from someone who has built a few wiring looms in his time...
If you are measuring the amperage draw on the pump, you need to set the multimeter to the AMPS setting, on the highest amperage range selectable, then place it IN LINE of the power wire (or earth wire)
Its important that all the current MUST flow through the multimeter, so not just resting the probes against two terminals, but physically disconnecting the wire and placing the multimeter between it.
Bear in mind that measuring in this way often requires some consideration of what you expect to see. No point having a multimeter thats only rated to a maximum of 20A and trying the same thing with the alternator output or starter motor, Clearly that sort of load will cause it to melt/break/explode ect.
If you have an Inductive Amperage meter, then its heaps easier, just clamp around the wire and measure, they are kinda expensive but they'll measure up to hundreds of amps....
The expected reading will be dependant on several factors, How big the pump is (how much it flows), how high the rail pressure is and often the pump is used.
Suffice to say, you should expect around 6-8 amps.
It will peak higher when it initally turns on, this is normal.
Even then, it'll only peak an amp or two over its nominal range.
Most Automotive pumps will be fine on 10-15A fuses.
Pretty much ALL the aftermarket pumps i have set up, i have used a 15A fuse.
While your doing that, it pays to ensure you have a good earth too, either measure for continuity to the chassis from the
pumps negative terminal (should be as close to 0 ohms as possible, 1-2 ohms is acceptable) Or physically remove the ground point and clean it.
