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Crankshaft position is the engine's timing reference
The controller must know how quickly the crankshaft is turning and where it is within each revolution. Closely spaced trigger events provide speed and angle information, while a missing tooth or unique pattern creates a reference point. Camshaft information then identifies which phase of the four-stroke cycle each cylinder occupies.
Without a plausible crank signal, many systems withhold fuel injection and ignition for safety and timing accuracy. A degraded signal can also disturb misfire detection, variable valve timing and transmission coordination.
How the signal is created and used
- A target wheel rotates with the crankshaft or flywheel.
- Teeth, slots or magnetic poles pass the stationary sensor tip.
- The sensor converts that change into an electrical waveform.
- The controller detects edges and calculates instantaneous speed.
- A missing or unique target feature establishes angular reference.
- Camshaft information establishes engine-cycle phase.
- Ignition, injection and diagnostic events are scheduled from these references.
Sensor technologies
| Technology | Electrical behaviour | Diagnostic characteristic |
|---|---|---|
| Variable reluctance/inductive | Two-wire passive coil generates an AC-like voltage. | Amplitude rises with speed and depends on gap and polarity. |
| Hall effect | Powered device switches a digital output as targets pass. | Check supply, earth and square-wave switching. |
| Magnetoresistive | Powered electronics sense magnetic field direction or strength. | May use current-modulated or specialised digital output. |
| Integrated smart sensor | Electronics condition or encode crank information. | Requires system-specific waveform and pin interpretation. |
| Distributor/internal reference sensor | Some older systems derive position within an ignition assembly. | Mechanical drive and distributor condition affect signal. |
| Combined engine-speed sensor | One crank sensor supplies both speed and position. | A single failure can cause complete no-start. |
Trigger wheels and reference patterns
Missing-tooth wheels
A wheel described conceptually as 60-2 has positions for sixty teeth with two omitted to form a reference gap. Other counts and missing-tooth arrangements are common. The controller calibration must match the actual pattern and angular orientation.
Flywheel and flexplate targets
Teeth or windows can be formed in the flywheel, flexplate or attached ring. Installation of an incorrect flywheel, a bent flexplate or a damaged target can create repeating waveform errors that a new sensor cannot correct.
Magnetic encoders
Alternating magnetic poles may be moulded into a seal or ring and can be visually featureless. Metal debris, impact, excessive heat and incorrect installation orientation can damage the encoded pattern.
Pulley or reluctor mounting
A front pulley or press-fit reluctor must be indexed securely. Slipped bonded pulleys, loose fasteners and incorrectly installed crank gears can move the reference relative to true piston position.
Fitment variables
| Check | Possible variation | Why it matters |
|---|---|---|
| Engine code | Target pattern, location and signal type. | Vehicle model alone is insufficient. |
| Build date | Connector, electronics or mounting revision. | Similar bodies can produce different outputs. |
| Transmission/flywheel | Manual flywheel or automatic flexplate target. | Sensor depth and reference pattern can change. |
| Wire count | Passive two-wire or powered multi-wire sensor. | Testing and controller interface differ. |
| Mounting depth | Tip length, flange and spacer. | Controls air gap and physical clearance. |
| Connector/cable | Key, terminal order, shielding and lead length. | Prevents polarity and interference faults. |
| Seal | O-ring size and material. | Prevents oil leakage without holding sensor proud. |
Air gap, run-out and signal quality
The distance between sensor tip and target affects magnetic coupling. An inductive sensor with excessive gap produces low amplitude, particularly at cranking speed. Too little clearance risks physical contact as shafts move and components expand. Some sensors set their own gap during installation; others use a fixed machined mounting.
Target run-out makes the gap vary each revolution, creating amplitude modulation. Bearing wear, bent flywheels and debris on a magnetic tip can cause the same effect. Measure mechanical condition where waveform variation repeats at a consistent crank position.
Inductive sensor polarity controls the direction of waveform edges. Reversed wires may still produce voltage, but the controller can time from the wrong edge or struggle to recognise the missing-tooth reference. Repair twisted-pair and shielded wiring in the approved orientation.
Electrical supplies and wiring
Active sensors commonly receive a regulated supply and earth, but the voltage may be 5 V, battery voltage or another defined level. Some output circuits are pulled up inside the controller and the sensor switches them low. Testing against a generic expected voltage can misdiagnose a healthy system.
Crank sensor wiring runs near starters, alternators, injectors and ignition coils, all sources of electromagnetic noise. Twisted conductors, shielding and routing protect the small signal. Connecting a shield at the wrong end or substituting ordinary untwisted wire can introduce intermittent faults.
Heat, oil and vibration harden insulation near the engine or gearbox. Check terminals for drag and retention without spreading them. Wiggle testing should be monitored on a live waveform and performed safely away from belts, fans and hot exhaust parts.
Diagnostic evidence
| Evidence | What it can reveal | Limitation |
|---|---|---|
| Engine-speed scan data | Whether the controller detects cranking rotation. | A displayed value may hide waveform distortion. |
| Fault codes | Circuit, plausibility or crank/cam correlation issue. | Do not identify the failed part by themselves. |
| Inductive resistance test | Open or grossly shorted winding. | Cannot reproduce hot, gap or insulation faults. |
| Supply/earth voltage | Power integrity for active sensors. | Must be checked under correct connection/load. |
| Oscilloscope waveform | Amplitude, edges, tooth pattern, noise and dropout. | Needs correct probe, scale and pattern knowledge. |
| Crank/cam comparison | Mechanical timing and synchronisation consistency. | Reference waveform must match exact engine. |
| Hot soak reproduction | Temperature-related sensor or wiring failure. | Heating must not damage components or create fire risk. |
A disciplined no-start diagnosis
- Confirm battery state and actual cranking speed.
- Record all controller codes and freeze-frame information.
- Observe engine-speed and synchronisation data during cranking.
- Inspect sensor wiring, connector, oil contamination and target area.
- Check active-sensor supply and earth or passive-sensor continuity.
- Capture the crank waveform at the controller and sensor where accessible.
- Compare crank and cam patterns when correlation is suspect.
- Check mechanical timing, compression and fuel/ignition only with safe methods.
- Reproduce hot or vibration conditions if the fault is intermittent.
A tachometer that remains at zero can support the diagnosis but is not definitive because dashboard filtering varies. Likewise, absence of spark and injector pulse may be the controller's reaction to a missing crank signal, an immobiliser or low voltage.
Fault patterns
| Symptom | Possible cause | Priority response |
|---|---|---|
| Starts cold, fails hot | Temperature-sensitive sensor winding/electronics or wiring. | Capture signal during the failure. |
| Sudden stall then restart | Signal dropout, supply interruption or other controller power loss. | Check relay, power and crank waveform together. |
| Long crank before starting | Weak crank signal or delayed cam/crank synchronisation. | Inspect both sensors and mechanical timing. |
| Correlation code | Timing movement, wrong target, sensor signal or adaptation. | Verify mechanical references before replacing sensors. |
| Misfire code at one speed | Target damage or waveform interference possible. | Inspect repeating tooth pattern and mechanical causes. |
| No signal after clutch work | Wrong/bent flywheel, damaged sensor or harness. | Check transmission-area installation. |
| Oil leak at sensor | Damaged O-ring, bore or incorrect seating. | Renew correct seal and inspect mounting. |
Removal and installation
Allow hot components to cool and isolate electrical power where specified. Clean around the sensor so dirt cannot enter the engine, bellhousing or oil passage. Release the connector by its latch and remove harness clips before the retaining bolt.
Some sensors seize through corrosion or O-ring swelling. Twist and extract only with approved methods; levering against a thin casting can break it. Compare tip length, flange, connector, cable and seal. Remove any protective shipping spacer only as instructed.
Lubricate the O-ring with the approved medium where specified and seat the sensor fully by hand. Do not pull it into the bore with the bolt. Tighten the small fastener to its stated torque, route shielding away from the starter and exhaust, then inspect for leakage and contact.
Relearn and verification
Some controllers learn small variations in crank trigger manufacturing to distinguish combustion acceleration from misfire. A crank variation relearn may be required after sensor, target, engine, flywheel or controller work. This is distinct from correcting mechanical timing.
The relearn can require correct coolant temperature, no active faults, transmission position and a controlled engine-speed event. Follow the diagnostic tool instructions with the vehicle secure and clear of people. Never perform a high-speed procedure when basic engine condition is uncertain.
Verify cold start, hot restart, stable speed data, absence of dropouts and correct cam/crank synchronisation. Clear codes only after preserving diagnostic evidence, then check that relevant monitors complete.
Common mistakes
- Replacing the sensor solely because its name appears in a fault code.
- Assuming every two-wire device is an inductive sensor.
- Checking resistance across a powered Hall sensor.
- Ignoring low cranking voltage and starter interference.
- Measuring voltage without examining waveform shape or tooth pattern.
- Reversing inductive wires during harness repair.
- Using untwisted unshielded cable near ignition and starter circuits.
- Forcing a long sensor into a shallow bore.
- Resetting correlation faults without checking mechanical timing.
- Skipping a specified crank variation relearn.
UK safety and MOT relevance
The crankshaft sensor is not normally inspected as a separate MOT component, but its failure can cause stalling, non-starting, an emissions warning, misfire or abnormal emissions. Warning-lamp status and emissions performance can be assessed under current requirements.
Unexpected engine shutdown can remove power assistance after its stored assistance is depleted and leave the vehicle stranded in traffic. Do not continue driving with repeat stalling. Keep hands and test leads away from rotating belts, pulleys, fans and high-voltage ignition while diagnosing.
Crankshaft sensor FAQs
Q: What does a crankshaft sensor measure?
A: It reports crank speed and angular position to the engine controller.
Q: Can a failed sensor stop the engine starting?
A: Yes. Many controllers withhold ignition and injection without a valid signal.
Q: Does a crank-sensor code prove the sensor is faulty?
A: No. Wiring, target, timing and voltage faults can set the same code.
Q: Why does the engine fail only when hot?
A: Sensor windings, electronics or connections can open as temperature rises.
Q: What is an inductive crank sensor?
A: A passive coil that generates a waveform as ferrous teeth pass it.
Q: What is a Hall crank sensor?
A: A powered electronic sensor that switches an output in response to its target.
Q: Can air gap affect starting?
A: Yes, especially for an inductive sensor at low cranking speed.
Q: Can a damaged flywheel cause a sensor fault?
A: Yes. Bent, missing or incorrect target features distort the signal.
Q: Should a new sensor be relearned?
A: Perform a relearn only where the vehicle procedure requires it.
Q: Can resistance testing prove a sensor is good?
A: No. It misses many dynamic, hot and active-electronics faults.
Q: Are crank and cam sensors interchangeable?
A: Usually not; their fit, output and target duties differ.
Q: Can a crank sensor cause misfire codes?
A: Yes, but mechanical, ignition and fuelling causes remain possible.
Q: Can the fault affect the MOT?
A: Indirectly through warning lamps, misfire or emissions performance.