arpunk

A few questions about this fuel kill setup since I’m intending to use the same source for power to pin 86 as the line that runs through 87/30:
1) Is the same wattage running through the switch as if I hadn’t used a relay?
2) I think it would be smarter to put the switch on the hot side instead of the ground. Would this mean that the wattage running through the switch is the same as if I hadn’t used a relay?
3) Should I move the fuse to between pin 87 and the kick panel?

lawsoncl

You're good. The relay is still switching the main current to the fuel pump. This setup is pretty common when the switch is a distance away from the relay (say under the hood and the switch inside). Basically what you have with the starter solenoid. If you're close to the relay, why not just grab a switch rated for at least 10-amps and skip the relay?

robsjeep

using ground for switch is smart. You only need run one wire from relay to switch rather than two. Plus a short to ground in wire from relay to switch wont burn up the wire if not fused. Win win!

arpunk

I prefer to err on the side of caution. I put an inline switch on my old integra’s fuel pump ground after extending it and the switch used to heat up and eventually became intermittent. Probably used a switch which wasn’t the proper rating but I have no idea it was probably 17 years ago.

arpunk

How does putting a switch on the hot side change the wiring? This is the other configuration I was thinking of:


Switch on hot side


Either way, I think I’ll move the fuse to the LINE said instead of the LOAD side, although in practice I don’t know that it will make a difference.

robsjeep

If you switch with hot side you will need to bring your hot wire too the switch and then back again Where as using ground you can pick up ground off any body part near the switch.


of course if you got another hot source near switch you may use that, but that is another circuit perhaps

arpunk

Ah! I see your point.

I know that generally the hot side is switched but perhaps it is different for vehicles? As you can see from the photos, I’ve double heat-shrunk the connectors and will make sure that any pinch point will be reinforced so I’m not particularly concerned with the hot wire touching ground. I’ve also used 12g wire which is a bit beefier than stock (14? 16?).

What size fuse would you recommend? Do you agree that it is better to put the fuse on the line side (between relay and fuel pump relay) as opposed to load side (between relay and fuel pump)?

bluejeep2001

Either way you run your switch, the amps needed to close the relay/ run through the switch is minimal compared to the amps needed to run the pump. Thats why they make relays instead of switches. If you put the fuse on line side it protects the relay and the pump wiring. For future reference, the wires to 86/85 do not have to be nearly as big as 87/30 because it only takes an amp or two to trigger the relay. If you have time give me the resistance across 86/85. 1 ohm draws 12 A...10 ohm draws 1.2 amp... 50 ohm draws 0.6 amp..you get the picture

arpunk

The higher the resistance the less current it pulls? That seems backwards, no?

doublechaz

No. The more plugged up your garden hose is, the less water flows through it.

arpunk

I saw it as: "the more resistance, the more current needed to push through to the other side."

So let's pretend that the fuel pump pulls 12 amps and the relay pulls 100mA from 85 to 86 in order to close the connection between 87 and 30. Are there 12 amps on the line side of the switch and 100mA going through the switch?

bluejeep2001

Using your picture of you holding the toggle switch....you will have 12a going into contact 87, and 100ma on the wire to the switch. Technically you will be pulling 12A + 100mA thru the fuse. You will have 12V on all red wires. A simple example is your starter relay....you need a huge 4 or 6 gauge wire to the relay power contacts but the trigger wire is a tiny 18 ga

bluejeep2001

V=IXR. I is amps. V/R= Amps. Your volts are preset at 12 ish. If you stick your screwdriver across battery terminals, you get a huge spark and it melts your screwdriver...(low resistance) high current. You hook up a dome light across terminals...High resistance...lower current

arpunk

Thank you @bluejeep2001

What size fuse would be appropriate? I was thinking 15 amps. I don’t know how much the fuel pump draws.

Sworvoo

Just start small and keep bigger ones on hand

Also, something to remember that people don’t think of is what is an in-line fuse for. It is not to protect the device. If it blows from the device having an issue, the device is probably toast already. A fuse is to protect the wires and prevent 600amps from your battery going through them and burning your car. That is why you want your fuse as close to the battery as possible. If the fuse is 6 feet from the power source what is protecting that length or wire?

And, relays are just switches controlled from somewhere else or if you want a smaller rated switch. If you don’t need a relay don’t bother. But your setup does look very nicely executed.