Today almost every modern diesel and also petrol engines are equipped with an exhaust gas turbocharger to increase engine performance and efficiency. To ensure that the optimum boost pressure is always present, it has to be adjusted to suit the particular load condition. In modern vehicles, this is done by the electronic boost pressure control.
The turbocharger uses the energy of the exhaust gases to draw in and compress fresh air for combustion. This allows a greater volume of air and thus more oxygen to enter the combustion chamber. Engine performance and engine torque are subsequently increased. Simply explained, the exhaust gas turbocharger contains an exhaust gas turbine and a compressor turbine, which are connected to each other by a shaft. The exhaust gas flowing out of the engine drives the exhaust gas turbine and thus also the compressor turbine.
So as to adapt the boost pressure to suit any given prevailing load condition and also to protect the engine and turbocharger, a boost pressure control or regulator is required. Depending on the type of turbocharger in question, a mechanical-pneumatic control device or alternatively an electromechanical one can be used. In the further course of the chapter, we will primarily deal with electromechanical control.
As part of troubleshooting, a visual inspection of the turbocharger in the engine compartment should first be carried out after the ECU diagnosis.
The turbocharger with its individual components should always be considered and diagnosed as one unit. Most vehicle manufacturers do not supply spare parts for VNT/VTG turbochargers. This is not because the technicians in the workshop are not trusted to replace or rework individual components, but because the turbocharger and the electromechanical actuator have to be precisely matched to each other before installation in the vehicle. This calibration is carried out in the dismantled state on a flow bench for turbochargers. On this special test bench, the vehicle-specific flow rate (MIN/MAX flow) is determined and set as part of a basic adjustment. For this purpose, depending on the model, it may be necessary for some actuators, prior to calibration, to be first activated using a special map so that they can then be recognised in the vehicle by the engine control unit
The following diagnostic information is given as an example using a Mercedes-Benz E350 24V CDI (212) from the year 2014.
The function of the turbocharger actuator is monitored by the respective higher-level system control unit. Any occurring errors are stored in the engine control unit's error memory and can be read using a suitable diagnostic unit. Depending on the vehicle and on the system, additional functions such as parameters or actuator tests can be selected and displayed in the diagnostic device. The data from control unit communication forms the basis for actual troubleshooting and for successful repair work.
In this function the error codes stored in the error memory can be read out and deleted. In addition, information on the error code can be called up.
In our case study, the electrical plug connection on the turbocharger actuator was disconnected and consequently the error code P004500 was stored in the error memory.
P004500 / charge limiting flap
> electrical fault or interruption
In this function, current measured values such as atmospheric pressure and boost pressure can be selected and displayed.
In this function, the stop values of the new turbocharger unit can be programmed into the higher-level control unit.
In this function, the electromechanical turbocharger actuator can be controlled by the diagnostic device. This allows a functional check of the periphery and of the relevant system components to be carried out without much dismantling work.
System-specific circuit diagrams can be taken from vehicle information and used for troubleshooting purposes. Here, for example, the PIN assignment on the turbocharger actuator can be read and used for further troubleshooting.
