I understand that the fuel pump relay provided power to the fuel pump when the ignition switch is on.
But there is a wire from the computer that turns off the pump under certain conditions.
Anyone know what those are?

On some motorcycles there are relays for the lights and fans. That is because these take a lot of power and you don't want that running through the ignition switch.
But the power to the fuel pump comes from a wire to the ignition switch, so that shouldn't be the reason here.

My relay quit and I jumped it till I can get a new one.

Jim

 

The fuel pump has to work harder as the pressure from the bowls increases, this drives the amperage up which will turn the relay off, you would better off bypassing the pump (by running fuel hose from tank to "T" connector at carbs) than to bypass the relay, you could blow the fuse, which also feeds power to the rest of the ignition system, which wouldn't be good for your health in traffic....

 

Relay is there to cut the fuel to prevent fire I beliieve. On my bike, the pump only operates when the bike is actually running (having the ignition on does not activate the pump).

So the theory is if you high / low side the pump stops when the bike stops, keeping the pump from spewing gas into an already dangerous situation

 

Fuel cut off relay. The name says for itself. It prevents the pump from over pumping too much gas and flooding the carbs. Although the pump only pumps when there is need for fuel . I might be wrong but as I know the pump doesn't pump when there is enough fuel. Even if relay is bypassed. It could be better if somebody explained how the pump operates in details.

 

OK, I'll give it a try. The fuel pumps in these bikes are low pressure, low volume plunger style pumps. They get their power from a Fuel Cut Relay and it's controlled by the ICM. With the ignition key on, power is supplied to the ignition coils, as well as the Fuel Cut Relay. The Fuel Cut Relay is a solid state device, meaning it doesn't have any moving parts. The ICM is what controls a ground path for the Ignition Coils (that drive the spark plugs) and the Fuel Cut Relay (which provides power to the fuel pump). When the ICM provides a ground to the circuit, a magnetic field builds up in the Ignition Coils, and the Fuel Cut Relay provides power to the Fuel Pump. When the ICM takes the ground away, the magnetic field in the Ignition Coils collapses and generates a spark. The Fuel Cut Relay also turns off, removing power to the Fuel Pump. So the Fuel Cut Relay is turning on and off as fast as the spark plugs fire.




So, while the pump has power, the coil inside the pump energizes and pulls up on a diaphram. It also lifts up on a plunger which pushes up on a set of contacts at one end of the fuel pump (behind the black cap and held on with one screw). The contacts open and this momentarily turns off the pump, alowing the spring on the plunger to push the plunger back down, closing the contacts again. The up and down motion is what pumps the fuel to the carburetors. The part that is failing on these pumps is the contacts wear out and the pump works intermittently.

I hope this clears it up a little.

 

Thanks, finally a good explanation. I had numerous problems with the pump. I guess R/R and fuel pump are the weakest parts on our bikes.
So does it mean that if the pump fails we should buy a new one...I mean brand new, not used one off Ebay. Its like 130 bucks. Oh....

 

No, don't buy a new one just yet. If you can hold off until the beginning of next week, I may have a solution that will help many F3 owners. It doesn't involve bypassing the fuel pump, and you don't have to replace the pump either.

 

Hey desperado,

Just bypass it for now. I dont have a pump/relay atm just like quite a few others on the forums here and it seems to be just fine. Sounds like Idodirt has something interesting in the works, cant wait to hear since this seems to be a common problem for us.

Blake

 

Hey,
I ve had it bypassed for a good month while changing my float valves and rebuilding the carbs. Some WERA racer told me that damaged float valves can stick causing the pump to stop pumping. So I ordered brand new set of float valves. Yes, the bike runs OK without the pump but I wanted to give another try. I finished my carbs work and put the pump back in. So far it runs jsut awesome. If the pump stops pumping again I guess i ll just bypass it for good.

 

Well here it is, I've found some replacement Switch Contacts for the fuel pump that allows you to repair your fuel pump for less than $40. It works like a charm. Here is a link on a How To that I did in this forum.

Fuel Pump Repair