Testing a vacuum: operated solenoid valve
This issue, Damien is showing how a methodical process will mean a successful diagnosis on an actuator-related problem
Published: 29 April, 2022
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.
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.
- Diesel diagnostics for the workshop
I’m mindful of several recent diagnostic topics that focused on cutting edge opportunities such as noise and vibration analysis. It also reminded me of the most important aspects of fault finding; to focus on the symptoms, ask relevant questions and conduct a methodical approach based on systems knowledge, accurate data and a proven process.
All of this really boils down to training, experience, and confidence. There are no short cuts, cheap fixes or internet gurus. There are however basic steps that are easily introduced into your workshop procedures.
This brings me to the topic in hand. Can we conduct relativity simple tests on common rail diesel systems? Not only can we, but we must! Remember, the foundation rule of fault finding is a simple methodical approach. Don’t expect a magical fix-all in less than 1,000 words. However, I can provide a pathway that will illustrate the area of responsibility and potential investment in time and money.
Vital information
The first vital step is to listen and ask questions. Owners often have vital information. Remember this is not a recipe for short cuts or silver bullets for your machine gun. Your approach will always depend on the extent of problems. Will it run? are there any mechanical noises? Is there a loss of power? if so when? Is the fault intermittent and how did it start? There is an endless list of questions that will help establish a hidden history.
I often find that a physical examination or health check helps understand the way the vehicle has been driven and serviced. This will often expose basic problems especially with charge pressure circuits.
Try to explore all non-intrusive tests first. They may not be entirely logical in order of priority, but do provide results in the minimum time period. With experience, you will hone these steps into a razor-sharp intuitive process.
Serial investigation
Serial investigation is without doubt the correct first step. Do not jump to premature conclusions as serial data often shows symptoms, not cause. For example, a faulty air mass meter will cause EGR calculation error values, incorrect load and boost calculation. This is a common problem with many causes.
The volumetric efficiency relies on the intake system, swirl flap control, turbo spooling, and a free-flowing exhaust system. Please note that I keep my thoughts non-specific yet focused on all possible causes. This is a very important reaction in any diagnostic process.
Assuming a non-run condition, excluding any serial clues as often there are none, I would always check for the correct rail pressure. This can be done with a DMM. Expect around 1-1.5v with a quick rise time of 0.5-1sec. If it is slow to rise or low, check the priming system including the filter. This should be done with a gauge. Remember pressure, flow and pump current. This will depend on system type so check the schematics carefully. Most systems now prime at 5-6bar.
Isolate components
A slow rise time may be due to an internal leak or worn components within the high-pressure system. This includes the HP pump, rail limit valves, and injectors, as well as volume and pressure regulation devices. Always isolate various components and conduct a blind or proof test before suspecting the pump. They rarely fail, unless run dry or have contaminated fuel.
The PCM requires camshaft position data to sync the injectors and crank position once running. If recent belt replacement or engine repairs have been carried out, add this to your list. To check the injector sync against cam and crank position is a bit technical. To perform you will require a scope and current clamp.
Quite often the serial data identifies the incorrect timing sensor for position error. This is due to the PCM looking at the camshaft first. Slow rotation speed may be due to a faulty or incorrect battery, so check charge and health status with a suitable conductance tester. Yuasa have a fantastic free online training academy.
Next check relative compression. This is a simple cylinder balance check but when compared with current and rotation calculation will accurately predict correct compression.
Identify
A blocked exhaust or failed open EGR will prevent the correct combustion properties. Exhaust back pressure can easily be proven from the map and DPF pressure sensors. Plotting them with a scope will quickly identify intake or exhaust restrictions. The maximum DPF sensor value cranking or at idle should be 0.5-1.25 volts, 100mbar-1.5psi.
Injector type, solenoid or piezo faults will normally be identified within serial data. A single faulty injector circuit will normally shut down all fuel delivery. It is also worth noting that if a minimum rail pressure is not reached, the injectors will not be activated.
So back to priming. Leaks, faulty rail sensors will all contribute to a non-start.
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