Fuel pump switch

[02GF74]

I have wired a halo LED switch to the fuel pump. This serves 2 purposes:
1. A simple anti theft device
2. Drain carbs of fuel before lay up when engine is running and switch is off.

Now here's the unexpected thing.

The LED is connected between power and ground to the fuel pump, after the switch, so should light up when switch is on.

When ignition is on, (carbs drained from last year) pump ran for a couple of seconds, then stopped and the LED went off. Turn ignition off, then on and the sequence repeats.

So now LED is off.(pump not running)

Push starter, engines fires up and LED is on, obviously the pump is running to top up the carbs.

OK, so what I think is happening is the ignition unit turns off fuel pump when engine is not running - I believe this correct operation and is a safety feature.

The LED being turned off when pump stops due to carbs being full is because the ground is switched by the pump so there is no electrical path to complete the earth circuit.

(Thinking about it now, I'm wondering if I should have wired the LED ground prior to the switch)

A piss poor photo in the dark, the switch fits inside one of the holes in the alloy knee protectors.

 

[02GF74]

I feel a bit of a fool, wiring diagram shows the earth is before the switch, so that explains why the LED goes off.
I suppose when the LED is on, that indicates the pump is actually ruining, as opposed to just being powered. I'll probably leave it instead of redoing the connector wiring.

Incidentally the fuel pump connector is standard 2.8 mm motorcycle connector so no need hack the loom.

 

[one2dmax]

The pump is dependent on a couple of conditions.
Of course the main condition is that the key is on AND in the run position. When this happens the relay activates for 5 seconds while it's looking for a tach signal. If no signal then the pump shuts back off. During cranking this drives a tach signal so the pump will again turn back on and continue running until there is no tach signal again (either by a no start condition and you've let go of the start button, or you've killed the ignition with the stop/run switch and or key switch).

There is a dependency on the fuel level as well. If the switch is on the ON position (to the right) and there is fuel in the tank (above the thermistor on the low fuel switch) then the pump will run. Again provided there is a tach signal. In the ON position if there is low fuel in the tank, then the thermistor will heat up and disconnect the pump's circuit (also illuminating the light slowly at first) causing the pump to shut down. Once you flip to the RES position (on the left) then you will be in a bypass mode where the pump will be always on (again provided there is a tach signal). ***NOTE***, the low fuel lights resistance is a component in the functionality where a low resistance bulb like an LED can cause the pump to have a no signal condition and not work unless in the RES position where it ignores the sending unit. ***ALSO NOTE*** If you swap around gauge clusters between 85-89 and 90-07 year ranges the fuel circuits will not work as they changed the grounding schemes when they went to digital in 1990.

To add even more complexity there are more redundancies that can affect operation including the 7 wire diode assembly (lays on top of the CDI box).

The oem pump also has a built in regulator so if it's full of fuel it will not pump even though it has the full power to it.

 

[Fire-medic]

Sean, thanks for the tutorial about the fuel pump and its actuation/non-actuation.