The Machine Outfitter will give you a key item Vat of martello fuel to go and repair the Martellos. The martello's fuel level must be 89% or below of maximum capacity for you to be able to replenish it.
Choose to start replenishing.
Choose to replenish from the direction offered. The direction affects the reward you get.
Regular replenishment of the martellos located throughout the Konschtat highlands is vital to the resistance effort in this area. Refuel a martello and return the empty vat to the machine outfitter when your task is complete.