155

TOYOTA TUNDRA – NEW FEATURES

JSTEERING 1. Description D A rack and pinion type steering gear with a hydraulic power steering assist is used on the models with 1UR-FE engine. D This hydraulic power steering uses a Variable Flow Control (VFC) power steering that controls the discharge flow rate in accordance with the steering angle, vehicle speed, and engine speed.

12DCH038I

"

Specifications A

Gear Ratio (Overall)

18.0

No. of Turns Lock to Lock

3.71

Rack Stroke Fluid Type

mm (in.)

198 (7.8) ATF Type DEXRON® II or III

156

TOYOTA TUNDRA – NEW FEATURES

2. Variable Flow Control (VFC) Power Steering General The VFC power steering is provided with a solenoid valve, which is controlled by the power steering ECU, in the vane pump. The solenoid valve controls the discharge flow rate of the power steering fluid in accordance with the operating conditions of the steering wheel, vehicle speed, and engine speed. Thus, it reduces the load on the engine applied by the actuation of the vane pump, and improves fuel economy. "

System Diagram A

ECM D Engine Speed Signal

Skid Control ECU D Vehicle Speed Signal Power Steering ECU

Steering Angle Sensor D Steering Angle Signal

VFC Solenoid Valve Vane Pump

Combination Meter CAN (V Bus)

Power Steering Warning Light 12DCH039I

157

TOYOTA TUNDRA – NEW FEATURES Layout of Main Components Power Steering Warning Light

Steering Angle Sensor

Power Steering ECU

DLC3

ECM Skid Control ECU

Reservoir Tank

Vane Pump D VFC Solenoid Valve

Steering Gear

Oil Cooler

12DCH040I

158

TOYOTA TUNDRA – NEW FEATURES

Vane Pump D A compact and lightweight vane pump is used. D A VFC solenoid valve is provided to control the discharge flow rate.

VFC Solenoid Valve

A

To Steering Gear

A

To Reservoir Tank

VFC Solenoid Valve

From Pump

12DCH041I

A – A Cross Section

Flow Rate

Current

08LCH188Y

Flow Rate Characteristic of VFC Solenoid Valve

"

Specifications A

Theoretical Discharge Rate Relief Pressure

l/min (@1000 rpm)

15

(kgf/cm2,

10 (102, 1450)

MPa

psi)

08LCH187Y

TOYOTA TUNDRA – NEW FEATURES

159

System Operation 1) General The power steering ECU effects the following controls: Control

Flow Rate Control

Fail-safe

Function Controls in accordance with the vehicle speed, steering angle, and steering speed conditions and also effects assist control when the steering wheel is being operated and standby control when the steering wheel is not being operated. Thus, it prevents the vane pump from discharging excessive flow, which lightens the load on the engine and improves fuel economy. Also, the following corrections are made to determine the optimal discharge flow rate:

Steering Angle Correction

Determines the discharge flow rate in accordance with driving conditions, such as steering at standstill, cornering, or straightline driving.

Steering Return Correction

Determines the discharge flow rate for returning the steering wheel to its neutral position after the steering wheel is operated.

Engine Speed Correction

Determines the discharge flow rate in accordance with the engine speed. If the steering control ECU detects a malfunction in the VFC power steering system, it makes the steering feel heavier than normal, in order to inform the driver of a system failure.

160

TOYOTA TUNDRA – NEW FEATURES "

Control Diagram A Steering Control ECU

Steering Angle Sensor Steering Angle

Calculates Steering Angle

Neutral Position

Calculates Steering Speed

Determines Standby Control or Assist Control

Skid Control ECU Vehicle Speed

Calculates Vehicle Speed

Calculates Flow Rate

Steering Angle Correction Steering Return Correction

ECM Engine Speed

Engine Speed Correction

Calculates Engine Speed

Flow Rate Limit Process Calculates Time to Achieve Target Flow Rate Assist Control Standby Control Time

Calculates Control Current Solenoid Valve

Flow Rate Current

12DCH045I

161

TOYOTA TUNDRA – NEW FEATURES 2) Flow Rate Control a. Standby Control or Assist Control

D The standby control or assist control is performed in accordance with vehicle conditions such as vehicle speed, steering angle, and steering speed. Control

Vehicle Condition

Vehicle Speed 0 km/h (0 mph) Vehicle Speed More than 0 km/h (0 mph)

Standby Control

Assist Control

Steering Speed = 0 deg/sec (for 10 Seconds or More)

Steering Speed > 0 deg/sec

Steering Angle < 4.5_ (for 10 Seconds or More)

Steering Angle y 4.5_

D The assist flow rate is determined in accordance with the vehicle speed and steering speed. D For 3 seconds after the ignition switch is turned to ON, the flow rate is controlled at 10.5 l/min. After that, the flow rate is controlled as shown in the graph below:

Assist Control Assist Control

Flow Rate (l/min)

Steering Speed (deg/sec) Assist Control

Assist Control

Standby Control Vehicle Speed (km/h) 12DCH044I

162

TOYOTA TUNDRA – NEW FEATURES b. Steering Angle Correction D Based on the steering angle and vehicle speed signals, this function corrects the flow rate in accordance with the driving conditions. Thus, an optimal steering feeling is ensured for every steering condition, such as steering at standstill, cornering, or straightline driving. D The correction flow rate is 0 l/min while the steering is operated with the vehicle at standstill or driving straight (steering angle is ±4.5_ or less). D The correction flow rate decreases when the vehicle speed and steering angle increase.

±0

0

Steering Angle (deg) Correction Flow Rate (l/min)

0

Vehicle Speed (km/h) 12DCH049I

c. Steering Return Correction To return the steering wheel to the neutral position quickly after a sudden lane change, the ordinary flow rate is insufficient, as it makes the steering feel heavy. The steering return correction increases the correction flow rate in accordance with the steering speed and vehicle speed, in order to ensure an optimal steering force and vehicle stability.

Correction Flow Rate (l/min)

0

0

Vehicle Speed (km/h)

08LCH192Y

163

TOYOTA TUNDRA – NEW FEATURES d. Engine Speed Correction

With the conventional vane pump, the flow rate increases when engine speed increases. The engine speed correction reduces the correction flow rate in accordance with the engine speed. This ensures optimal steering force, reduces engine load, and improves fuel economy.

0

Correction Flow Rate (l/min)

0

Engine Speed (rpm) 12DCH050I

164

TOYOTA TUNDRA – NEW FEATURES 3) Fail-safe D If the power steering control ECU detects a malfunction in this system, it illuminates the power steering warning light to alert the driver of the malfunction. D If a malfunction occurs in any of the sensors, the power steering control ECU effects the following fail-safe control: "

Fail-safe Control List A Control Correction

Malfunction

Flow Rate Control*

Steering Steering Engine Angle Return Speed

Steering Angle Signal

D Fixes steering angle at 0_ D Fixes steering speed at 330 deg/sec D Effects 330 deg/sec steering speed control

×

×

f

Neutral Position Signal

D Fixes steering angle at 0_ D Effects normal control

×

×

f

Vehicle Speed Signal

D Fixes vehicle speed at 80 km/h (49.7 mph) D Effects 80 km/h (49.7 mph) vehicle speed control

f

f

f

Engine Speed Signal

D Fixes engine speed at 0 rpm D Effects normal control

f

f

×

Steering Angle and Neutral Position Signals

D Fixes steering angle at 0_ D Fixes steering speed at 330 deg/sec D Effects 330 deg/sec steering speed control

×

×

f

Steering Angle and Vehicle Speed Signals

D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes vehicle speed at 80 km/h (49.7 mph) Controls flow rate at 8.5 l/min and over constantly

×

×

f

Steering Angle and Engine Speed Signals

D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes engine speed at 0 rpm Effects 330 deg/sec steering speed control

×

×

×

Neutral Position and Vehicle Speed Signals

D Fixes steering angle at 0_ D Fixes vehicle speed at 80 km/h (49.7 mph) D Effects 80 km/h (49.7 mph) vehicle speed control

×

×

f

Neutral Position and Engine Speed Signals

D Fixes steering angle at 0_ D Fixes engine speed at 0 rpm D Effects normal control

×

×

×

Vehicle Speed and Engine Speed Signals

D Fixes vehicle speed at 80 km/h (49.7 mph) D Fixes engine speed at 0 rpm D Effects 80 km/h (49.7 mph) vehicle speed control

f

f

×

*: Fixes at assist control during fail-safe. (Continued)

165

TOYOTA TUNDRA – NEW FEATURES Control Correction

Malfunction

Flow Rate Control*

Steering Steering Engine Angle Return Speed

D D D D

Fixes steering angle at 0_ Fixes vehicle speed at 80 km/h (49.7 mph) Fixes engine speed at 0 rpm Effects 80 km/h (49.7 mph) vehicle speed control

×

×

×

Steering Angle, Vehicle Speed and Engine Speed Signals

D D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes vehicle speed at 80 km/h (49.7 mph) Fixes engine speed at 0 rpm Controls flow rate at 8.5 l/min and over constantly

×

×

×

Steering Angle, Neutral Position and Engine Speed Signals

D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes engine speed at 0 rpm Effects 330 deg/sec steering speed control

×

×

×

D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes vehicle speed at 80 km/h (49.7 mph) Controls flow rate at 8.5 l/min and over constantly

×

×

f

D D D D D

Fixes steering angle at 0_ Fixes steering speed at 330 deg/sec Fixes vehicle speed at 80 km/h (49.7 mph) Fixes engine speed at 0 rpm Controls flow rate at 8.5 l/min and over constantly

×

×

×

Neutral Position, Vehicle Speed and Engine Speed Signals

Steering Angle, Neutral Position and Vehicle Speed Signals Steering Angle, Neutral Position, Vehicle Speed and Engine Speed Signals

*: Fixes at assist control during fail-safe.