Driver-Controlled Differential Lock (DCDL)
Some vehicles are equipped with a driver-controlled differential lock (DCDL) that is operated by a dash switch in the cab (see Ignition Switch and Key for more information about the switch). The differential lock provides maximum traction for slippery conditions by forcing the wheels on a drive axle to rotate together. The differential lock should be engaged when the vehicle is stopped or moving at a low, constant speed and the wheels are not spinning, slipping, or losing traction. Do not exceed 25 mph (40 km/h) when the differential lock is engaged.
On tandem axles there are three possible DCDL options:
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Forward-rear carrier only
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Rear-rear carrier only
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Both rear carriers.
An indicator light comes on when the DCDL is engaged. An optional buzzer can also be used to indicate DCDL engagement.
The DCDL provides maximum traction under slippery conditions. When the DCDL is engaged, the clutch collar completely locks the differential case, gearing, and axle shafts together, maximizing traction of both wheels and protecting against spinout. Under normal traction conditions, do not engage the DCDL; operate the axle with differential action between both wheels.
Warning: Be especially careful when driving under slippery conditions with the differential locked. Though forward traction is improved, the vehicle can still slip sideways, causing possible loss of vehicle control, personal injury, and property damage.
Main Differential Lock Operation
Notice: The differential lock should be engaged when the vehicle is stopped or moving at a low, constant speed and the wheels are not spinning, slipping, or losing traction. Engagement at high speed or power can damage the axle(s).
Do not permit rear wheels to spin freely for more than ten seconds when traction is lost. Shift into LOCK to prevent damage to interaxle and main differentials.
To lock the main differential and obtain maximum traction under slippery conditions, move the control switch to the lock position.
Note: On some vehicles, the differential lock system is connected through the low speed range of the transmission. If this system is used, the transmission must be in the low speed range for the differential to fully lock.
If the vehicle is moving, maintain a constant vehicle speed while engaging the differential lock. Briefly let up on the accelerator to relieve torque on the gearing, allowing the differential to fully lock. The indicator light should come on and the buzzer should sound on vehicles so equipped. When the differential is fully locked, the turning radius will increase because the vehicle understeers. See Fig.. Drive cautiously and do not exceed 25 mph (40 km/h).
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Turning Radius When Differential is Locked (engaged)—Understeer Condition
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Turning Radius When Differential is Unlocked (disengaged)
Fig. 1, Turning Radius
To unlock the main differential, move the control switch to the unlock position. Briefly let up on the accelerator to relieve torque on the gearing, allowing the differential to fully unlock.
Note: If the differential lock system is connected through the low speed range of the transmission, shifting out of low speed range will also unlock the differential.
When the differential lock disengages, the indicator light will go off and the buzzer will stop.