How do i run an LED to show that my fuel pump is on?

 

You can install one to show that it is getting power. To show that it is running would require a circut to measure resistance through the motor and alert you when the circut becomes open.

If all you want is a light to show that the pump is running and making pressure, I would drill and tap the fuel rail and install an oil pressure switch in it.

 

you can switch a resistor in series with the pump.
if current flows through the pump there is a voltage drop over the resistor. If you use a low current LED with a matching resistor you can see if the pump "runs".... do not use a resistor with a high value, because the voltage drop is to high and your pump don't turns..... this is quick and dirty.......
better is a circuit with an OP-amp and a low resistor in series to the pump... but more tricky

 

Cool idea Thomas. But I think I'll fine tune that for ya a bit. The problem I see is your LED get's it's ground through the pump. If it craps out you most likely won't get the ground any more and the light won't come on. Better if it had it's own ground

Also, you probably won't need R1 at all. As long as R2 had a value approx 1.5 greater than the resistance of the motor, the voltage would not be forced through the resister unless the pump crapped out.

 

no Reiner, the LED's current is only 1....2 mA if it is a low current typ!
You need R1 to make the voltage drop!
the LED shows only this voltage drop.
if the pump goes open, you have no current through R1 -> no voltage drop -> no LED light!
thats all.....

never use the LED without R1!
It does not work with this (high current) pump

for example: R1 must be very low.... only a few Ohm's
R2 must be in the range of hundred Ohm maybe.....