The pick-up ring references below relate to a number of possible components:
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This test evaluates the operation of an inductive Crankshaft Position Sensor (CPS) with an earth return circuit (non-floating ground).
Note: The correct operation of the inductive CPS depends on the integrity of the CPS internal coil windings, the CPS circuit, the fitment of the CPS in relation to the pick-up and the air gap between the CPS and pick-up ring.
The test procedure below assumes the conditions mentioned above are all in order and the CPS is functioning correctly. Any failures identified with the operation of the CPS while conducting these tests does not necessarily indicate a fault with the CPS itself.
The CPS will display operational characteristics that are inconsistent due to circuit faults, electromagnetic interference, mechanical failures (pick-up ring) or measurement/connection errors. The results obtained are therefore symptoms of underlying conditions and not the result of a faulty CPS. It is therefore paramount to carry out a basic inspection of all the items above before measurements are taken to prevent incorrect diagnosis of the CPS.
All numerical readings quoted in this help topic are typical and not applicable to all engine types.
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Accessories |
PicoScope settings
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Using the vehicle wiring diagram, identify the signal wire of the CPS.
The inductive type CPS will generally utilise two wires, one of which carries the signal during cranking and the other, zero volts.
All values included in the Example waveforms are typical and not applicable to all vehicle types.
Channel A. Figure 2 (example waveform 1) indicates the AC voltage output of the CPS while cranking at 50 ms/div to display 1 full crankshaft revolution on one screen.
Channel A. Figure 3 (example waveform 2) indicates the AC voltage output of the CPS while cranking at 20 ms/div to highlight individual teeth and the missing tooth/teeth of the pick-up ring.
Refer to vehicle technical data for specific test conditions and results.
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① |
Signal ruler at zero volts should pass through the centre of the CPS waveform. |
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② |
Indicates the missing teeth from the CPS pick-up ring, often mistaken for TDC (see Diagnosis notes below). |
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③ |
Minimum amplitude of the CPS output signal voltage due to reduction in cranking speed as a result of the compression stroke (see More information below). |
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④ |
Maximum amplitude of the CPS output signal voltage due to increase in cranking speed as a result of the expansion stroke (see More information below). |
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⑤ |
Time, and rotation rulers denote one revolution of the crankshaft between the missing teeth of the pick-up ring (35 teeth in one crankshaft revolution with one missing tooth denoting crankshaft position). |
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⑥ |
Indicates the cranking speed of the engine based upon the position of the time rulers ⑤. Time rulers placed at subsequent missing teeth of the pick-up ring allow PicoScope to calculate the frequency of 1 full cycle (revolution) of the crankshaft. Both the frequency and more importantly, the RPM are then displayed at point ⑥. |
The CPS is one of the fundamental components of the modern engine management system. While beautifully simple in construction, the correct operation of the CPS is essential to an efficiently running engine. To fully appreciate the role played by the inductive CPS we must understand the operational principle behind the AC voltage generated by such a critical component. Once we understand how this component functions, we can evaluate the cause of any errors within our waveform.
The inductive CPS consists of two essential components: a wire coil wound about a permanent magnet.
Figure 4 - CPS Operating principle
Each end of the wire coil terminates at the PCM where the generated AC output signal voltage (+) is utilised by the PCM to indicate crankshaft speed and position, while the remaining coil wire (–) is connected to ground (see Figure 4).
The permanent magnet contained with the CPS naturally produces a magnetic field about the coil windings. Should a metallic object be introduced into this magnetic field (in the form of our pick-up ring) the intensity of the magnetic field will vary and either increase or decrease depending on the speed and direction of travel of the pick-up ring (see Figure 5). The variation in the magnetic field has the desired effect of inducing an AC voltage into the coil winding that can be utilised by the PCM to derive engine speed and position.
N.B The variation in the magnetic field is solely responsible for inducing an AC voltage into the coil windings. Should the pick-up ring come to a halt, no voltage is generated regardless of the position of the pick-up ring in relation to the CPS.
Figure 5
Figure 6
The CPS waveform will include a dropout in the signal where the pick-up ring has a deliberate gap (missing tooth or teeth) in the even spacing between the teeth (see Figure 6 and Figure 3).
The PCM uses this dropout in the signal to identify the position of the crankshaft which may or may not refer to TDC. Manufacturers use the missing teeth from the pick-up ring to indicate a variety of crankshaft positions. Such as: Pistons in line, (engine safe position) TDC, the No. of degrees before TDC, or they may choose a combination of missing teeth at 90 degree intervals. For accurate evaluation of the crankshaft position reference signal refer to the relevant workshop manual.
Crankshaft speed calculation is based on the frequency of the AC output signal from the CPS. As the speed of the crankshaft increases the frequency of the CPS output signal increases in direct proportion. The amplitude of the signal also increases with engine speed, with amplitudes above 20 V AC at higher engine speeds (see Figure 7).
Figure 7
The rise and fall in the frequency/amplitude can be seen in both Figure 2 and Figure 3 where the load placed on the crankshaft during the compression stroke results in a momentary reduction in crankshaft speed ③. Here we can see a decrease in frequency and a fall in the amplitude in direct contrast to ④ where the crankshaft will momentarily increases in speed just after TDC compression (expansion stroke) resulting in an increase in frequency and a rise in amplitude.
The rise and fall in both frequency and amplitude can be used as an indication on the condition of engine compression. Assuming all engine compressions are equal, we should obtain a uniform frequency and amplitude in the CPS signal with a constant cranking speed.
When the complaint concerns poor starting or prolonged cranking, it is vital to consider the cranking speed input to the PCM.
Cranking speed can be calculated from the CPS signal by placing the time rulers at each consecutive dropout points in the captured signal (missing tooth/teeth) see Figure 2 ②.
Most pick-up rings contain 36 or 60 teeth with 1 or 2 missing teeth at a single point about their circumference. Placing the time rulers directly at the dropout points will indicate 1 x crankshaft revolution. In the Example Waveforms above we have 35 teeth between the time rulers, so indicting a pick-up ring with 35 teeth about the circumference with 1 missing tooth for engine position reference (36 teeth − 1 tooth = 1 x crankshaft revolution).
Placing a time ruler at consecutive dropout points within the waveform allows PicoScope to calculate the cranking speed based upon the frequency of the crankshaft (cycles per second).
In Figure 2 each time ruler is positioned at the point of the missing teeth ② (one crankshaft revolution) where the frequency between the time rulers is calculated at 4.022 Hz in the frequency legend ⑥.
In order to convert Frequency to RPM we simply multiply 4.022 Hz by 60 to obtain 241.3 RPM. Both the frequency and RPM value of the crankshaft are displayed in the frequency legend ⑥.
Crankshaft position sensor installed and disconnected from the vehicle harness
The resistance value of the wire coil wound around the permanent magnet provides an indication of the integrity of the CPS and is essential to the correct output voltage/signal. Incorrect resistance values or a short circuit to vehicle ground will result in output signal failure.
Typical CPS wire coil resistance values vary between manufacturers. Refer to the relevant workshop manual for the correct specifications.
In order to test the resistance value of the CPS, disconnect the sensor from the vehicle harness and connect an ohmmeter as illustrated in Figures 8 below:
Figure 8
Figure 9
While the CPS resistance may be in specification, a check for a short circuit to ground can also be performed very quickly at the same time as the resistance check. Figure 9 indicates how the ohmmeter is connected to confirm no short circuit to vehicle ground is present. A value greater than 10 kilohms is sufficient to confirm no short circuit exists (an infinite or open circuit resistance value is ideal).
Crankshaft position sensor installed and connected to the vehicle harness
Depending on manufacturer a bias voltage may be present on the signal wire from the CPS. The bias voltage is provided by the PCM and is used as an aid to diagnose CPS circuit errors and noise reduction (interference from neighbouring electrical components).
In order to measure the bias voltage, connect PicoScope exactly as mentioned above in How to perform the test (Figure 1) and follow Steps 1 through 5, followed by:
Figure 10
If the waveform indicates a fault, make a physical inspection of the CPS. The list below highlights some key areas for inspection.
AT017-3
Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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oscar Carranza
October 28 2015
excellent information source
