Diagnostics of modern automotive engines is impossible without a deep understanding of the processes occurring inside the cylinders and the precise operation of control systems. A key tool for the master in this field is a motor tester, which allows visualizing electrical signals from sensors of the position of the crankshaft and camshaft. It is the shape of the signal, its amplitude and time characteristics that allow us to judge the technical condition of the nodes without disassembling the engine.
Many novice diagnosticians make the mistake of relying solely on error codes read by the scanner through an OBD connector. However, standard The device is able to tell about the problem long before the ECU (electronic control unit) decides to light the Check Engine lamp. Understanding the physics of signal formation opens up access to a level of diagnostics that is not available with simple error readers.
In this article, we will discuss in detail what the perfect signal looks like, what anomalies indicate faults, and how to interpret the data correctly. You will learn to distinguish software failures from mechanical damage to the cog crowns and wiring breaks, which will significantly reduce the time for finding a defect.
Physics of the process: how the signal is formed sensors
For the correct interpretation of the oscillogram, it is necessary to understand the principle of the sensors. In the automotive industry, two types are most common: inductive (magnetic-strictive) sensors and Hall sensors. Inductive sensors, often installed on DPC (sensor position crankshaft), generate variable voltage, the amplitude of which depends on the speed of rotation of the rotor.
The principle of operation is based on the law of electromagnetic induction: when a metal tooth passes the magnetic core of the sensor, the magnetic flux changes, causing the appearance of EMF. The faster the shaft rotates, the higher the frequency and amplitude of the signal. That is why when starting the engine by the starter, the amplitude of the WPC signal can be low, and at high revs, reach tens of volts.
Hall sensors that are more common as DPR (Sensor of the position of the camshaft), work differently. They require external power supply and produce a rectangular signal that changes its potential from 0V to the voltage of the onboard network (or 5V). The shape of the signal here does not depend on the speed of rotation, which makes them more stable at low revs, but sensitive to the quality of power and "ground".
⚠️ Note: When diagnosing inductive sensors, never test them with a conventional multimeter in resistance measurement mode. This will only give information about the integrity of the winding, but it will not say anything about the sensor’s ability to generate a signal when it is rotated.
Why does the signal amplitude drop at high speeds?
Inductive sensors have their own inductance and capacity. At high frequencies (high revs), the reactive resistance of the winding increases, which can lead to distortion of the signal shape and a decrease in the amplitude if the impedance of the oscilloscope input is selected incorrectly.
Analysis of the WPC reference oscillogram
The reference oscillogram of DPC is a sinusoidal or close to it signal, following with a certain frequency. Engines with a 60-2 cog (58 teeth and 2 missing teeth) are characterized by 58 full pulses and one long pause interval corresponding to the missing teeth. This pass is the reference point for ECU synchronization.
When analyzing the signal, it is necessary to pay attention to the uniformity of the amplitude. All teeth should generate pulses of approximately the same height. If you observe a “failure” of amplitude on a certain tooth, this may indicate mechanical damage to the flywheel or pulley crown, as well as the presence of metal shavings on the end of the sensor.
The most important parameter is the absence of "shaking" of the signal front. Sharp voltage surges or additional emissions on the sine wave indicate tips from high-voltage wires or malfunction in the supply chain of the ignition coils. Clean signal without noise is the key to accurate calculation of the angle of advance of ignition by the control unit.
The key sign of serviceability of the DPC is the stability of the signal shape when changing engine speeds and the absence of parasitic emissions in the area of missing teeth.
Diagnosis of DPRV and phase synchronization
The sensor of the camshaft position serves for phasing injection and determining the engine stroke. Its signal is usually a rectangular pulse. The reference oscillogram of the DPRV should have clear vertical fronts and a stable level of logical "0" and logical "1".
The most important stage of diagnosis is the analysis of synchronization of DPCV and DPRV. ECU compares the position of missing teeth on the crankshaft with the pulse from the camshaft to determine in which cylinder the compression stroke occurs. On the oscillogram, this is seen as a shift in the momentum of the DPRV relative to the group of DDPV prongs.
If the angle between the signals floats or the DPRV pulse "floats" relative to the WPCV reference tooth, this is a direct sign of stretching the GRM chain or jumping the belt. Even if the scanner does not show error in phases, the oscilloscope will detect desynchronization in real time.
Below is a table of typical signal parameters for different types of sensors:
| Type of sensor | Signal shape | Amplitude (start) | Amplitude (idle) |
|---|---|---|---|
| Inductive DPC | sinusoid | 1.5 - 3.0 V | 4.0 - 15.0 B |
| Hall sensor (DPRV) | Meander (rectangular) | 0 - 5.0 V | 0 - 5.0 V / 12B |
| Optical DWR | rectangular | 0 - 5.0 V | 0 - 5.0 V |
Typical malfunctions and their reflection on the oscillogram
Wiring defects are often masked as a faulty sensor itself. A break in the “ground” circuit will cause you to see a signal with a floating “zero” line on the oscillogram or a complete loss of signal when powerful consumers are turned on (for example, headlights or a fan).
Short circuit of turns inside the winding of the inductive sensor will lead to a sharp decrease in the signal amplitude. In this case, the ECU may lose synchronization with a sharp set of revolutions, when the signal frequency increases, and the pulse energy is insufficient to overcome the threshold of sensitivity of the input cascade.
Mechanical backlash of the sensor itself or its improper installation (too large gap to the crown) will cause instability of amplitude. The gap is a critical parameter: too much distance will weaken the signal, and too little can lead to physical contact and sensor destruction.
Use the function "Trigger" on the oscilloscope, configuring it on the front of missing teeth. This will allow you to "freeze" the picture and consider in detail the shape of each tooth in the static.
Method of troubleshooting: step-by-step algorithm
The diagnostic process should be systematic. First, visually inspect the connectors for oxidation and integrity of the wire harness. Then connect the motor tester and start the engine, comparing the resulting picture with the standard. If there is a signal but the engine is not starting, check the synchronization.
If the signal is missing completely, check for power on the Hall sensor connector or the resistance of the inductive sensor winding. Don’t forget to check the “mass” – often the problem lies in the poor contact of the sub-conductor wire.
If there is a signal, but unstable operation of the engine, use the oscillogram accumulation mode. This will allow you to see rare failures that occur, for example, only when the engine is heated or when vibration at certain speeds.
☑️ Algorithm of check of the sensor circuit
Effects of tips and interference on the signal
The electric car is full of interference. High-voltage discharges in the ignition system, the operation of the generator and the pulsed currents of the nozzles create powerful electromagnetic fields. These tips can be superimposed on a useful sensor signal, creating false pulses.
On the oscillogram, this looks like "noise" or "needles" on the tops of the sinusoid or on the flat areas of the meander. If the interference is large, the ECU can count an extra spike for the passage of the tooth, which will lead to an erroneous calculation of the position of the shaft and the engine twitching.
To combat this, screen diagnostic probes and use a short ground. Sometimes it helps to install ferrite rings on the sensor wires, if the problem is massive for a particular car model.
⚠️ Warning: Do not place sensor wiring harnesses in the immediate vicinity of high-voltage spark plug wires. The minimum distance should be 5-7 cm to avoid inductive tips.
FAQ: Frequently Asked Questions
Can the Hall sensor be checked with a conventional tester?
Yes, you can check for food and "mass." To check the signal at the output of the tester in the voltmeter mode, you need to slowly rotate the shaft (manual) and watch the voltage surges, but the oscilloscope will give a much more accurate picture of the signal shape.
What does it mean if the amplitude of the DPC falls when heated?
This is a classic sign of a defect in the winding of an inductive sensor. When heated, the resistance of the winding changes, or a micro-break occurs inside the coil, which leads to loss of signal and stop the engine "hot".
How often should I change the DPA and DPA?
These sensors do not have a replacement. They change only when they are in trouble. However, when replacing a belt or chain of HRM, it is recommended to diagnose them, since access to them is often open.
Why does the scanner not see the error, but the engine is twitching?
The scanner reads errors stored in the ECU memory. If the desynchronization or signal omission occurs briefly and does not meet the criteria for recording the error (for example, less than 2 seconds), the lamp will not light up, but the oscilloscope will show a failure in real time.
Can stretching of the GRM chain affect the WPC signal?
The circuit itself does not affect the WPC signal, as it is on a knee. But it affects the DPRV signal and, most importantly, the mutual position of the DDPV and DPRV signals. It is the shift of the phase of the DPRV relative to the DDPV that will indicate the stretching of the chain.