Nibbles wrote:
Flow/pressure/resistance is just like an electrical circuit - The pump provides pressure and the 'load' draws flow. If the pump can't cope with the flow then the pressure will drop.
Current draw relative to pressure and flow depends on the pump type. An impeller pump will provide a relatively steady pressure and the current draw will go up as you draw more flow. A vane / gear etc. type pump tends to provide a steady flow and current draw goes up as you increase pressure. Either way - when you go on boost the flow goes up and pressure goes up (maintained at a steady figure above manifold pressure) so you will draw more current on boost.
Carried out a few more tests tonight..
I fitted a new earth cable direct from the pump to the battery, it was going through the chassis but i haven't noticed any difference by doing this.
with the diagnostic FP and B+ bridged and the fuel pump disconnected in the boot im seeing the same voltage as the battery.
When i reconnect the fuel pump this voltage does drop lower than the battery voltage, surely this is always going happen though?
The fuel pump draws 7~Amps at idle/normal driving and goes up to around 8.5~Amps on boost, this does seem a little lower than what Walbro shows, this is in keeping with what you mention above.
i have noticed that walbro list two 255LHR Fuel pumps, normal and High pressure, of course i can not for the life of me remember what one i have?
It turns out the two DVM's that i have been using read around .1v different to each other, the lower reading being the one i have been using to read Fuel pump voltage, all of todays voltages are taken with two good DVM's.
I have come to the conclusion that the fuel pump and battery have a difference of .3V (using two accurate DVM's, which i think is OK?
The Fuel pump does not drop below 13Volts now and i would say drops to around 13.20V when on boost?
Resistance of the fuel pump is 0.6 Ohms, this sounds about right to me.
I can only assume that the ECU is taking its power from under the bonnet somewhere, closer to the alternator, which is why im seeing 13.6 Volts once up to temperature.
Thank you once again for all your input