748s fuel pump not priming (sometimes)

Crank possistion sensor.

 

Dave, your problem could be one of a number of things and may require some careful circuit tracing to resolve, however often these intermittent faults are eventually obvious and you just need to eliminate as many possibilities as you can.

The fuel pump is energized in the following way:

Current for the fuel pump switching comes from the battery through the main 30A fuse in the fuse box to the ignition switch

Ignition key in the run position allows +12v to go to the yellow 40A ignition relay switch coil (not to be confused with the starter solenoid), this closes the relay contacts allowing a +12v supply to the fuse box (fuses 2-8 or B-H on later 748s).

From the fuse box (3A fuse H on the later 748s) +12v goes to the safety relay switch and supply side contacts. When the sidestand is in the up position the safety relay switch coil is earthed closing the safety relay contacts to supply +12v to the RH handlebar switch cluster (engine kill switch and starter).

With the kill switch in the run position +12v goes through the kill switch to the switch side of the fuel pump relay located under the seat. +12v gets to the supply side of the fuel pump relay direct from the battery bypassing the main 40A fuse on the RH of the battery. It goes through the 20A fuse by the relays under the seat.

From what you describe the most likely fault could be a number of things but it cannot be the alarm (you removed it) or the crank position sensor and it should not be the ecu. For what its worth I think your problem is on the switch circuit for the pump, it is most likely between the kill switch and the fuel pump relay switching supply.

Assuming you replaced the yellow ignition relay on the LH side of the battery and the fuel pump/ecu relay under the seat (the one with 20A fuse) and also the other under seat relay (5A fuse) for good measure (you can swap them in a quick fault diagnosis), here is a list of easy things to check.

1. Fuses A and H in the fuse box are seated correctly
2. The 20A fuse under the seat is seated correctly.
3. The connector for the fuel pump relay under the seat is clean with no corrosion
4. The 8 way connector under the front RH side of the seat (the Orange/blue lead is the fuel pump switching supply) is clean (separate thetwo halves and look for dirty/wet or corroded pins)
5. The kill switch is clean (use contact cleaner or wd40if you have to)
6. The connection plug for the RH switch is clean and sound
7. The main multi connector for the front of the bike (the round one below the front RH side of the tank) has clean contacts and no water in it. Separate the two halves and look carefully at the pins and sockets for contacts U (brown wire to the kill switch) and V (orange blue wire supply to the fuel pump relay switch contact)
8. The ignition switch contacts are cleaned out with a jet of contact cleaner (or WD40 if you must) down the key slot and some rapid switching on and off with the key
9. Check the operation of the side stand switch and the side stand switch loom connections are clean and sound, make sure the side stand switch is fully home in the on position and is clean.
10. Check or replace the safety relay which is the small black one LH side of the battery (the other two are the yellow ignition relay and a big one which is the flasher unit).
11. Lift the tank and check the tank 4 way plug
12. Check that the 3 way ecu power supply connector under the seat close to the 8 way one mentioned above is clean and seated


One final suggestion is that when alarms are fitted often the orange/blue lead is cut between the 8 way plug under the seat front RH side and the round front main connector under the tank, it is then fed into the alarm/immobilizer as one of the immobilized circuits (this is typical for a Meta alarm installation). Have a look to see if the black tape around the part of the loom below the seat and tank has been disturbed close to the 8 way connector. If it has been carefully investigate by opening up the black loom tape, you may find a cut blue/orange wire has been twisted together or connected with a crappy Halfords style bullet connector. It needs to be soldered and properly insulated with heat shrink or carefully applied self amalgamating insulating tape.

Good luck on this, I hope you get it sorted soon

 

Its not a bad thing to go round all the loom and check /clean connections .Rubbish weather and all that.

 

Did you get it sorted?

 

Ah..................................:wink: