The down motion on most dump trailers is just a bleed to let the hydraulic oil go back to the tank and the pump doesn't run. . . . . .
I'd set the safety up. . . and use an ohm meter to test the wiring from the plug back to the pump assembly. . . . .
Usually they are a 3 wire system. . . . one wire is a power lead and the other two are going back to the pump. . . .
One wire trips a solenoid to power the pump in the up position and the other wire trips the drain back to the tank. . .
Usually there is enough oil on the shaft of the cylinder so you don't have to worry about the weather. . . .
These are straight forward systems but don't work on the system if its under the dump bed unless you have the safety up. . . .
Retired Roaming America, 05 3500 4x4 Dually Molten Red stick, 125K miles, PacBrake Exhaust Brakes, 05 Jeep Grand Cherokee Limited, 05 Honda Rincon 650 ATV, 2001 Honda Goldwing