1 8/19/04 4:40 PM Page 4. Honda/Acura VTM- 4. by Sean Boyle, Southern Illinois University W. ith advanced computer control and electronics The carrier on today's automobiles, it's houses an oil no surprise that the once pump, and simple differential assembly two, multiple- has been modernized disc clutch through electronic and assemblies, hydraulic controls. Most one on each manufacturers are out to end of the rear build a better 4WD or AWD axle housing. system, which incorporates Figure 1. a variety of technology, such as hydraulics, electronics, or impressive mechanical physics. You'll Acceleration: The VTM-4s control How Does It Work? typically find these advanced drive- module provides a duty-cycled signal About the only things the VTM-4 . trains on higher-end SUVs and sports to control the coils in the axle assembly, has in common with a conventional rear cars, but expect them eventually to find based on a variety of inputs, such as axle assembly are the drive pinion and their way to your service bay.
2 Engine load and speed. This controls ring gear. The carrier houses an oil This article covers the new, elec- the amount of torque transfer to the rear pump, and two, multiple-disc clutch tronically-controlled, limited/locking wheels; a proactive measure to prevent assemblies, one on each end of the rear axle found in the honda Pilot and the wheels from losing traction while axle housing (figure 1). acura MDX. This is called the Variable accelerating. The carrier drives a clutch housing, Torque Management ( VTM-4 ) system. Cruise or Coast: When in cruise or with 10 plates that spline to the housing The VTM-4 system contains a control coast mode, the VTM-4 eliminates and 10 plates that spline to a hub module, which controls current to two torque to the rear wheels to improve (which, in turn, is splined to the axle electromagnetic coils that act on a fuel efficiency and driveability.)
3 Shaft). A thrust plate with three ramped mechanical wet clutch. The system can Limited Slip: If the drive wheels slots and three ball bearings is connect- operate from 0 percent torque delivery lose traction, the VTM-4 control mod- ed to the axle shaft. to up to 53 percent of torque delivery, ule controls each coil in the rear axle The VTM-4 housing has an elec- based on conditions and preemptive separately, to deliver torque to the tromagnetic coil that magnetizes a pole strategies. wheel with the most traction. piece (figure 2). The pole piece also has One of the great benefits of the Locking: If you press the VTM-4 three slots that ride with the ball bear- VTM-4 is its ability to initiate a variety button with the transmission in manual ings that are engaged with the thrust of operational strategies, including 1st or 2nd, the differential initiates full plate.
4 The pole piece has a plate surface delivering torque to the rear wheels lock (5 amps to the coils) until the vehi- that rides close to a plate in the clutch before wheel slip even occurs. cle reaches 6 MPH. Then the control housing, similar to an A/C clutch. module decreases current flow to the When the electromagnetic coil Operation coils until the vehicle reaches 18 MPH, energizes, the pole piece is magnetized, We'll start by examining the VTM- where the control module turns the which attracts the plate that rides with- 4s operating modes. coils off. in the clutch housing. The rotating 4 GEARS September 2004. 8/24/04 1:37 PM Page 6. Honda/Acura VTM- 4. clutch housing now drives Clutch Housing Pole Piece Electro-magnetic the pole piece, which causes Clutches, half Coil the ramped areas of the pole splined to the piece and the thrust plate to housing, the separate.
5 The separating others splined Ramped force compresses the clutch to the axle shaft area on pack, which transfers the Pole Piece rotating torque of the clutch housing to the axle shaft. Thrust plate, connected to Maintenance the axle shaft Like most rear axle and establishes assemblies, regular service the force intervals are part of the vehi- against the cle's maintenance program clutch plates (although, if history is any during VTM4. indication, the regular fluid operation change intervals will proba- bly be missed). The fluid Figure 2. change intervals are: IMPORTANT: According to Normal Conditions Maintenance American honda , the first fluid change Schedule interval is essential for the VTM-4 to 15,000 miles or 6 months hold up. In addition, only use honda 30,000 miles or 24 months VTM-4 fluid in the differential assem- 60,000 miles or 48 months bly.
6 90,000 miles or 72 months The VTM4 differential assembly 4 BLK:PGM Tester Ground 120,000 miles or 96 months has a drain plug and a fill plug on the 5 BRN/BLK: Signal Ground for communication network rear cover. Fill the differential assembly 7 GRY: K-line Severe Conditions Maintenance on a level surface until the fluid reach- 9 BRN: SCS circuit Schedule es the level of the fill hole. 12 RED/WHT: Programming Many customers fall within the 14 LT BLU: Input/Output for VTM-4 and severe maintenance category, which Diagnosis immobilizer control include the following driving condi- Unless you have a honda 16 WHT/GRN: Power, 4 amps max tions: Diagnostic System (HDS) or a PGM-FI Figure 3. Driving less than 5 miles per trip tester at your shop, you'll have to rely Driving less than 10 miles per trip on flashing the trouble codes using the in freezing temperatures VTM-4 light on the dash.
7 Here's how: DTC Code Description Driving in extremely hot [over Retrieving the Codes: 21-22 Front wheel speed sensors 90 F (32 C)] conditions 1. Park the vehicle on level 23-24 Rear wheel speed sensors Extensive idling or long periods of ground. 26 ABS control unit or harness stop-and-go driving 2. Put the transmission in park. 37-38 RPM signal circuit Trailer towing, driving with a 3. Turn the engine off. 41 PCM communication (A/T. rooftop carrier, or driving in moun- 4. Release the parking brake control system). tainous conditions pedal. 42-43 Differential temperature Driving on muddy, dusty, or deiced 5. Connect the SCS circuit to 44 VTM-4 relay roads body ground (connect DLC 45 ABS modulator power supply pin 9 to pin 4, figure 3). 51-54 Left clutch coil circuit 7,500 miles or 6 months 6. Turn the key on, engine off, 55-58 Right clutch coil circuit 15,000 miles or 12 months and watch the VTM-4 indica- 59 Right or left clutch power 30,000 miles or 24 months tor light.
8 Supply 45,000 miles or 36 months 73 MAP/TP/PCM circuits 60,000 miles or 48 months Record all DTCs and refer to the 76 Rear differential clutch warn- 75,000 miles or 60 months DTC Troubleshooting Chart. Codes ing 90,000 miles or 72 months above ten will have long blinks indicat- 77 PCM. 105,000 miles or 84 months ing the first digit and short blinks indi- 78 VTM-4 control unit 120,000 miles or 96 months cating the second digit. Figure 4 shows Figure 4. a list of the DTCs. (continued on page 11). 6 GEARS September 2004. 8/24/04 1:37 PM Page 11. Honda/Acura VTM- 4 (continued from page 6). button and the VTM-4 indica- If the VTM-4 indi- tor on the dash should light. 3. Turn the key off. cator light doesn't 4. Turn the key on, engine off. 5. The VTM-4 indicator and switch LED should light for blink twice quickly, about 4 seconds, then turn off.
9 The VTM-4 control module is now 2 WHT: Temp coil high the memory hasn't initialized. 3 Male GRN;. Female RED/GRN: Right coil low been cleared. Mechanical Testing 2WD: In 2WD, the clutches aren't 4. 5. Male BRN/BLK; Female BLK. BLK: Temp coil low 6 Male BRN;. Turn the key off engaged, so, with the vehicle raised, each wheel should spin freely; the actu- Female BRN/BLK: Right coil high al specification is 15 ft-lbs of rotating and repeat the force. If the wheels don't spin freely, procedure. check for a short to power by dis- connecting the electrical connec- tors to the coil. Erasing the Codes: If the wheels still won't turn freely, 1. Transmission in park, key off. check for mechanical binding in 2. Release the parking brake. the associated clutch pack. 1 Male BLK. 3. Connect the SCS circuit to Female BLU/YEL. body ground (connect DLC 4WD: When activated, the electro- 2 Male BLK.)
10 Pin 9 to pin 4). magnetic coils in the VTM-4 force the Female GRY/WH. 4. Key on, engine off. ramps in the thrust plate and pole piece Figure 5. 5. When the VTM-4 indicator firmly apart as the wheel or ring gear light turns on, press and hold carrier rotates. Here's how to test the the VTM-4 lock switch. holding capacity of the clutches: 8. To check the right side clutch 6. When the VTM-4 indicator 1. Lift the vehicle and place it on pack, apply 12 volts to pin 6 of light goes off, release the jack stands. the 6-pin connector and VTM-4 lock switch. 2. Remove the rear wheels. ground pin 3 of the 6-pin con- 7. When the VTM-4 indicator 3. Engine off; transmission in nector. light turns back on, press and park. 9. Now use your torque wrench hold the VTM-4 lock switch 4. Release the parking brake. to try to rotate the right side until the VTM-4 indicator 5.