25 May 2024
The voice of the independent garage sector

Testing a vacuum: operated solenoid valve

Actuators on modern vehicles can be difficult to diagnose, with many technicians resorting to replacement rather than diagnosing the component correctly. However, by using a methodical fault-finding process, we can quickly and accurately diagnose these components. Actuators can be controlled by a vacuum solenoid or a reverse polarity DC motor. In the next issue we will investigate the operation of motor control.  
Getting back to today, in this issue we will test a vacuum operated solenoid used for controlling the wastegate on a turbocharger fitted to a 1.5L K9K Renault engine. This is a compression ignition engine, so the intake manifold pressure never becomes negative (vacuum) due to the absence of a conventional intake throttle. These engines due use a throttle valve to manipulate intake manifold pressure for exhaust gas recirculation (EGR), diesel particulate filter (DPF) regeneration and shutting the engine off smoothly. Testing the electrical and pneumatic operation of a vacuum-operated valve can be beneficial when troubleshooting issues such as over and under boost concerns.

System operation
Fig.1 shows the electrical operation of the turbocharger. The diagram illustrates the ‘command’ and ‘feedback’ part of the circuit. The turbo boost control solenoid has a constant supply (system voltage) from the engine bay fusebox and is controlled on the ground side by the engine control module (ECM).
Closed loop control is via the turbo boost pressure sensor. This component has a constant 5-volt supply and ground from the ECM. The signal wire has a 5-volt biased voltage from the engine control module which is manipulated by an integrated circuit, within the sensor, to 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           1013mBar

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.

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.