Crankshaft sensor inductive (floating) voltage during cranking

The purpose of this test is to check the signal from an inductive Crankshaft Position (CKP) sensor, with no reference voltage, during engine cranking.

How to perform the test

View connection guidance notes.

1. Connect PicoScope Channel A and Channel B to the CKP sensor circuits.
2. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
3. Start the scope to see live data.
4. Crank the engine for around 3 seconds to capture your waveforms.
5. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveforms.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

• Channel B mirrors Channel A.
• There is no excessive noise, or any inconsistent breaks in either waveform.
• At the start of cranking, there is an almost immediate response to crankshaft rotation.
• As engine speed increases, amplitude and frequency increase until normal cranking speed is reached.
• The waveform shows a cyclic variation in engine speed indicating the effect of the 4-stroke engine cycle: compression strokes decrease the engine speed, whereas expansion strokes increase the engine speed.
• The oscillations are punctuated by a periodic gap caused by the timing reference mark, which indicates a fixed position within each crankshaft rotation.
• The engine reaches its cranking speed within about one turn of the crankshaft.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Crankshaft sensor (inductive).

Further guidance

A CKP sensor provides an Engine Control Module (ECM) with its primary engine timing reference signal. The ECM uses it to calculate the engine speed and position for accurate injection and ignition control. The signal is also used to detect engine speed anomalies from misfires etc.

An inductive CKP sensor consists of a circuit with a wire coiled around a magnet. The sensor is accompanied by a pulse wheel, typically arranged about the flywheel circumference. The pulse wheel passes through and disturbs the sensor magnetic field inducing a circuit voltage. The induced voltage depends on engine speed: the faster the pulse wheel rotates, the greater the magnetic field disturbance.

When either the tooth or gap centres align with the sensor, there is an equal and opposite magnetic field disturbance and no voltage is induced. Conversely, as either a tooth leading or trailing edge aligns with the sensor, the magnetic field disturbance and induced voltage are greatest.

Positive voltage is produced when a tooth leading edge is closer than its trailing edge, and a negative voltage is produced in the opposite case.

The missing tooth on the pulse wheel provides the main timing reference mark. As the gap passes through the magnetic field, there is a period of reduced disturbance and voltage. Furthermore, the trailing and leading edge of the teeth that immediately precede and follow the gap are further apart, thus they produce a larger net magnetic field disturbance and induced voltage.

A two pin CKP sensor and ECM circuit can be arranged in two ways, with either:

• a constant reference, non-floating, voltage to one side of the sensor and the sensor output signal on the other; or
• a floating voltage, with mirrored output signals on each side of the sensor.

The CKP sensor signal is critical to ECM operation and failures can cause symptoms such as:

• Engine cranking but not starting
• Engine cutting out symptoms
• Malfunction Indicator Lamp (MIL) illumination
• Diagnostic Trouble Codes (DTCs)

Possible faults are:

• Short or open circuits or high circuit resistances.
• Signal errors resulting from excessive dirt and detritus on the sensor housing or pulse wheel.
• Incorrect fitment or operation of the sensor or crankshaft components, causing:
  • Excessive gaps between the sensor and pulse wheel
  • Damage to the sensor housing or pulse wheel
  • Excessive crank or flywheel movement or vibration

GT428