Facet Fuel Pump

[matt205xs 0]

i have just bought a facet fuel pump to run my bike carbs, would i be ok in running a switch from the dash to the pump then the live from the pump to the coil with its own separate fuse?

[Richie-Van-GTi 71 2 Cars]

it actually works OK if you take the a feed from negative on the coil to live on the fuel pump, just put an inline fuse in just in case the coil goes beserk. That way it shuts the fuel off when the engine isnt running.

[matt205xs 0]

so thered be no need for a switch?

[Guest ashnicholls]

thats a class idea, about putting it off the coil.

i was thinking about how to cut the pump off if the engine stops, without buying that expensive oil pressure cut of switch.

[MartinM 0]

I'm no auto electrician, but feeding anything from the coil -ve seems a very BAD IDEA .

 

It might work, but have a look at the picture of the voltage on the -ve side of the coil in a typical installation at http://www.picotech.com/auto/waveforms/primary_ignition.html, and especially note:

- that for some of the time (depending on dwell angle) and for proportionately more of the time at higher rpm, the voltage is zero

- every time there is a spark, the voltage peaks to 100-350 volts, then goes to ~40volts for 1mS, and then oscillates back to +12v over the next 1mS or so

 

Would I put this sort of supply voltage on a Facet pump, which I believe has some solid state electronics inside it? Er....no!

 

Also:

- when the voltage is +12v, you are feeding the load (ie fuel pump) through the coil - and the electrical and thermal design of the coil assumes that there is no current flowing through it when the -ve is at +12V

- the whole design of the ignition system assumes that the coil -ve is essentially just switched to ground and made floating very quickly (either by points or the ECU). Putting an additional electrical load at this point must invalidate a whole load of ignition system design assumptions

 

...and if putting the fuel pump on the coil -ve is a GOOD IDEA then you would have thought that it would be standard practice by the manufacturers/engine designers (which it is not!)

[Guest ashnicholls]

surely he didnt mean running it off the sparkin side of the coil, because thats stupid, because as you say it needs a constant supply,

 

i know nothing about electrics, and coils, so i didnt understand what he said clearly, i just thought he meant off a constant feed that is going to the coil, and which is not there when the engine is running.

[MartinM 0]

The coil +ve is always at 12v when the ignition is on, whether the engine is running or not - so putting the fuel pump there is the same as putting it at any other ignition switched +12v point. Might be an easy take-off point I suppose.

 

It's Richie-Van-GTi that was suggesting taking it from the coil -ve since "That way it shuts the fuel off when the engine isnt running"...which isn't true either, since when the engine is off, most ignition systems leave the -ve floating above ground (ie +12v, if measured) so the fuel pump would still be powered on.

 

 

 

 

 

...and just to flog this horse completely dead...

 

...the OEM 205 design is to use the tachymetric relay to power the fuel pump - this relay is only energised (ie the fuel pump powered) when the engine is turning....

[matt205xs 0]

ok then thats killed that,

 

now what about mounting the fuel pump near the pump, as i believe facets push better than they pull fuel??

 

then i could use the live from the OE pump?

[Richie-Van-GTi 71 2 Cars]

Martin after measuring mine I found the -ve never gets above 12v when the engine is running and its definatley dead when the engine isnt cranking. Obviously my equipment wont be sensitive enough to measure a peak for a millisecond and based on your links etc I have no reason to disbelkieve you, but you could easily enough use a zener diode inbetween to hold the voltage back to 12v to ensure the pump doesnt get damaged. That or wire in a relay to act the same as the tachy.