got a newer model gradall XL 4100 starts at the top compartment but on the lower one doesn't start unless there is air pressure in the system already. If no air pressure in system won't start at bottom compartment. These newer models also only have one engine this is the Mercedes model engine also.
Go to www.gradall.com . Under product support, you can print off wiring and air schematics for your machine. You will need your machine serial number to get the proper schematics. There is a lot of electronic circuitry, relay systems, and electric/pnuematic switches in the gradalls. Certain switches detect for the computer the proper parameters for start up between the travel and remote modes. You have something in the system somewhere that is either not completely engaging or has failed, or has bad connections