Diagram of sensors attached to Haldex Controller

 Sensor Is the error Shown on VAG COM 'Auto-Scan' ? What's the error code?
Engine Speed Sensor Yes  
Acceleration / Throttle  Position Sensor Yes  
Wheel Speed Sensors (x4) Yes  
Longitudinal Acceleration Sensor Yes  
Brake Light Switch Yes Address 22 AWD : Brake Light Switch - 00526
Handbrake Switch No N/A

 

 Sensor  Where is it, What does it do, What happens if it fails?
Engine Speed Sensor This sensor is installed near the oil filter on the left-hand side of the engine. The sensor records the exact angular position of the crankshaft to determine the ignition and injection point, as well as engine speed. If the engine speed signal supplied by the engine sensor fails, the engine will not start or run.
Acceleration / Throttle  Position Sensor This sensor sends a signal corresponding to the accelerator pedal position. The Haldex control module uses this signal in combination with other signals to determine when and how much power should be applied to the rear axle. There are two sensors, one acts as a backup. If one of the two fails, the warning lamp on the instrument panel will illuminate and the vehicle will enter emergency running mode.
Wheel Speed Sensors (x4) This sensor detects the change in speed of the wheel and sends this information to the ABS control module, this information is then sent to the Haldex control module. The wheel speed sensor is mounted in the vicinity of the axle flange. If this signal fails then the ABS control will fail (as well as the 4WD control).

To access these sensors using VAG COM, go to ABS(03), measuring block(08), channel(001). After 4mph all four wheel speeds should read the same speed, if not then you know which sensor to check out.
Longitudinal Acceleration Sensor This sensor is attached to the right side of the A pillar. The value is used to verify the calculated road speed. Without this value, it is not possible to determine the true vehicle road speed exactly in unfavourable conditions. As a result, the electronic stability program (ESP) and anti-slip regulation system (ASR) will not operate.
Brake Light Switch This sensor is located at the upper end of the brake pedal and is secured to the pedal support. This sensor sends the 'brake activated' signal to the ABS control module which then informs the Haldex control module. When the brake is applied, the Haldex control module immediately opens the pressure regulator via the positioning motor and the Haldex coupling clutch is opened. If this signal fails then the information provided by the CAN bus is used as an alternative.
Handbrake Switch This sensor is located under the hand brake lever. The handbrake switch sends the 'handbrake engaged' signal to the ABS control module and simultaneously to the Haldex control module. Whereas the ABS control module transfers this information to the Haldex control unit in 'filtered' form along the CAN bus, the Haldex control module also receives the information directly from the handbrake switch. If the signal generated by the handbrake switch is picked up, the Haldex coupling clutch is opened. If the switch remains closed, then no all-wheel-drive control is available and restrictions are placed on the ABD control.

 

Info on Haldex Coupling Temperature Sensor

The Haldex coupling temperature sensor is installed near the regulating valve in the Haldex control module housing and is immersed in hydraulic fluid. This sensor senses the momentary hydraulic oil temperature and sends this information to the Haldex control module. This information is used for adapting to the changing viscosity of the hydraulic fluid.

If the hydraulic fluid temperature exceeds 100oC, the clutch is released. If the temperature of the hydraulic fluid drops below 100oC, the clutch is again pressurised. In the event of signal failure, the 4WD is shut off.
Temperature Hydraulic Fluid / Viscosity Regulating Valve
In the minus range High Viscosity Slightly more open
Normally 20oC Normal Normally open
Over 20oC Low Viscosity Slightly less open

 

Info on Haldex Control Module
The Haldex control module is mounted directly on the housing of the Haldex coupling and combines with the positioning motor and the regulating valve to form the Haldex Control unit. The Haldex coupling control module is connected to the engine and the ABS control unit via the CAN bus. From the signals that are generated the Haldex control module decides what oil pressure to apply to the plates of the Haldex coupling clutch. The oil pressure acting on the plates determines what torque is to be transmitted to the rear.

The VAG COM Address to access the Haldex Control Module is 22

 

Info on Motronic Engine Control Module (ECM)
The Motronic engine control module (ECM) is mounted in different areas on the various vehicles but is normally accommodated in the plenum chamber. The operating mode of the Motronic ECM is torque-oriented. The ECM provides the following 3 signals to the Haldex control module: Engine speed signal, Accelerator pedal position & Engine torque. If this unit fails the Haldex unit will not operate.
The VAG COM Error Code Address to show a fault in the ECM is

Address 39 Right Light : Internal Control Module Memory Error - 65535

 

Info on ABS Control Module
The anti-lock brake system (ABS) control module is combined with the hydraulic unit as a module and mounted in the engine compartment on the left-hand side. When the ignition is turned on, the ground control modules carry out a self-test. The control module consists of two processor systems. This ensures a high level of fail-safety. In addition to mounting individual components, the two processor systems monitor each other. The following 4 signals are supplied to the Haldex control module from the ABS Control Module: Wheel speed sensor, Brake light switch, Handbrake switch & Longitudinal acceleration sensor. In the even of total failure of the control module, the Haldex unit will not function properly. Note, if the vehicle also has the electronic stabilisation program (ESP), then ESP control takes precedence over the all-wheel drive function.
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