Absorption refrigeration was invented by a man named Albert Einstein.
Charging a system requires a precise mix of water, ammonia and hydrogen. If you haven't had formal training in this and years of experience, I wouldn't even try.
I take it it's a Servel unit? If so make sure the solution pump, fluid pump, burner and all controls are doing what they should before you jump the gun. That's unless you know there is a leak for sure.