Thread: How to wire an electric fuel pump

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I am finishing up a complete rewire of my 28 PU with a harness kit. I wired the electric fuel pump through a relay to the designated wire to the fuse block, plus a wire to a "12volt battery constant with a inline fuse". To me this means to my battery.
Well, of course, when the battery is connected the fuel pump runs even though the ignition is off.
Where am I going wrong. I know I'll want to give myself a head slap when I get the answer. But I'm stuck. And, yes, there is a good ground to the chassis and hot lead to the pump
Can someone give this rookie electrician some help?

The " 12volt battery constant with a inline fuse" should go to the relay and the ignition switch will turn on the relay and then turn on the pump.
Does your relay have numbers near the electrical connectors?

#86= power from ignition switch(turn the pump on)

#87= +pump

#30+ 12volt battery constant with a inline fuse

#85= ground

Yup, that is how it is wired. I tried another relay, but same result. The pump runs when the 12 volt from the battery is connected with the ignition off. Any ideas?

disconect the constant 12v to the pump and turn on the key see what happens

Welcome to CHR - hope we can help you out!

You are describing a Normally Open (NO) 12 volt relay. It sounds as if you have it hooked up correctly –
30 - should be a #12 wire fused at 20 amps to the battery (+ 12 Volts)
86 – should be a #16 wire to the switched side of the ignition, i.e., power when the key is on
85 – to ground
87 – should be a #12 to the pump

The image below depicts a standard relay. The “blue circuit” shows a square with a diagonal line running through it. This represents the coil circuit. The center numbers (86 and 85) are part of the coil circuit and will energize the the relay “on”. The relay will work when 12 volts dc is applied to these inner pins. If you follow the “red circuit” lines that extend from the outer numbers you will notice a gap in the line. This gap represents that these two pins are not connected or “Normally Open” in this circuit. The red circuit will close and allow voltage across it after the coil circuit is powered.

To test it apply 12 volts the coil (blue) circuit then place an ohm meter on the high amp (red) circuit. Select a low ohms or tone setting on your ohm meter. As the coil is powered you should see numbers close to zero displaying on the meter (or hear a tone indicating continuity). When you disconnect the 12 volts to the coil (blue) circuit the meter should show zero (no tone).

This will happen if the relay is working correctly. If not – the relay is bad.

Good Luck,
Glenn

Like Glen says, you should be looking at a NO relay. Do you by chance have a NC relay? It would be closed when the ignition circuit is off, running the fuel pump as soon as the battery is connected.

Most relays can be wired NO or NC. A quick check ignition switch on the pump stops, this tells you that the relay is wired NC. On the bottom of the relay you will see 5 jugs, take the wire for the pump and move it to the vacant lug.

Thanks for the input. The relay is good. If i disconnect the battery lead, then plug in the wire from the ignition, i get an audible click, like a switch as opened or closed. The pump doesn't run with this hook up. I'm gonna try angrystrokers suggestion next.
Paul

Sounds like you have the relay wired NC

Check out this photo
12 VDC Automotive 5-Pin Relay SPDT 30/40A Bosch Type 330-073

Look at this relay you will two lugs 87A and 87

I think you have the pump wired to 87A

It needs to be on lug 87

The path to the pump is from lug 30 (battery) through the contact connecting lug 87A, ignition switch off. Pump will run when battery is connected. Ignition switch on the dotted line pushes the contact up and opens the circuit, no power for the pump

When you turn on the ignition switch that dotted line pushes the contact up to lug 87 now making the path, This is where you want the pump wired.

Hope this will clear the fog, your close just need to move a wire.

Just figured it out! The wire that runs to the ignition, was touching a wire connector that is hot with the key off. I had changed out these connectors and not insulated soldered joints carefully enough when tracing another issue. Proper insulation on both wires and the pump runs with the ignition on.
Thanks to all for your time and advice

There ya go! Good job on the trouble shoot. Keep us in the loop on your build and we'd enjoy some pictures when you get a chance.

Regards,
Glenn

Glad you got it working lots of people don't bother coming back and saying so

That's cool I love stuff that works, x2 on getting back saying the problem was solved.

Did you add an oil pressure switch to shut pump down if the engine isn't running? I uploaded a pic of my basic wiring that's now mounted under the seat.