you would more than likely need a recording volt-ohm-amp meter so you would be able to see the spike as the circuit turn on, there is a much higher but very quick spike in an electromagnetically energized device as the magnetic field builds up and as it collapses, but I don't think you would not be able to read it unless you had a graphing function that you could read almost instantly.
also, if something was keeping the lock-off solenoid's armature from moving as quickly as it is supposed to, that higher current draw spike could last until the armature had moved it's complete stroke. someone putting a washer under the magnet coil may cause this sort of draw.

Hi, a third party a few years ago (and before my time) came in and converted some of the LPG machines (various older Hysters using the ignition relay circuit) to NG. The wire to the + of the solenoid comes off the coil and is fused after the coil. (When the machine died it had burned up the wire as there was either no fuse or too high a rating, not totally sure). So I put an in-line fuse holder to prevent melting wires. Additionally I had to run a new wire from the coil to the solenoid so there is nothing else coming off that run.

Yes, I did in fact do a "direct to battery test" and that is when I discovered that the solenoid was ok (or so I thought). Then I took it's wire and connected to the 5 amp fuse and it popped it.

The machine is running now, but I'm still perplexed at why I have to put such a high rated fuse in when ohms law tells me that a 2 amp is about right and a 5 amp would give lots of wiggle room (in case of a momentary high current draw upon being energized).

I'll dig up a more capable amp meter and do some tests. I'm curious now!

well depending on what system is on this truck there could be other components in that circuit as edward hinted on.
If we had a serial number so we could get the correct series to make it easier to find the correct wiring diagram that would help.

in general though if this truck has an ECM on it the lpg/ng solenoid shutoff is controlled by it but there are some other common circuits that are tied into that positive side like the starter relay and vsw but those circuits all loop back to the ECM. So if this unit has the ECM control something in the harness or one of those components may be causing the higher resistance blowing that fuse.

If it has the older circuits then it would probably pick up the lockoff solenoid circuit off the ignition relay circuit.

in either case it sounds as if you have something shorted causing the circuit to go high resistance and blowing the fuse.

check the connectors across the main harness at the transmission housing area or just under the dash at the floor plate, where ever the main harness has those large connectors between the engine compartment and the front of the truck, they are known for corrosion problems and i've seen a few harnesses get smashed/pinched because of tech's not paying attention when they are working on them and putting them back together haphazardly.

You could get tricky to prove to yourself that the valve coil is ok by disconnecting the hot wire to it and running a jumper wire from the battery to it with a in-line 5 amp fuse to see if it turns on without blowing that fuse. Also, if the valve was sticking, it wouldn't cause more of a amp draw.

there most likely is some thing else on this circuit that is causing the fuse to blow, how sure are you that the NGV is the only device with a draw on this circuit?