Do I need a relay here?
I want to add an electric fuel pump in my 1953 Kaiser in the back near the gas tank as you suggested in a previous Mechanic on Duty column because I have had trouble with vapor lock this summer. I want to hook it up to an on-and-off switch under the dash so I can use it only as needed, as you advised in the column. However, at a club meeting a few days ago I told a friend about what I planned to do and he said I should use a relay when I hook up the wire to the ignition. Why? What do they do?
Your friend is right about using a relay for your auxiliary fuel pump. That’s very good advice. The reason is, if you power the fuel pump using a wire directly from the ignition switch you are asking more of the ignition switch than it was originally designed to do. But if you use a relay, which is in reality just another switch, it will only need a fraction of the amperage required by the fuel pump itself. That’s because the power for the fuel pump will be coming from a fuse panel connected directly to the battery.
The relay will be connected to the ignition using a toggle switch under the dash that will merely activate the relay, which will then complete the circuit allowing current to flow from the fuse panel to the fuel pump. This decreases the demand on the ignition switch and at the same time supplies power directly from the battery to the fuel pump. In other words, the switch under the dash just activates another switch (relay) in the main wire, but the actual current to power the fuel pump comes directly from the battery and does not go through the ignition switch. For safety reasons you will want this direct wire fused either through a fuse panel or by an in-line fuse.
Also, in the event that there isn’t a slot on a fuse panel, instead of trying to hook up your fuel pump wire to the big battery terminal, you can attach it down where the battery cable attaches to the starter motor. You will want to protect any wire going through the frame and engine area using snakeskin or other insulation, and make sure it is clamped securely into place so it won’t get snagged. And you won’t want the wire or the fuel pump anywhere near a hot exhaust system.