2001 Honda CR-V -- Transmission

9/15/2000 6:43:10 PM

Honda engineers set the following goals for the CR-V transmission and 4-wheel- drive system:

  • An automatic 4-wheel-drive system that required little or no management by the driver
  • Minimize the traditional undesirable effects of 4-wheel-drive systems, including lower fuel economy, higher noise and vibration levels, higher weight and difficulty in adapting the system to ABS

In order to achieve their goals, the following systems were developed for the CR-V:

  • An entirely new Real Time 4WD system was designed
  • A newly developed 4-speed automatic transmission with Honda Grade Logic Control was adapted to the CR-V

The CR-V's Real Time 4WD offers the following advantages:

  • Operates automatically, only when needed
  • Requires practically no maintenance (90,000-mile oil-change interval)
  • Weighs less than conventional 4WD systems
  • When not in use it has minimal noise, vibration or handling intrusion
  • Better fuel economy
  • Operates with ABS

The CR-V's Real Time 4WD system only sends power to the rear wheels when there is insufficient traction for the front-wheel-drive system. The system consists of the conventional front-wheel-drive system, a compact transfer case that distributes drive to a propeller shaft running the length of the vehicle, a dual-pump system, the rear differential and left and right rear-wheel driveshafts.

The heart of the system is the dual-pump unit. It consists of two hydraulic pumps, one driven by the front wheels via the propeller shaft and one driven by the rear wheels via the rear differential. A hydraulically actuated, multi-plate clutch, similar to the clutches used in Honda automatic transmissions, connects the propeller shaft to the rear differential.

When the CR-V is operating with the front and rear wheels turning at the same speed, for example, on dry pavement, the front and rear hydraulic pumps operate at the same speed. Hydraulic fluid circulates between the two pumps; however, no pressure is generated. In effect, the fluid fed by the front pump is absorbed by the rear pump.

If the front wheels begin to turn faster than the rear wheels, as would be the case if they were spinning on snow or ice, the two hydraulic pumps would turn at a different rate and hydraulic pressure proportional to the difference in their speeds of rotation would be generated. The resulting hydraulic pressure opens a valve body and feeds pressure to the multi-plate clutch, which engages the front propeller shaft to the rear differential. The rear differential feeds the drive torque to the right and left rear wheels.

System operation is completely automatic; no electronics or driver intervention is involved. The greater the degree of front-wheel slippage, the greater the amount of torque fed to the rear wheels.

Real-Time 4WD is also practically maintenance- free, requiring only a scheduled fluid change (using Honda original CVT-F-type fluid) at 90,000 miles, like an automatic transmission.

Another big advantage of the CR-V's dual-pump Real Time 4WD system is that, unlike conventional 4WD systems, Real Time 4WD automatically disengages under braking, thereby allowing the ABS system to engage.

A 4-speed electronically controlled automatic transmission (ECT) with a lockup torque converter and Grade Logic programming is standard on the CR-V. The transmission is a constant-mesh unit, incorporating a first-gear, low-hold feature and an overdrive fourth gear. Overdrive improves fuel economy and helps reduce noise at highway speeds. The low-hold function allows the transmission to be held in low gear for climbing or descending steep grades or when pulling a trailer.

The lockup torque converter helps minimize fluid-coupling slippage, which can adversely affect mileage. The lockup feature works in third gear as well as fourth and also maintains lockup in third and fourth gears during deceleration.

Shifting is electronically controlled by a 16-bit microprocessor located in the Electronic Control Module (ECM). The microprocessor controls a linear shift solenoid, that in turn controls hydraulic pressure to the gear clutch packs. For optimal control of hydraulic pressure, the ECM monitors engine torque and controls transient-condition shifting. Since the control is linear, the engagement is more progressive. The result is smoother shifting and power transmission over a greater variety of power and gear settings.

The ECM is also programmed to minimize shift shock during full or part-throttle upshifts by momentarily retarding ignition timing. The ECM also controls downshifts.

The ECM also controls the CR-V transmission's Honda Grade Logic shift programming and is even used to monitor automatic-transmission fluid temperature. It uses the temperature data to alter clutch-pack hydraulic pressure to compensate for temperature-related viscosity differences.

The CR-V automatic transmission uses Grade Logic to determine when to shift. Grade Logic differs from more conventional computer-controlled shift programming in two important ways: It can determine the actual driving situation and then, based on six different stored "shift maps," select the appropriate shift points for second-, third- and fourth-gear upshifts and downshifts.

The Grade Logic Control System on the CR-V automatic transmission uses throttle position, road speed, rate of deceleration and rate of acceleration to determine driving conditions. It then chooses the appropriate shift map for that situation in D4 and D3. For example, when driving uphill, Grade Logic will sense a large throttle opening without any increase in speed. Based on this, Grade Logic will determine that the CR-V is going uphill and it will determine how steep the grade is. It will then choose a shift map that will downshift and hold third or even second gear, thereby eliminating hunting between gears.

Grade Logic also uses brake-pedal application as a control input. For example, Grade Logic will determine that the CR-V is driving down a grade, if it receives a closed throttle signal and a brake-pedal activation signal. It then selects a shift map that will downshift and hold third or second gear to allow the CR-V to utilize engine braking.

Similarly, Grade Logic can use a rapid deceleration signal and closed throttle to determine that the CR-V is entering a tight curve in the road or a corner. It then chooses a shift map that downshifts early for more responsive acceleration.

This same set of inputs occurs in the abrupt braking followed by quick acceleration that sometimes accompanies stop-and-go city driving, thus promoting smoother driving.