With advanced computer control and electronics found on modern automobiles, it is no surprise that the once simplistic differential assembly has been modernized through electronic and hydraulic control. Most manufacturers are out to build a better 4WD or AWD system, which incorporates a variety of technology, such as hydraulics, electronics, or impressive mechanical physics. You will typically find these advanced drivetrains on higher end SUVs and sports cars, but expect them to eventually find their way to your service bay. This article covers the new electronically controlled limited/locking axle found in the Honda Pilot and Acura MDX termed as the Variable Torque Management (VTM-4) system. The VTM-4 system contains a control module, which controls current to two electro-magnetic coils that act on a mechanical wet-clutch. The system can operate from 0 percent torque delivery up to 53 percent of torque delivery, based on conditions and preemptive strategies. One of the great benefits of the VTM-4 is the ability to initiate a variety of operational strategies, including delivering torque to the rear wheels before wheel slip even occurs. The following defines the operating modes of the VTM4.

Operation:

Acceleration: Based on a variety of inputs, such as engine load and speed, the VTM-4's control module pulse-width-modulates (PWM) the coils in the axle assembly to control amount of torque transfer to the rear wheels. This is a proactive measure to prevent the wheels from loosing traction on acceleration.

Cruise or Coast: When in cruise or coast mode, the VTM-4 reduces the torque transfer to the rear wheels to zero in order to improve fuel efficiency and driveability.

Limited Slip: In the event of a loss of traction, the VTM-4 control module has the ability to control each coil in the rear axle in order to deliver torque to the wheel with the most traction.

Locking: When the vehicle is placed in Manual 1st or 2nd and the VTM-4 button is pressed, the differential initiates full lock (5 amps to the coils) until the vehicle reaches 6mph. At that point, the control module decreases current flow to the coils until the vehicle reaches 18mph, where the current flow is decreased to zero amps.

How does it work?

About the only thing that the VTM-4 has in common with a conventional rear axle assembly is a drive pinion and ring gear. Upon disassembly, you will find an oil pump incorporated within the carrier and two multiple disc clutch assemblies on each end of the rear axle housing.

The carrier drives a clutch housing that contains 10 plates that spline to the clutch housing and 10 plates that spline to a hub (which is splined to the axle shaft). Connected to the axle shaft is a thrust plate that has three ramped slots with three ball bearings. The VTM-4 housing has an electro-magnetic coil magnetizes a pole piece that also has three slots that ride with the ball bearings that are engaged with the thrust plate.

The pole piece has a plate surface that rides closely to a plate in the clutch housing, kind of like an A/C clutch. When the electro-magnetic coil energizes, the pole piece is magnetized, which attracts the plate that rides within the clutch housing. The rotating clutch housing now drives the pole piece in the direction of housing rotation, which causes the ramped areas of the pole piece and the thrust plate to separate. The separating force compress the clutch pack in the clutch housing and transfers the rotating torque of the clutch housing to the axle shaft.

Maintenance:

Like most rear axle assemblies, regular service intervals are part of the vehicle's maintenance program. Although if history repeats itself, the regular fluid change intervals will probably be missed. The fluid change intervals are a follows:

Normal Conditions Maintenance Schedule.

Severe Conditions Maintenance Schedule.

Many customers fall within the severe maintenance category, which involve the following driving conditions:
  • Driving less than 5 miles per trip
  • Driving less than 10 miles per trip in freezing temperatures
  • Driving in extremely hot [over 90F(32C)] conditions
  • Extensive idling or long periods of stop-and go driving
  • Trailer towing, driving with a roof top carrier, or driving in mountainous conditions
  • Driving on muddy, dusty, or de-iced roads
  • Note: According to American Honda, the first fluid change interval is essential to the longevity of the VTM-4. In addition, use only Honda VTM-4 fluid in the differential assembly. The VTM4 differential assembly has a drain plug and a fill plug on the rear cover. Fill the differential assembly on a level surface until the fluid is at the level of the fill hole.

    Diagnosis:

    Unless you have a Honda Diagnostic System (HDS) or a PGM-FI tester at your shop, you will have to rely on flashing the trouble codes using the VTM-4 light on the dash. The procedure is as follows:

    Retrieving:
    1. Park the vehicle on level ground
    2. Shift to the PARK position
    3. Turn off the engine
    4. Release the parking brake pedal
    5. Connect the SCS circuit to body ground (connect pin 9 to pin 4 of the DLC)
    6. Turn the ignition switch to the "ON" position and observe the VTM-4 indicator light
    7. Record all DTC(s) and then refer to the DTC Troubleshooting chart
    8. Codes above ten will have long blinks indicating the first digit and short blinks indicating the second digit

    Erasing:
    1. Shift the vehicle in the PARK position and turn the engine "OFF"
    2. Release the parking brake
    3. Connect the SCS circuit to body ground (connect pin 9 to pin 4 of the DLC)
    4. Turn the ignition switch "ON"
    5. When the VTM-4 indicator light turns "ON", press and hold the VTM-4 lock switch
    6. When the VTM-4 indicator light goes off, release the VTM-4 lock switch
    7. When the VTM-4 indicator light turns back "ON", press and hold the VTM-4 lock switch again until the VTM-4 indicator light goes off
    8. The indicator light will blink twice quickly to confirm that the DTCs have been cleared in the VTM-4 control unit memory.
    9. If the VTM-4 indicator light does not blink twice quickly, the memory has not been cleared. Turn the ignition switch "OFF" and repeat the procedure.

    Initializing:

    The VTM-4 control module must be initialized after replacement. The following are procedures for manually initializing the VTM-4 control unit.
    1. Start the vehicle and place the gear selector in "Manual Low"
    2. Press the VTM-4 button on the dash to lock the differential. The VTM-4 LED on the button and the VTM-4 indicator on the dash should illuminate
    3. Turn the ignition to the "off" position
    4. Turn the ignition to the "on" position
    5. The VTM-4 indicator and switch LED should turn "on" for approximately 4 seconds, then turn "off"
    6. The VTM-4 control module is now initialized

    Mechanical Testing:

    2WD: In 2WD, the clutches are NOT engaged and each wheel should spin freely by hand while on jack stands. The actual specification is 15 ft lbs of rotating force. If the wheels do not spin freely, check for a short to power by disconnecting the electrical connectors to the coil. If the wheels continue to have excessive turning resistance, check for mechanical binding in the associated clutch pack.

    4WD: When activated, the electro-magnetic coils in the VTM-4 force the ramps in the thrust plate and pole piece to separate with considerable force when the wheel or ring gear carrier is rotated. To test the holding capacity of the clutches, use the following procedure:

    1. Lift the vehicle and place on jack stands
    2. Remove the rear wheels
    3. The engine should be "OFF" and the transmission selector in "PARK"
    4. The parking brake should be "RELEASED"
    5. Disconnect the 2 pin and 6 pin connectors
    6. To test the left clutch/coil, place full battery power to pin 1 of the 2-pin connector and ground to pin 2 of the 2-pin connector. NOTE: The VTM-4 controller typically Pulse Width Modulates current to the coils. Providing full battery voltage to the coils will get them HOT and could potentially cause damage to the coils if left "on" for too long. DO NOT energize the coils for more than 3-minutes
    7. Set a click type torque wrench to 148 ft lbs and attempt to rotate the rear hubs in both directions. The torque wrench should click before the hub/axle rotates. If the Hub rotates before 148 ft lbs, the clutch pack is slipping
    8. Repeat the procedure for the right side clutch pack
    9. Place 12 volts on pin 6 of the 6-pin connector and ground on pin 3 of the 6-pin connector.

    Future Advances with VTM-4 Technology:

    Check out the '05 Acura RL for an interesting look at a rear axle assembly. Acura combines the VTM-4 technology with the Active Torque Transfer System (ATTS) technology found earlier Honda Prelude SH models. The RL is an AWD vehicle that has the capability of overdriving or underdriving either rear wheel to correct for an oversteer or understeer situation. This technology complements their Vehicle Stability Assist (VSA) system that has already been in place since 2000. If you are at all interested in Honda or Acura, check out the website www.vtec.net.