Tandem Drive Axles with Interaxle Differential Lock
An interaxle differential lock is standard on vehicles equipped with tandem drive axles. It is driver-actuated by means of a LOCK/UNLOCK control valve switch located on the dash in the cab (see Ignition Switch and Key for more information about the switch). Engagement can be at any speed, provided the wheels are not slipping or spinning. A red indicator illuminates when the interaxle differential lock is engaged.
In the UNLOCK position, there is differential action between the two axles. The differential compensates for different wheel speeds and variations in tire size. Keep the interaxle differential unlocked for normal driving on roads where traction is good.
In the LOCK position, the interaxle differential is locked out and the driveshaft becomes a solid connection between the two axles. Power entering the forward axle is also transmitted straight through to the rear axle, so both axles turn together at the same speed. The LOCK position should be used when the vehicle encounters poor traction conditions; however, it also increases drivetrain and tire wear and should be used only when improved traction is required.
Interaxle Differential Lockout Operation
Notice: Do not actuate the interaxle differential control valve while the tires are slipping. Do not operate the vehicle continuously with the interaxle differential locked during extended good road conditions. Using the IAD lock when wheels have good traction can harm the axle, wear the tires excessively, and increases fuel consumption. Release the IAD lock as soon as road conditions permit it.
To lock the interaxle differential and achieve maximum pulling power when approaching slippery or poor road conditions, move the lockout control valve to LOCK while maintaining vehicle speed, before encountering the poor road conditions. Let up momentarily on the accelerator to engage the differential lock. Proceed over poor road conditions with caution. Do not wait until traction is lost and the tires are spinning before locking the interaxle differential.
To unlock the interaxle differential, move the lockout control valve to UNLOCK while maintaining vehicle speed, after leaving the poor road conditions. Let up momentarily on the accelerator to allow the shift, then resume driving at normal speed.