Programming – Soft Vehicle Speed Limit

Both Vehicle Speed Limiting (VSL) and Soft Vehicle Speed Limiting (Soft VSL) set the maximum speed of the truck. Vehicle Speed Limiting cuts off the fuel to the injectors when the truck exceeds the VSL programmed mile per hour value. When you choose Soft VSL, fuel delivery is modulated when VSL is reached. When the truck reaches the programmed VSL value, the fuel is not abruptly cut off. Instead, it is gradually reduced. Soft VSL may allow the truck to exceed VSL by a maximum of 2.5 MPH (4 km/h), when the truck is using less than 50% of the available engine horse power.

All electronically controlled on-highway engines covered in this handbook


Using Soft VSL feature not only allows you to “gear fast – run slow” to optimize a balance between fuel economy and performance or simply specify for the best fuel economy, but also allows the driver to “run at hills” by “warming” the turbo at the bottom of the hill.
This means that there is enough fuel to keep moderate boost levels even though fuel delivery is tapered off by Soft VSL.

Not setting VSL allows the truck operator to “run fast” and as a result get poor fuel economy. Also, some drivers may not like driving with Soft VSL enabled. Some common complaints are that there seems to be lower power and the truck may not feel like it can hold a constant speed. However, these effects are due to the modulated fuel delivery, and once a driver gets used to them, will still result in the advantages listed previously. This perceived power loss can be even more noticeable on trucks that are spec’ed near minimum gradeability requirements.

Do not use Soft VSL unless you expect the truck to operate at or above VSL in uneven or flat terrain.




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