Node:Autorenewal of old vehicles, Next:Rebuild ttdpatch.ovl on every run, Previous:Show Vehicle Speed, Up:The Patches
Using the switch "autorenew", all vehicles will be automatically renewed the given number of months before (or after) they reach the maximum age. Note that they will be replaced by an identical vehicle, it is not (yet?) possible to change the engine type. Vehicles with an obsolete engine will not be renewed.
Normally, with "autorenew", the vehicles will be renewed at the next service, or whenever they enter a depot. If you also enable "forceautorenew", they will be sent to a depot and renewed immediately, as soon as they are too old. Setting "autorenew" without "forceautorenew" will not work properly if you are also using the new service intervals, in which case a vehicle will never go to the service where it would be renewed.
The value given with either of these switches determines the number of months after the maximum age when they are renewed. This value can be negative, in which case vehicles will be renewed before they reach the maximum age. Since you get the first warning of a vehicle getting old a year before it reaches the maximum age, you have the following options for the value of this switch:
The default value is -6, which means that after the first warning, you have half a year to manually replace the vehicle, otherwise it will be renewed with the same vehicle type. If you do get the second warning, it means that something is going wrong: the vehicle could not be renewed and you should pay more attention to it.
The way this renewal works is that when it is time to renew a vehicle, and if you have enough money to actually do the renewal, it will start looking for a depot, as if it were going for a standard maintenance. However, in addition to the maintenance, the vehicle (actually, the entire consist) is also renewed automatically, and the cost is deducted from your account. Afterwards, the vehicle continues its regular schedule, it even keeps the cargo if it had any.