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You should have one terminal that’s hot all the time. It should be 12v or more if the vehicle is running.
When commanded the other large terminal should also read the same. It may be slightly less since the glow plugs are using quite a bit of juice. But if you key on and your glow plugs are cycling you should have a similar voltage reading on both large terminals.
Replace it with a quality gpr. I wasted more time with cheap ones. Motorcraft, Stancor
One way that the solenoid fails, it will not make a good contact inside it. Due to burnt or worn contacts. Results are lower than adequate voltage across the solenoid.