2004 AWD Question

[Paulguy]

I had my Pilot up on 4 jackstands, put it in gear, and only the front wheels spun. Question is, doesn't this simulate the vehicle on ice? Why didn't the rear wheels kick in?

[N_Jay]

Did you step on the gas?

[mudbog]

How fast was the speedo reading when you had it on jackstands? I believe the AWD system gradually cuts out and is completely off by the time you hit 18MPH. Also, did you try it in gear 1 or 2 with the VTM4 locked? This will force some power to the rear axle and lock the back axle.

[Paulguy]

It was in gear but maybe it sensed greater than 18mph. Didn't try thr VTM4, shoulda.

[mozzer]

I am no expert on the pilot (new owner) but isn't the AWD system based on a viscous coupling to move power front to back ? If so i think it needs some resistance to allow it to work, better to park up on some gravel and step on it to see if the front wheels spin much or not.

Also, does the VTM button lock front to back or just lock the rear diff side to side ?

[jamesgarner83]

Originally Posted by mozzer

I am no expert on the pilot (new owner) but isn't the AWD system based on a viscous coupling to move power front to back ? If so i think it needs some resistance to allow it to work, better to park up on some gravel and step on it to see if the front wheels spin much or not.

Also, does the VTM button lock front to back or just lock the rear diff side to side ?

Uses electro magnets to engage wet clutches built into the differential instead of a viscous coupling. Engagement is based on having an electrical charge and not wheel slippage.


Should not have to have resistance should engage during acceleration.

"VTM - 4 Lock" locks front to back not side to side.

See below

Variable Torque Management 4-wheel-drive (VTM-4)

"Ensuring a high level of all-weather stability, traction and control is the Pilot's VTM-4 (Variable Torque Management 4-Wheel Drive) system. Unlike conventional on-demand systems that work only when the wheels are slipping, VTM-4 proactively delivers torque to all four wheels during acceleration for excellent dry-road vehicle dynamics as well as outstanding control in wet, icy and snow conditions. A unique "lock" feature is provided to maximize traction for extremely slippery or "stuck" conditions. A compact transfer case is bolted directly to Pilot's front-mounted transaxle. A two-piece propeller shaft delivers torque from the transfer case to a rear axle drive unit. Two computer-controlled, electromagnetically-powered clutches engage as needed to provide torque to the rear wheels."

"The Pilot's innovative VTM-4 four-wheel drive system was designed to deliver outstanding traction, stability and control in all weather conditions as well as good medium-duty off-road performance. It was also designed to minimize the weight and packaging penalties associated with conventional four-wheel drive systems.

The VTM-4 system is unique in its operation. Unlike many competitive systems that use an engagement strategy triggered by wheel slippage, VTM-4 anticipates the need for all-wheel drive and engages the rear wheels whenever the vehicle is accelerating. Additional torque is applied to the rear wheels when wheel slip is detected, up to an approximate maximum of 50-percent in low gear. Another unique feature of the system is the VTM-4 Lock function.

Activated by a button on the instrument panel, the VTM-4 Lock mode delivers maximum torque transfer to the rear wheels to aid extraction from extremely slippery or "stuck" conditions. The feature works only when the vehicle is in first, second or reverse gears, and automatically disengages at speeds above 18 miles per hour.

When cruising under normal conditions, the Pilot provides front-wheel drive power for improved efficiency. Torque is proactively distributed to the rear wheels when the vehicle is accelerating or wheel slip is detected. The level of torque delivery, front to rear, is determined by the amount of acceleration (rate of change in velocity) and wheel slip (difference in rotational speed) and is controlled by a dedicated CPU with sensors in the braking, engine and transmission systems.

To avoid the weight and bulk of a conventional transfer case, VTM-4's torque transfer unit is a compact cast-aluminum housing bolted directly to transaxle. The transfer case is a single-speed, permanently engaged device without a low-range, reducing weight and space penalties while maintaining excellent on- and off-road capabilities. Attached to the front wheel differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the drive ninety degrees, move it to the vehicle center line and lower its axis by approximately 3.75-inches.

VTM-4 Engagement Modes:

There are three distinct modes of VTM-4 engagement:

(1) The first mode, called Acceleration Torque Control (ATC), works whenever the vehicle's throttle is depressed, even on dry pavement - a feature unique to the VTM-4 system. Sensors in the engine and transmission monitor vehicle speed and acceleration. The amount of torque applied, as directed by the system's ECU, is determined according to vehicle speed, the amount of acceleration and transmission status (gear setting). This benefits not only the Pilot's ability to gain traction from a standing start, before wheel slip occurs, but also its overall dynamic stability on both dry and slippery roads. Reducing the propulsive force carried by the front tires under acceleration reduced torque steer and cornering adhesion. Rear wheel torque rises smoothly from zero to a preset maximum in proportion to vehicle acceleration (both forward and reverse). During constant-speed driving, all power is driven to the front wheels for improved fuel efficiency.

(2) The second engagement mode occurs when wheel slip is detected. Differences in rotational speed between front and rear wheels are measured by sensors in the ABS system and monitored by the ECU. In response, the ECU commands an increase in torque delivery to the rear wheels. Torque application is adjusted according to the amount and the rate of change in wheel slip. As slip increase, more power is delivered to the rear wheels for improved traction.

(3) The third mode of engagement is VTM-4 Lock. Lock mode occurs when the driver shifts into first, second or reverse gears and depresses the VTM-Lock button on the instrument panel. When lock mode is selected at vehicle speeds below 18-mph, the system ECU commands a preset maximum amount of rear-drive torque to be delivered to the rear wheels for improved traction in very low-speed, low-traction, conditions. As control is regained and vehicle speed increases, the system gradually reduces rear axle torque until it is completely disengaged.

The maximum torque delivered to the rear wheels is sufficient to climb the steepest grade observed on any public road in America - 31-degrees (60 percent slope) - with a two-passenger load on board. The Pilot will also move from rest up a 28-degree (53 percent slope) dirt grade. On a split-friction grade (different amounts of traction at each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help the vehicle climb a steep, slippery driveway to enter a garage.

Propeller Shaft:

The two-piece propeller shaft that carries torque from the transfer case to the rear-drive unit is made of high-strength steel tubing to permit a smaller diameter. Minimizing driveline dimensions improves both ground clearance and interior room. The cross yokes attached at each end by friction welding are forged steel for high strength and low weight. The center support bearing is rubber isolated to block the transmission of driveline noise from the interior of the vehicle. A low-friction plunger joint located near the center of the propeller shaft accommodates relative motion between front- and rear-mounted driveline components.

A tuned-mass damper inside the front portion of the propeller shaft cancels any bending tendency in response to powertrain vibrations. Equal-length, front-wheel half-shafts have a plunger joint at their inboard end and a ball-type universal joint at the wheel end. Rear half shafts are similar in design but use a double-offset joint at the inboard end and a ball joint at the outboard end. All universal joints are constant-velocity type.

Rear Axle Drive Unit:

The Pilot's rear axle drive unit consists of a hypoid ring-and-pinion gear set supported by a cast-aluminum housing which switches torque from the propeller shaft's longitudinal orientation to the lateral orientation necessary to drive the rear wheels.

A connection from the ring gear to each wheel's half-shaft is made by left- and right-side clutches. Each drive clutch consists of three elements: an electromagnetic coil, ball-cam device and set of 19 wet clutch plates which are similar in design to clutches used in an automatic transmission. Ten of the plates are splined (mechanically connected) to the ring gear while nine of the plates are splined to a half shaft.

When the VTM-4 system's electronic control unit (ECU) determines that torque should be distributed to the rear wheels, an electric current is sent to the two electromagnetic coils. The resulting magnetic field moves a rotating steel plate toward each fixed coil. Friction between that steel plate and an adjoining cam plate causes the cam plate to begin turning. As it does, three balls per clutch roll up curved ramps, creating an axial thrust against a clutch-engagement plate. This thrust force compresses the wet clutch plates, engaging the corresponding rear wheel.

Unlike mechanically actuated four-wheel drive systems, the VTM-4 system is infinitely variable. The amount of torque provided to the rear wheels is directly proportional to the electric current sent from the ECU and can be adjusted from zero to a preset maximum. This current constantly changes to deliver the optimum rear torque calculated by the ECU. An internal gear pump circulates VTM-4 fluid to cool and lubricate the clutches, bearings and gears within the rear drive unit. Use of high-strength, low-weight materials - such as die-cast aluminum for the housing - minimizes the bulk and weight of the hardware."

[theirishscion]

Well, effectively the VTM-4 Lock button locks front to back and side to side at the back, but since there's no center diff, the job is simply done by coupling both rear wheels to the driveshaft directly. The system is really quite elegant in it's simplicity. There's no way to couple the front wheels though, they always have an open diff.

[jamesgarner83]

Originally Posted by theirishscion

Well, effectively the VTM-4 Lock button locks front to back and side to side at the back, but since there's no center diff, the job is simply done by coupling both rear wheels to the driveshaft directly. The system is really quite elegant in it's simplicity. There's no way to couple the front wheels though, they always have an open diff.

The rear is open as well. Nothing locks the two half shafts together. Both clutches can be fully engaged but they still have an open diff between them. The lock sends maximum voltage to the electro magnets which ensures the clutches are fully engaged allowing for torque to be equally split between the front and rear diffs. VSA can help in the side to side split, but it does not have a mechanical lock in the diff itsself and unfortunately VSA was not available on 2004 model.

[N_Jay]

Originally Posted by jamesgarner83

The rear is open as well. Nothing locks the two half shafts together. Both clutches can be fully engaged but they still have an open diff between them. The lock only locks torque to the rear wheels. VSA can help in the side to side split, but it does not have a mechanical lock in the diff itsself and unfortunately VSA was not available on 2004 model.

No, there is no "differential" (open or not) in the rear.

Each half-shaft is independently connected or rel;eased from the drive line via it's electrically operated wet-plate clutch.

[jamesgarner83]

N_Jay likes to call it a rear drive unit for some reason.

[N_Jay]

Because if you are familiar with what a traditional mechanical differential is, and the various modifications of them that impact their operation between being completely "open", and being "locked" (and no longer a differential), you find that the rear drive unit is NOT a differential (in the classic sense), and is simply a hypoliod ring gear which directly drives the input of two INDEPENDENT wet-plate clutches, each driving one half-shaft.

Unlike a differential;
1) The torque applied to one side does not impact the torque available on the other side.
2) Except for the limiting affect of the speed on the input drive (the ring gear) speed changes on one side do not affect the speed of the other wheel.

If you understand how a differential (classic mechanical differential, if you care to be picky), carrying that knowledge into the operation of the VTM-4 unit will only cause confusion.

[N_Jay]

Around The Corner (1937) How Differential Steering Works - YouTube

Around The Corner (1937) How Differential Steering Works - YouTube

[theirishscion]

Originally Posted by jamesgarner83

The rear is open as well. Nothing locks the two half shafts together. Both clutches can be fully engaged but they still have an open diff between them. The lock sends maximum voltage to the electro magnets which ensures the clutches are fully engaged allowing for torque to be equally split between the front and rear diffs. VSA can help in the side to side split, but it does not have a mechanical lock in the diff itsself and unfortunately VSA was not available on 2004 model.

N_Jay has beaten me to this, but it's a pet peeve of mine as well so I've going to throw my oar in, in case there's still any confusion. I detest misinformation on forums if it's at all avoidable.

There is no differential gear, open or otherwise, in the VTM-4 system. There is just a solid bevel gear translating the torque and rotation from the drive shaft 90 degrees toward both back wheels.To repeat, this bevel gear is solid, no planetary gears or carrier. No possible differential effect. When the drive shaft turns, both left and right sides after the translation turn at the same speed. Always and without exception.

Under normal circumstances, the clutches only actuate when wheel spin is detected at the front of the car, or under certain acceleration conditions to combat torque steer and enhance traction. When not actuated, the rear wheels spin independently of each other and the drive shaft. Any differential-like effect (both wheels powered but moving at different speeds) is achieved through clutch slippage.

When the VTM Lock feature is active (lock button pressed, 1st or 2nd gear selected, under 18MPH) the two back wheels are locked together by the two clutch packs (full 5 Amps of current to both coils) to the degree that the clutches are mechanically capable of achieving from 0 to 6MPH, transitioning from there to fully unlocked by 18MPH. As such, insofar as mechanically possible, the rear wheels are locked together, much like a locked differential. In actual fact, the clutches are only capable of transmitting up to 50% of total engine torque, so in that sense it's possible for one to stall while the other one turns, but this is not a differential effect. The energy is not being transferred to the moving wheel, it is being dissipated as heat and wear on the stalled clutch pack.

There's a fabulous writeup of the system here if anyone disbelieves us or wants to see pictures. I'm happy to clarify any of this if anyone has questions.


Dermot (the apparently very grumpy old man)

[jamesgarner83]

I get what you guys are saying but why does Honda call it a differential?

http://www.globalautoparts.biz/image...VTM4_small.jpg

[theirishscion]

Originally Posted by jamesgarner83

I get what you guys are saying but why does Honda call it a differential?

Presumably because the mechanic's vernacular for 'the mechanism that transfers power to half-shafts and wheels' is 'differential', and there's no short, commonly understood descriptive word or phrase for what the VTM unit actually is. I don't have serious issue with referring to the part as a differential so people understand what you're talking about, but when the mechanism of operation is being described authoritatively (like in this context) it should be described correctly.