Contents: Starting and accelerating in forward…⇊ Driving in forward gear at a…⇊ Slowing down in forward gear (2WD) ⇊ Driving and accelerating in reverse…⇊ Driving in reverse gear at a…⇊ Slowing down in reverse gear (4WD) ⇊ Thermal switch operation (2WD) ⇊ Operation of the bypass valve ⇊
Starting and accelerating in forward gear (4WD)
When starting off or accelerating in forward gear, the dual-pump system can engage all-wheel drive.
If the front wheels spin faster than the rear wheels, the front oil pump spins faster than the rear one. The front oil pump draws fluid through check valve B and drains it. Part of the fluid at the outlet of the front oil pump is sucked in by the rear oil pump. The remaining fluid passes through the check valve E into the clutch piston, where the hydraulic pressure is regulated by two diaphragms.
Adjustable hydraulic pressure on the clutch piston compresses the clutch plates and discs together to form a bond. In this case, the engaged clutch transfers traction force from the transfer case to the rear wheels and creates four-wheel drive (4WD).
Driving in forward gear at a constant speed (2WD)
When driving in forward gear at a constant speed, the dual pump system operates in two-wheel drive mode.
The speed of rotation of the front and rear wheels is the same, therefore the speed of the front and rear pumps is also the same. The fluid at the outlet of the front oil pump is sucked in by the rear oil pump and circulated through the system. Since no pressure is supplied to the clutch piston, the clutch does not engage and the vehicle remains in 2WD mode (front wheel drive).
Slowing down in forward gear (2WD)
When decelerating in forward gear, the dual-pump system operates in two-wheel drive mode.
Due to braking characteristics, when decelerating, the rotation speed of the rear wheels may exceed the rotation speed of the front wheels. In this case, the rear oil pump rotates faster than the front one.
The fluid at the outlet of the rear oil pump is simply sucked back into the rear oil pump and recirculated. Since no pressure is applied to the clutch piston, the clutch piston does not engage and the vehicle remains in 2WD mode (front wheel drive).
Driving and accelerating in reverse gear (4WD)
When starting off or accelerating in reverse gear, the dual-pump system can engage all-wheel drive.
If the front wheels spin faster than the rear wheels, the front oil pump spins faster than the rear one. The front oil pump draws fluid through check valve A and drains it. (It should be noted that in the case of reverse gear, the direction of operation of the pumps is opposite to the direction when moving in forward gear).
Part of the fluid at the outlet of the front oil pump is sucked in by the rear oil pump. The remaining fluid passes through the shut-off valve F into the clutch piston cylinder, where the hydraulic pressure is regulated by two diaphragms. Adjustable hydraulic pressure on the clutch piston can compress the clutch plates and discs together to form a bond. In this case, the engaged clutch transfers traction force from the transfer case to the rear wheels and creates four-wheel drive (4WD).
Driving in reverse gear at a constant speed (2WD)
When driving in reverse gear at a constant speed, the dual pump system operates in two-wheel drive mode. The speed of rotation of the front and rear wheels is the same, therefore the speed of the front and rear pumps is also the same. The fluid at the outlet of the front oil pump is sucked in by the rear oil pump and circulated through the system, but since there is a difference in the performance of the pumps, the fluid flows through the check valve E and then through the diaphragms. This fluid lubricates and cools the clutch assembly and bearings.
Under these conditions, only a small amount of pressure is applied to the clutch piston, so the clutch does not engage and the vehicle remains in 2WD mode (front wheel drive).
Slowing down in reverse gear (4WD)
When slowing down in reverse gear, the dual-pump system can engage all-wheel drive.
When slowing down in reverse, the rear wheels may spin faster than the front wheels (due to engine braking). Under these conditions, the rear oil pump draws fluid through check valves B and C. The fluid at the rear oil pump outlet then flows through check valve E to the clutch piston, where the pressure is regulated by two diaphragms.
Adjustable hydraulic pressure on the clutch piston can compress the clutch plates and discs together to form a bond. In this case, the engaged clutch transfers traction force from the transfer case to the rear wheels and creates four-wheel drive (4WD).
Thermal switch operation (2WD)
When operating in 4WD mode (four wheel drive) the fluid under controlled pressure comes into contact with the clutch piston and the thermal switch. When the temperature of the fluid in the differential increases, the thermal switch opens the bypass valve R. In this case, the pressure on the clutch piston decreases and the 4WD mode is exited.
Operation of the bypass valve
When the fluid pressure becomes greater than the resistance of the bypass valve spring, the stop valve R opens. The pressure supplied to the clutch piston is maintained at a constant level. This prevents excess torque in the rear-wheel drive system and improves operating stability.
(Original published on this resource: HONDABOOK)