Does your problem disappear when you run the engine?
I just went through something similar and discovered it was one of my battery connections. My hydraulic pump would't operate properly and the jack panel would turn on but would go off when I tried to raise the jacks. If I ran the engine everything worked. So I figured I had a battery problem.
I have interstate house batteries which drive the hydraulic pump. The batteries have typical + / - post but have a stud beside it. The stud and post are manufactured as one piece. The battery cable connections are to the stud, not the post. While trying to figure out my problem, I was checking the battery voltage using the posts, not the studs. The voltage always appeared correct. I decided I would charge the batteries, so, I disconnected one of the batteries and lo and behold, the stud was loose. It actually came out in my hand. It was acting like a bad ground and the battery bank was not providing enough voltage. It cost me $5.00 for a part to fix it. Once fixed everything worked and still works.
So, check your batteries, their water level, voltage, and connections.
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