electric fuel pump

shawser · Post by **shawser** » Thu Nov 24, 2011 12:00 am

which part of the ignition does the power need to be connected to on an electric fuel pump? has it to be wired so it comes on as ignition is switched on or when engine is turning on starter?

jefferybond · Post by **jefferybond** » Thu Nov 24, 2011 7:45 am

Ideally, it needs to be wired so that it only runs when there is oil pressure (so that it stops if you find yourself in a ditch upside down!). I think some rv8 oil pressure switches has three terminals for this purpose. You'd also need for the pump to run while cranking, or it'll be trouble to start.

Running straight off the ignition feed will work, but won't be that safe in a crash.

DaveEFI · Post by **DaveEFI** » Thu Nov 24, 2011 8:38 am

shawser wrote:which part of the ignition does the power need to be connected to on an electric fuel pump? has it to be wired so it comes on as ignition is switched on or when engine is turning on starter?

If it is wired from the ignition, it will run while cranking. As the ignition must.

Many are designed not to run if the engine is stopped. So are controlled by oil pressure or air flow or the ECU. If you switch it via a relay, you can use diodes to feed the relay coil from both the oil pressure sensor or whatever and the starter circuit. The diodes prevent the two circuits interacting. You may also consider fitting an inertia switch which will trip on an impact and stop the pump.

shawser · Post by **shawser** » Thu Nov 24, 2011 2:52 pm

so i need to wire it via the oil pressure switch and conect to ignition so that pump comes on not when ignition lights are on, but when im cranking the engine?

kiwicar · Post by **kiwicar** » Thu Nov 24, 2011 3:15 pm

Hi
when cranking you need it fed directly off the ignition supply without the oil pressure switch (you will never get enough pressure when cranking to close the oil pressure switch) and under normal running via the oil pressure switch. Hence Daves' diodes!

Best regards
Mike

ChrisJC · Post by **ChrisJC** » Thu Nov 24, 2011 8:57 pm

shawser wrote:so i need to wire it via the oil pressure switch and conect to ignition so that pump comes on not when ignition lights are on, but when im cranking the engine?

The SD1 had a 3 terminal oil pressure switch (as did the 101 I think) for that reason.

Chris.

ged · Post by **ged** » Thu Nov 24, 2011 9:12 pm

(you will never get enough pressure when cranking to close the oil pressure switch) ]

You're wrong there Mike,

My oil pressure light goes out after a few seconds of cranking. Usually start up is from fuel in the float bowl.

regards Ged

shawser · Post by **shawser** » Thu Nov 24, 2011 10:38 pm

my oil light goes out aswell when cranking.

DaveEFI · Post by **DaveEFI** » Thu Nov 24, 2011 10:39 pm

ChrisJC wrote:
shawser wrote:so i need to wire it via the oil pressure switch and conect to ignition so that pump comes on not when ignition lights are on, but when im cranking the engine?
The SD1 had a 3 terminal oil pressure switch (as did the 101 I think) for that reason.

Chris.

Only carb cars. And later carb cars used a 'normal' oil pressure switch with a relay for the pump - much better than the earlier three terminal type, which I'd avoid like the plague.

DaveEFI · Post by **DaveEFI** » Thu Nov 24, 2011 11:03 pm

Here's the circuit for the later SD1 with pump relay:-

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