Your car’s crankshaft position sensor is a key part of the ignition and fuel management system. Without it, the engine control unit (ECU) cannot determine piston position or engine speed — two essential inputs for accurate ignition timing. Learning how to test a crank sensor helps you diagnose starting issues, rough idling, and engine stalls before they become costly repairs.
What Is a Crank Sensor?
The crank sensor, also called the crankshaft position sensor (CKP), monitors the exact position and rotation speed of the crankshaft. It works closely with the camshaft sensor, ignition coil, and fuel injectors to keep the engine synchronized.
Every time the crankshaft turns, the sensor sends a signal to the ECU, allowing it to:
- Time the ignition spark
- Control fuel injection
- Detect engine misfires
- Adjust idle speed and timing advance
Crank sensors are found on all modern engines.
How Does a Crank Sensor Work?
The sensor typically operates on one of two principles:
- Magnetic (Inductive) Type: Generates an AC voltage signal as a toothed reluctor wheel on the crankshaft passes the sensor tip.
- Hall-Effect Type: Uses a 5V reference supply to produce an on/off DC voltage signal based on the crankshaft’s rotation.
This signal tells the ECU how fast the engine is spinning (RPM) and where the pistons are in their cycle. Together with the camshaft sensor, it ensures that fuel injectors and spark plugs fire at the correct moment.
If the crank sensor fails or its signal becomes erratic, your engine will struggle to start, idle roughly, or stall unexpectedly.
Common Symptoms or Signs of Crank Sensor Failure
A bad crank sensor often shows the following symptoms:
- Engine Cranks but Won’t Start: The ECU cannot detect crankshaft position.
- Intermittent Stalling: Engine may die randomly while driving.
- Check Engine Light: Codes such as P0335, P0336, or P0339 indicate sensor malfunction.
- Rough Idle or Misfires: Inaccurate timing causes poor combustion.
- Reduced Power and Fuel Economy: ECU switches to a default mode that reduces performance.
Because these symptoms overlap with other problems (such as a faulty ignition coil or camshaft sensor), proper testing is crucial.
How to Test a Crank Sensor
Testing a crank sensor requires a digital multimeter or oscilloscope. Always consult your car’s service manual for correct specifications.
1. Visual Inspection
- Disconnect the sensor connector and check for corrosion, oil leaks, or broken wiring.
- Inspect the reluctor ring for dirt or damage.
2. Resistance Test (for Magnetic Sensors)
- Set the multimeter to the ohms (Ω) range.
- Connect probes to the sensor terminals.
- A good sensor usually reads 200–900 ohms.
- Infinite resistance or zero reading indicates an open or shorted sensor.
3. Output Voltage Test
- Switch the multimeter to AC voltage.
- Crank the engine and measure output voltage.
- You should see at least 0.5V AC while cranking.
- If there’s no signal, the sensor is likely defective.
4. Hall-Effect Sensor Test
- With the ignition ON (engine off), back-probe the signal wire.
- Rotate the crank manually or crank the engine.
- A good sensor produces a pulsing 0V–5V DC signal.
5. OBD-II Scanner Test
Run a diagnostic scan to check for codes like P0335 (Crankshaft Position Sensor “A” Circuit) and monitor live data. No signal = replacement needed.
Crank Sensor Replacement Cost in South Africa
Replacing a crank sensor in South Africa typically costs between R600 and R1,500, depending on your vehicle make and model.
Related Car Parts and Maintenance Tips
When working on your crank sensor, it’s wise to inspect or service other related engine parts:
- Camshaft sensor: Works directly with the crank sensor for timing.
- Ignition coil and spark plugs: Misfires can result from weak spark delivery.
- Timing belt or chain: Worn components can affect synchronization.
- Oil filter and 5w30 engine oil: Keep lubrication clean to prevent wear on sensor housing.
- Air filter and fuel filter: Ensure efficient combustion and smooth operation.
For further learning:
These guides can help you diagnose related issues before replacing parts unnecessarily.
Conclusion: Why Crank Sensor Maintenance Matters
The crankshaft position sensor plays a vital role in controlling your vehicle’s ignition and fuel systems. Knowing how to test a crank sensor helps you spot potential problems early, saving time and money while keeping your car running reliably.
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