SAAB WIS ONLINE

Rear Drive Axle Description and Operation

Rear Drive Axle Description and Operation

Components

The all-wheel drive system consists of 6 main components:


• transfer case

• propeller shaft

• rear drive clutch solenoid valve

• differential clutch pump

• rear differential clutch solenoid valve

• rear differential clutch control module


Transfer Case

The transfer case is mounted on the side of the transmission. Its function is to take power from the gearbox and transfer it via a hypoid gear and a propeller shaft to the differential clutch.

Differential gear

The function of the differential clutch is to control the torque to the rear wheels. It can be varied between 0-100% drive via a hydraulically controlled multi-disc clutch. The differential clutch is bolted to the limited slip differential with 4 bolts and has a dedicated oil supply separate from the differential.

Propeller Shaft

The function of the propeller shaft is to transfer power from the transfer case to the differential clutch.

Limited Slip Differential

The limited slip differential, also referred to as the rear drive module, is located in the subframe and the drive shafts extend from it to each rear wheel. The limited slip differential consists of a hypoid gear and a differential which has a dedicated oil supply separate from the differential clutch. The limited slip differential is fully mechanical. The limited slip differential clutch, if installed, is bolted with 6 bolts to the left-hand side of the limited slip differential. Its function is to fully or partly lock the left-hand drive shaft to the differential housing. When this takes place the drive torque can be distributed between the rear wheels. The limited slip differential clutch is a disc type, and it is electro-hydraulically controlled by the rear differential clutch control module via a pressure control valve which is fitted on the limited slip differential clutch housing. The rear differential clutch control module calculates drive torque distribution. There are two variants: with and without limited slip differential clutch for the rear wheels.

Differential Clutch Pump

The differential clutch pump consists of an electric motor and a pump. It is supplied with voltage from the rear differential clutch control module. The differential clutch pump produces an operating pressure of 2700-3200 kPa (392-464 PSI). The rear drive clutch solenoid valve is electrically controlled and regulates the hydraulic pressure between 0-3000 kPa (435 PSI). It is directly connected to the rear differential clutch control module which supplies it with a pulse width modulated voltage. The rear differential control module activates the output to the differential clutch pump as soon as the the engine started. The output is active until the calculated pressure is approximately 3200 kPa (464 PSI). The pressure is calculated by the rear differential control module by measuring the differential clutch pump electric motor power consumption. The higher the pressure, the greater the resistance is for the electric motor to drive the pump therefore the current required increases. When the system pressure is approximately 3200 kPa (464 PSI), the rear drive clutch solenoid valve bleeds off the excess pressure so that the pressure does not increase any further. The current to the motor does not increase any further but remains constant. When the maximum system pressure is reached, the rear differential control module shuts down the pump. Restart takes place when the calculated pressure drops below approx. 2700 kPa (392 PSI).

Rear Differential Clutch Control Module

The rear differential clutch control module is fitted on the housing. The hydraulic pressure is built up by the differential clutch pump, and is controlled by the rear differential clutch control module via the rear drive clutch solenoid valve. Utilizing information on the serial data bus, the rear differential clutch control module calculates the drive torque distribution between front and rear axles as well as the distribution between the rear wheels for vehicles with limited slip differential clutch. The rear differential clutch control module is connected to the vehicle's high speed communication bus. The serial data messages are used to determine the operating/driving conditions for the vehicle and how much drive torque should be transferred to the rear wheels. Since the rear differential clutch control module and the rear drive clutch solenoid valve are calibrated as a matched set, they must be replaced together without exception.

The rear differential clutch control module is constantly monitoring for bus communication, inputs, outputs, certain mechanical faults and internal control module faults. If relevant control module functions are missing or other mechanical or electrical faults are recognized, diagnostic trouble codes are generated.


• Internal signals

The control module has a monitoring for internal control module faults.

• Input signals

The control module has an input for an internal temperature sensor. The temperature sensor is fitted on the rear differential control module circuit board and is monitored for electrical faults. The temperature sensor is also used by the rear differential control module to calculate the current fluid and disc pack temperature in the differential clutch.

• Output signals

The rear differential clutch control module has outputs for the rear drive clutch solenoid valve and the rear differential clutch solenoid valve for the limited slip differential clutch (if equipped) as well as for the differential clutch pump. These are all monitored for electrical faults.

• Mechanical functions

The control module can diagnose clogged pressure control valves and leaking check valves.