Originally Posted by
RyanP
If one pump is on they both run regardless of switch position.. You can deselect it all you want but it doesn't do anything if the other boost pump is running.
That is true, if one engine has a low fuel pressure both pumps will turn on. However as 80knots said, those switches arm the pumps. If you are running single engine, your left engine off will have no fuel pressure, thus causing the pumps to run. When you deselect the left pump, it disarms the low pressure sensor and kills the pump. Thus you get the low pressure and L fuel pump messages. The right EDP provides the pressure to turn the right pump off.