magnum

Getting ready to install an electric fuel pumpon a small block carbed motor.I've got all the parts,pump,reg,gauge,inline fuse,low oil pressure cut out switch and relay.The cut off switch came with a wiring diagram wich does not include a relay.Is a relay really needed or should I leave it out.The pump is a mallory 140 gph unit and calls for a 7.5 amp fuse.What do you think,yes or no?

Gary Anderson

Depends on the amount of current going through your ignition switch. If it's just the starter solenoid, Tbolt ignition, and the fuel pump, you're probably OK without the relay if your wiring is good (14g or bigger to the key switch). Just be sure you're not seeing a voltage drop across the ignition switch. Corrosion will cause this frequently in boats. Low voltage wont hurt the ignition, but ruins fuel pumps fast.
Gary

RumRunner

Yes, a realy should be used.

Running power from your battery up to the ignition switch, back to your oil pressure safety switch and then to your fuel pump will cut the voltage. Simple resistance.

Fuel pump us DC motors which are voltage sensitive, more voltage more fuel.

Run a relay, directly to the battery, and to the fuel pump, and then use your oil pressure safety switch to turn the realy on and off. This will ensure you get full voltage to the pump, and optimal performance.

Liberator21

In your post, you say it came with a relay. If that's the case, you would wire the low oil pressure cut off switch in series with the ignition switch, and relay coil. This way, if you lose oil pressure, power to the relay is cut, the relay drops out, and your fuel pump shuts off. You don't want to wire the cut off switch in the fuel pump line directly, because it probably can't handle the fuel pump's current.

Paul

Liberator21

Here's a little diagram.

Liberator21

Can't post attachments anymore?

magnum

Thanks guys,I'll be using the relay like RumRunner says.