Why can't I shift my transmission selector from Drive (D) to Neutral (N) when I am coasting? I understand some cars have a coasting function to save on fuel. While it is possible to select neutral on an automatic gearbox during coasting, the transmission is not designed to accommodate coasting or free-wheeling in neutral.
Oil pressure is critical for an automatic transmission's hydraulic system to lock a set of clutches to enable the correct gear to be selected.
What provides the oil pressure is a built-in pump that relies solely on engine power. When disengaged from the engine, the pump can no longer develop sufficient pressure to work properly.
This is not a problem when the car is stationary. But on the move, this will result in accelerated clutch wear at the instant the selector is shifted from N back to D, as it takes a few seconds for hydraulic pressure to build up before a firm clutch-lock can be achieved.
For the benefit of fuel economy, a number of automatic transmissions these days are capable of coasting in neutral without any driver intervention. This is possible because they are designed with a dedicated pressure accumulator that maintains a constant hydraulic pressure in the transmissions control system for immediate actuation of gear selection at the end of any coast-event.
An electronic control unit orchestrates the selection of N or D based on a number of driving parameters.
An interesting point to note is that the dual-clutch transmission - like the direct-shift gearbox (DSG) found in most Volkswagens - are, strictly speaking, not automatic in the traditional sense. Their internals are similar to a manual gearbox and hence can be manually shifted to neutral when coasting, without causing any damage.
The practice does save fuel such as when approaching a traffic junction where you know you will have to stop. However, this is a driving style that needs practice as you need to be fully aware that you are in neutral and remember to select Drive once your coasting is done.
The latest generation of DSGs also incorporates an automatic coast-in-neutral mode where the driver is not required to shift the gear selector.