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You should not operate a vehicle's differential lock when?
Driving uphill
The wheels are spinning
Driving on a straight road
Driving in reverse
The correct answer is: The wheels are spinning
When operating a vehicle's differential lock, the wheels are fixed, meaning they have to move at the same speed. This is useful for driving off-road and in slippery conditions, but is not recommended for regular driving on flat or uphill roads. Using the differential lock while driving on a straight road or going uphill may cause damage to your vehicle's drivetrain, as the wheels need to be able to rotate at different speeds in those situations. Similarly, using the differential lock while driving in reverse can also damage the drivetrain, as the wheels will need to rotate in opposite directions at times. Therefore, the only appropriate time to use the differential lock is when the wheels are spinning, e.g. in mud or snow, to get unstuck.