Thanks for the info rog747.
I operated B747-238 with P&W JT9-7AW engines. These aircraft used 2400Kg water, 600Kg per engine for 2.5 minutes on takeoff. We FEOs checked the gauges carefully coz we needed the lot.
There was only ONE water pump switch on the FE panel fwd knee panel for all pumps along with the lights as described. Of that I am absolutely certain, definitely no individual pump switches.
As I recall it the water tanks were plumbed such that one tank fed inboards and the other fed outboards. It was also necessary to ensure that Galley Power was switched off after start prior to using water coz the AC system could not support the load if the GP did not automatically switch off when Water On selected.
Calculating performance data for a wet takeoff was an absolute nightmare. Three of us would fish the numbers out of our performance manual independently and regularly came up with three different answers usually 3-4 tons MTOW difference. It mattered when we were operating PER-BOM return and BKK-ATH return coz every Kg of fuel was needed and we had lots of cargo offered.
Ah the Good Old Days!!