Theory of Operation
The ISB Gear Down Protection feature promotes increased fuel economy by encouraging the vehicle driver to operate as much as effectively possible in top gear, where fuel consumption is lower. It does so by limiting vehicle speed one gear immediately below top gear. Gear down Protection also senses engine load to ensure that the limit imposed does not have an adverse effect on performance.
There are two vehicle speed limits imposed by the Gear Down Protection feature: Gear down Protection Light Load Vehicle Speed and Gear Down Protection Heavy Load Vehicle Speed.
Under light loads, when the vehicle is not operating in top gear, the vehicle will be able to travel no faster than the Light Load Vehicle Speed. This parameter is configured to limit engine speed to between 1500 and 1650 rpm in the gear immediately below top gear.
When lugging uphill with heavy loads, Gear Down Protection allows access to the full operating range of the engine one gear down from top gear. Therefore, under heavy load, the operator can accelerate up to Heavy Load Vehicle Speed in the gear immediately below top gear. This allows the vehicle to perform well under heavy loads, but still encourages the use of the most efficient gear. The Gear Down Protection Heavy Load Vehicle Speed parameter must be set less than or equal to the Road Speed Governor imposed Accelerator Maximum Vehicle Speed (maximum speed the vehicle can achieve on level ground).