BMW X5: Load Control

Load control of the N54 engine is effected by means of the throttle valve and the waste gate valves.

The throttle valve is the primary component in this process. The wastegate valves are actuated to bring about a fine tuning of the boost pressure. At full load the throttle valve is completely open and load control is undertaken by the wastegate valves.

Fig. 43: Identifying Load Control Operations Diagram

Controlled Variables

The following variables, among others, influence control of the N54 engine's boost pressure:

• Intake-air temperature
• Engine speed
• Throttle-valve position
• Ambient pressure
• Intake-manifold pressure
• Pressure before the throttle valve (reference variable)

The electropneumatic pressure transducers are activated by the engine control unit on the basis of these variables. The result of this activation can be checked from the boost pressure achieved, which is measured before the throttle valve.

There follows a comparison of the boost pressure achieved with the set point data from the program map, which can if necessary give rise to an activation correction. The system therefore controls and monitors itself during operation.

Limp-home Mode

In the event during operation of malfunctions, implausible values or failure of any of the sensors involved in turbocharger control, activation of the wastegate valves is shut down and the valve flaps are thus fully opened.

Turbocharging ceases at this point.

The list below sets out those components or functional groups of the N54 engine in which a failure, a malfunction or implausible values result in boost-pressure control being deactivated.

The driver is alerted to a fault of this type via an EML indication.

• High-pressure fuel system
• Inlet VANOS
• Exhaust VANOS
• Crankshaft sensor
• Camshaft sensor
• Boost-pressure sensor
• Knock sensors
• Intake-air temperature sensor

One principle of vehicle repair is particularly important in this respect: It is important to focus on the causes rather than the effects.

With regard to diagnosis of the turbocharging system, always check the basics first. Look at such items as the vacuum hoses, the solenoids and the vacuum reservoir.

Don't overlook the fundamentals. Check compression, fuel system pressure (low and high), secondary ignition components etc.

The turbochargers are usually the last item to fail, but often one of the first items to be replaced.

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