Aftermarket March 2022

32 AFTERMARKET MARCH 2022 TECHNICAL/SNAP-ON www.aftermarketonline.net vary the output voltage depending on pressure measured at the sensor. The waveforms below the system layout show the voltage control (ground side) and current flow through the solenoid. This will be expanded upon later in the article. The image on the right shows the voltage from the boost pressure sensor under snap throttle conditions. Visual inspection An initial visual inspection can be used to observe the wastegate actuator on engine start-up. Fig.2 shows the rod fully extended with the key on and engine off. This is the minimum boost position. Should the electrical or pneumatic system fail, the turbocharger will return to this position to protect the engine from an overboost condition. When the engine starts, a vacuum is created by the engine vacuum pump. The boost pressure control solenoid is actuated by the ECM which closes the wastegate to allow the turbocharger to generate boost. It must be noted that the pressure in the intake manifold will equal athmospheric pressure at idle. As the engine speed increases, the turbocharger turbine will increase in speed due to greater exhaust gas flow. The impeller will also increase in speed to pressurise the air in the intake manifold. The turbine and impeller wheels are connected via a common shaft. To see it with the key on, and the engine at idle, please refer to Fig.3. Live data will display absolute pressure as opposed to gauge pressure. See example below: Data parameter Idle Wide open throttle Intake manifold pressure 1020mBar 2210mBar Barometric pressure 1013mBar 1013mBa r Turbo boost control solenoid valve The solenoid valve used to control the turbocharger is a three-way, normally closed valve. To see the turbo boost control solenoid valve, refer to Fig.4 There are three ports, although only two may be easily identifiable. There will be a supply (vac-in) from the vacuum pump, output to the turbocharger (vac-out) and an exhaust, which can sometimes be connected to the intake air filter housing. The purpose of the exhaust is to bleed atmospheric air pressure into the vacuum circuit to open the wastegate and control the boost pressure. To see a diagram of a three- way normally closed valve, please refer to Fig.5. Waveforms The waveform as seen in Fig.6 shows both the electrical and pneumatic operation of the solenoid valve upon engine start-up. The pressure transducer as seen in Fig.7 was connected between the solenoid valve and the turbocharger to sample the actual vacuum. Engine start-up Yellow channel: Solenoid valve duty cycle Green channel: Boost pressure solenoid voltage Blue channel: Solenoid valve current flow Red channel: Actual vacuum The waveform shows the duty cycle control of the solenoid valve increase to 90% when the engine starts. This results in a current flow of 0.87 amps and a vacuum of 13.5 inches of mercury (460 mBar) applied to the turbocharger actuator. The voltage on the boost pressure sensor signal wire is 1.6 volts at zero boost pressure. In Fig.8, we can see the vacuum deplete and increase after several applications of the brake pedal. The brake servo requires a large vacuum to operate and this can affect the overall vacuum system, however a good vacuum pump can re-instate the required vacuum quickly. Fig.9 shows the system under wide-open throttle conditions. As the boost pressure increases, the duty cycle control of the solenoid valve is reduced which causes the vacuum applied to the turbocharger actuator to reduce. Once the boost pressure stabilises after over-run, the duty cycle again increases. Fig.10 shows the Duty Cycle control of the solenoid. As the solenoid is supplied (electrically) with a constant supply, the current flow is controlled by varying the duty cycle on the ground side of the actuator. The current flow (blue trace) increases when the voltage on the ground side (yellow trace) is 0 volts. As the ground circuit is opened the current flow decreases. The duty cycle can be estimated by looking at the average voltage on the ground circuit and comparing it to the applied voltage. A lower voltage means a larger duty cycle. Testing the electrical and pneumatic operation of a vacuum- operated valve can be beneficial when troubleshooting issues such as over and under boost concerns ” NEXT ISSUE: In a direct follow- up to this article, Damien will be looking at the operation of motor control