Other Articles - May - 2019

Just Lock Me Up, A Look at E LOC: A Different Type of Lockup System

Steve Garrett just lock me up featured image

I’m often asked, “When are the constant changes in vehicle technology going to slow down?” The answer is quite simple: They won’t! 10 years from now, you’ll look back at today’s technology and think, “Wow! That stuff was simple when compared to what we’re working on today.”

Technology has permeated every part of today’s vehicles. It’s easy to wonder, “What was wrong with the system/components we used for decades? They seemed to work just fine.” That question has several answers, such as:

  • It was changed because Federal fuel economy standards or Federal safety standards are changing.
  • It was changed because the competition did it differently, and manufacturers wanted to be seen as a leader.
  • It was changed because the new way/component is less expensive.
  • It was changed to improve efficiency at the manufacturing plant.
  • It was changed because the car manufacturer’s management believed that the consumer would prefer the vehicle assume many of the tasks that the driver used to have to perform. Many feel that the more they can take the operator out of the equation, the better it is for the operator and society in general. There’s a feeling that the consumer loves gadgets and the more of them available, the more the customer will want to buy.
  • Some of you are old enough to remember when systems like power windows, A/C, and even automatic transmissions were rare. Not today; they’re standard equipment on nearly every vehicle sold. I think it’s safe to say today’s vehicles have so many bells and whistles that the owner isn’t even aware they’re there, which means they certainly don’t know how to operate them.

You might think that some systems and components would be safe from the technology juggernaut; you’d be wrong. One such system is the good old differential. But, like other vehicle systems, the differential is now high tech.

Ford, Toyota, Nissan, Ram, Land Rover, and GM all use electronically controlled differentials that mechanically lock the side gears to prevent wheel-to-wheel speed differences. Some applications use the ABS system to apply the brake on the wheel that’s slipping while others use an electronically controlled differential case to do the job. Many of the manufacturers incorporate their system with other powertrain functions, such as controlling transmission shifts, reducing engine power, and managing throttle opening.

Eaton and American Axle (AAM) offer electronically controlled systems that are used by various OEMs. Eaton’s E Locker and AAM’s TracRite EL electronically lock the side gears to the differential case (carrier) to provide 0 speed difference between the drive wheels. On some applications, E LOC is automatic, but with most applications, the E LOC system is selectable. The customer controls the system via a switch.

E LOC isn’t designed to be used on dry pavement as it could result in vehicle handling problems, tire wear, and powertrain damage. E LOC is typically a shift-on-the-fly system. The speed allowing E LOC selection depends on several variables, such as vehicle speed, wheel speed, wheel slip rate, steering wheel position, and load.

Note: Some applications inhibit operation in 2WD High Range (Ford), while others (GM) will allow E LOC operation in 2WD mode. Some applications use E LOC components in the rear and front differentials.

Note: With E LOC engaged, ABS (antilock braking) and ESC (electronic stability control) operation is typically degraded or disabled.

When engaged, an indicator lamp will light, showing the differential is locked. If the lamp fails to light, one of these conditions is likely present:

  1. The vehicle speed is too high.
  2. The vehicle is experiencing antilock brake activation.
  3. The left and right wheel speed difference is too high during an engagement attempt.
  4. The system has a problem and is accompanied by a CHECK LOCKING DIFFERENTIAL message on the driver information display.


The E LOC system is controlled by an electronic clutch located within the differentials. The Eaton GM system (figure 1) consists of:

  • An electromagnet actuator or electronic clutch.
  • A drag plate with three sets of double ramps built into the plate.
  • Actuator pins to transfer the drag plate movement to the locking collar and side gears.
  • A controller (typically the chassis control module, CCM)
  • A control switch(s) that acts as an input to the control module.
  • Control relays that control power to the electronic clutch engaging or disengage the E LOC system. The relays are controlled by a controller (typically the CCM).


To activate E LOC, the customer engages the switch. The switch design varies by manufacturer. Some applications use a lever, some you rotate a knob, while others have a simple pushbutton that activates/deactivates the system.

The switch typically sends the signal to the BCM. The BCM sends a requests for E LOC to engage via the CAN Bus to the CCM. The BCM also signals the EBCM (ABS) to change or disable ABS and stability control. In addition, the BCM signals the IPC/ Driver information center to light the differential lock indicator.

The controller (CCM) energizes a relay, which then provides power to the E LOC clutch. Energizing the clutch puts magnetic torque on the drag plate. The actuator pins ride in a series of ramps within the drag plate. As the drag plate moves, the pins move up the ramps, forcing the pins to move horizontally toward the side gear.

The pin movement then transfers their motion into the locking collar. The locking collar has a series of dog ears, which engage on the side gear. With the dog ears engaged, the side gears are locked to the differential case (carrier), forcing both wheels to turn at the same speed.

The CCM monitors the wheel speed sensors (via the EBCM) and the steering wheel position sensor (via the power steering control module) to determine whether the E LOC is working and that it’s interfacing with the TCM and ECM regarding gear commands, speed, and load values. DTCs, such as B272D, C11A5, C159A, C11B0, C159B, C1304, C1305, C1306, C1307, C1308, or C1309, may set if issues with system operation occur.


To disengage the system, the customer simply activates the switch. The CCM de-energizes the E LOC clutch. With the clutch deactivated, the return spring moves the shift collar, actuator pins, and drag plate into their released position. This disconnects the side gear dog clutches, which places the differential into open mode.

As you can see, changes in technology are a challenge to all of us. But technology is only a problem if you don’t understand it. Education is key to understanding, so thanks for taking to time to learn about these new systems.

Until next time remember, “We are told never to cross a bridge till we come to it, but the world is owned by men who have crossed bridges in their imagination far ahead of the crowd.”