7.4 Driver-Controlled Main Differential Locks (DCDL): Operation, Operating Rules, Sensor Switch, and Service

7.4 Driver-Controlled Main Differential Locks (DCDL): Operation, Operating Rules, Sensor Switch, and Service

The ASE T3 task list includes "inspect and replace components of locking differential case assembly." On heavy trucks, the most common version is the driver-controlled main differential lock (DCDL), sometimes called a cross-lock. The power divider (section 7.2) locks the two axles of a tandem together; the DCDL locks the left and right wheels of one axle together. This section uses Meritor's DCDL service information.

1. How a DCDL Works

Meritor describes carriers with DCDL as having the same gears and bearings as standard carriers, plus an air-actuated shift assembly mounted on the carrier:

• Unlocked (normal): the shift collar is disengaged and the carrier has normal differential action between the wheels at all times.
• Locked: air moves a piston and shift fork. The shift collar moves along the splines of the axle shaft toward the differential case. When the collar's splines engage splines on the differential case, the axle shafts and differential assembly are locked together. There is no differential action — both wheels turn at the same speed.
• A sensor switch in the carrier signals the cab indicator light when the collar is locked.
• A manual engaging capscrew in the shift cylinder cover can mechanically engage the lock for service or towing situations; it must be removed for normal operation.

Cab switch --> air valve --> DCDL cylinder/piston --> shift fork --> shift collar
                                                                  |
                                   collar splines engage differential case splines
                                                                  |
                                             axle shafts + differential locked together
                                                                  |
                                            sensor switch --> cab indicator light ON

2. Driver Operating Rules (Meritor Caution Label)

Meritor requires a driver caution label next to the DCDL switch and indicator light. It states:

• Engage DCDL only under poor road conditions.
• Do not engage DCDL during downhill operation.
• Do not engage DCDL or operate the vehicle at speeds above 25 mph.
• When DCDL is engaged, an understeer condition can occur in turns, so drive carefully; normal steering returns when it is disengaged.

Engage and disengage with the driveline unloaded. In Meritor's operational check, the driver lets up on the accelerator to remove driveline torque and permit the shift. Engaging while one wheel is spinning makes the collar teeth crash into turning splines, which rounds the teeth and can break the fork.

3. Sensor Switch Installation and Adjustment (Meritor)

1. Shift the differential to the locked position (with air, or with the manual engaging capscrew).
2. Connect a test light to the sensor switch and thread the switch into the carrier until the test light illuminates.
3. Tighten the switch one additional turn.
4. Tighten the jam nut to the specified torque.

A switch that is not screwed in far enough will not turn the light on when locked; one screwed in too far can hold the light on or be damaged.

4. Operational Check After Service (Meritor)

1. Start the engine and bring system air to normal.
2. With the cab switch unlocked, drive at 5–10 mph. The indicator light must be off.
3. Move the switch to locked and let up on the accelerator to remove driveline torque so the shift can occur. The light must come on.
4. If the light stays on with the switch unlocked, the differential is still locked. Verify that the manual engaging capscrew was removed from the cylinder cover.

5. Service Precautions and Component Inspection

• Removing axle shafts: Meritor says that on axles with DCDL, the DCDL must be shifted into and held in the locked position before removing the axle shafts. The locked position gives enough clearance between the shift collar and the axle housing to remove or install the shafts and the carrier.
• Raised wheels: during DCDL disassembly or carrier removal, if the DCDL is locked and one wheel is off the floor, do not start the engine and engage the transmission — the vehicle can move.
• Metric fasteners: Meritor's DCDL equipment uses metric sizes, so use the correct metric tools.
• Inspect: the shift collar and differential case splines for rounding or chipping (from engaging under load or while spinning), the shift fork and roll pins, the piston and O-rings, the return spring, the air cylinder and lines for leaks, and the sensor switch and wiring.
• Air leaks at the cylinder make the lock slow to engage or drop out. A weak or broken return spring can keep the collar from disengaging, leaving the truck locked with a lit indicator and hard steering.

6. Troubleshooting a DCDL

Because a locked DCDL causes understeer, a truck that "won't turn" after driving on a poor road may simply still be locked. Check the indicator and cab switch position before chasing steering components.