 3 and finally option 2.

Does anyone have advice on which route I should take?
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Old 11-24-2015, 01:03 AM
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alwaysFlOoReD alwaysFlOoReD is offline
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Could you use a relay for the alternator?
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Old 11-24-2015, 01:09 AM
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Output is 160 amps. That would take a very big relay. Of course I could probably cut the field wire via relay or perhaps cut ECM power. Probably a lot of ways to do it.
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Old 11-24-2015, 01:11 AM
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alwaysFlOoReD alwaysFlOoReD is offline
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Ford fender starter relay, also known as a solinoid?
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Old 11-24-2015, 01:32 AM
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Ah yes I forgot about that, but I'm trying to simplify. This will probably get used only in the rare case that a tech decides to test it or some punk thinks it would be funny to fiddle with when the car is parked.
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Old 11-24-2015, 01:42 AM
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I guess my real question is will cutting off the fuel pump while the engine is running cause any damage / problems?
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Old 11-24-2015, 01:53 AM
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This question is above my pay scale. But once in a while probably wouldn't hurt. I would think it would temporarily cause a lean condition. I await a more knowledgable answer too.
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Old 11-24-2015, 05:11 AM
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Red face

Just curious why not just break the ground for your disconnect.

With the battery in the trunk your all ready running a positive and a ground to the engine compartment .

That is what I did back in the good old days .
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Old 11-24-2015, 07:14 AM
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Go over to http://www.ronfrancis.com/ and ask the question. Order a catalog while you are there. It has a ton of how-to articles and information in the back of the catalog. Jim
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Old 11-24-2015, 11:11 AM
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Quote:
Originally Posted by wyoming9 View Post
Just curious why not just break the ground for your disconnect.
NHRA rules specify the switch needs to be on the positive side.
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Old 11-24-2015, 11:39 AM
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Quote:
Originally Posted by TheBandit View Post
I guess my real question is will cutting off the fuel pump while the engine is running cause any damage / problems?
That's actually how we generally start and stop the sprint car engine with no ill effects. Actually, shutting off the fuel first is a positive because it allows the majority of the fuel in the lines after the shutoff to get burned off so it can't weep into a cylinder with an open intake valve and cause a hydro-lock situation later.

As long as it isn't done under load, which I can't imagine happening with a manual disconnect, I don't see any issues.
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Old 11-24-2015, 11:57 AM
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Thanks for the reply.

I have been digging and it seems like this would be a perfectly acceptable way too shut the engine down, but why haven't I come across other people wiring it this way?
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Old 11-24-2015, 12:19 PM
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The only logical reason I can think of is to get the power shut off immediately in case of an electrical fire. If you're shutting off the fuel pump, the engine (and alternator) would take a few seconds to shut off because it would have to burn off the fuel in the lines...or at least bleed off enough pressure the injectors won't fire.
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Old 11-24-2015, 05:14 PM
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...the problems with the disconnect switch is that you have to be on the outside of the car and or, you have to be conscious to trip the switch.
Often the battery is in close proximity to the fuel tank and the switch is not attached to the battery, braking the connection at the battery terminal: thus there is a nice fat wire running from the battery to the shut off... Not a good design, maybe better then nothing, but IMO not much.
When it comes to fuel shut off I donít think I want to rely on ether the battery shut off or a separate switch alone! In the olden days the mechanical pump died with the engine. Then we used an oil pressure switch when we moved to electric pumps with our Carbs. EFI would let an engine run where a Carb wouldnít, like upside down and the Oil-pressure switch was no-longer an absolute dead-man.
Thus most OEM EFI systems today (electric pump) have an Impact (aka Inertia) Switch and thatís one piece of new tech that I can live with!
Main power disconnect
Fuel Pump switch within reach of the driver.
Impact switch as the dead-man.

As to shutting off the pump and running the lines to minimum operating pressure at which point the engine will starve and die, you run the risk of vapor locking the fuel lines on restart if you are running a dead head rather the a fuel return system. Iíve only seen it happen one time and I expect that it is a real low risk. Anywayz, food for thought.

E
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Old 11-24-2015, 05:22 PM
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Sounds like perhaps I should kill ignition to get an immediate shutdown. At this point I like option 3 because it would take care of that and allow use of a relatively small gauge wire. I can also add an inline switch as a security measure.

I'll see if I can find one of those inertial switches. I like that idea.
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Last edited by TheBandit; 11-24-2015 at 05:24 PM.
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Old 11-24-2015, 07:11 PM
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One of the switch manufacturers makes a switch that does the main battery and the alt in one switch.(I think its the alt?) I can find the link.

"Main power disconnect
Fuel Pump switch within reach of the driver."

This is what I did on my buggies. Ive seen the control wiring for the fuel pump get damaged/shorted and even after the main battery was shut off the fuel pump continued to pump fuel on the fire.

EDIT: heres the STYLE I was talking about. However I bought this brand and it failed rather quickly. I have pics if anyone is interested.

https://www.pegasusautoracing.com/images/L/4430.JPG

https://www.google.com/url?sa=i&rct=...48493470390242

Last edited by bullnerd; 11-24-2015 at 07:22 PM.
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