the truck should work without combi instrument, but only on low speed.
I think the problem is the 80v to 24 v converter, the one with the green light on it. check if it has output of 24v if not it will never work and the combi instrument will not work either.24 v supply is for curve sensors, lift sensors, combi instrument and for lift and drive electronis and for cooling fans. try to disconnect the coolingfans they might give problems on older trucks if they are very dirty.
converter only gives 3 amps so if the fans make a problem you loose 24v supply to the units.
Tinus