Technical Relay position in an old truck

I’m building a 46 Chevy pickup with a 55 Chevy fuel tank in the rear. I installed an in-tank fuel pump that came with a relay. So the instructions say mount the relay as close to the pump as possible but that puts it under the bed and it’s not a weather tight relay. If I mount it on the firewall by the time I route wires it would be about 10,-12’ feet away. What should I do?

 

I'd, most likely, mount the relay inside a weatherproof box.

 

There are two purposes for using a relay in a circuit. Take the current draw of the device off the switching device. Shorten the distance of the power circuit from the device.

Do you know how many amps the pump draws? If the wire feeding the relay for the pump comes from an ignition circuit, you will still have the source 10 or so feet from the pump. If the wire gauge feeding the relay is the same as the wire gauge from the relay feeding the pump, it really doesn't matter where you mount the relay. The power still has to travel 10 or so feet in probably the same gauge wire. In this case you are using the relay to insulate the switching device from the pump.

 

I have in-tank pumps in 3 of my current cars and all of them run relays and all of are located in or near the fuse panel. No where's near the tank. Maybe we can call in an expert @Crazy Steve

 

safety note-
Make sure the primary side of the relay is only active if the engine is actually running, not just with the ignition on. In an accident you don't want the pump to remain energized and pumping fuel. Oil pressure, alternator output, or rpm are some of the signals that can be used. An inertia switch from a 90s ford is also a good safety measure.

 

How much current does this fuel pump draw? Just curious.

 

It makes no difference where the relay is, if the wires are big enough to handle the distance.
You have an either /or situation with this wire from the battery to the pump.
1: A long wire from the battery to the relay [ post#30 ] with a rear mounted relay
Or
2: A long wire from the relay to the pump [ post#87 ] with a firewall? mounted relay

Personally the relay should be mounted as close to the battery as possible [so you don't have live wires running to the back] For Safety especially when working on the car.
On Race Cars we do the opposite "ends" with trunk mounted batteries [we use a Ford Starter relay next to the battery, and bridge the solenoid on the starter] so the main heavy cable is dead unless cranking.
Then run a 2nd wire from the battery to the ignition switch [with a fusible link]

 

Bosch makes water proof relays that can be mounted anywhere, https://www.ubuy.co.in/product/54CI...mp-12-volt-automotive-marine-relays-for-boats

 

I was always under the impression that a relay should be as close to a device as possible, so a light gage wire would be run from the switch to the relay , and the necessary, heavier gage wire from the relay to the device. This would allow savings to be made by enabling a lighter, less expensive, switch & wire to be used.
Is that not correct?

 

I believe that's broadly correct but you make if sound like the lighter wire somehow powers the high consumption device! Well it kinda does as it controls it but doesn't power it as such. There's always got to be a heavy power source wire to the relay. The length / cost of that wire depends on the location of a suitable power source.

Chris

 

If it were mine, relay would be next to the battery, 10 or 12 gauge wires would be run to the pump with the ground side right to the negative battery post, positive side to the relay. Same size wire from positive post of battery to relay. The switching side of relay be grounded to negative battery post with on/off source coming from a oil pressure safety switch being feed by the key switch.
It might have a momentary push button added for priming. Relay will need to be 30/40 amp capable. Even if battery is under the truck, relay would be next to it with grease covering the contacts if not in a water proof case. My battery is under the cab, and shows very little water spotting if any.
Run + & - to the pump also gives you a great place to ground the fuel sending unit.

 

Sorry - my poor explanation, but you're correct. The light gage wire triggers the relay, which is really just a switch, which in turn, energizes the heavier, required wire gage, from the battery, through the relay and to the device. Better?