The crank position sensor (CKP) is perhaps the most important sensor in the modern engine. It’s also referred to as the engine speed sensor (ESS or RPM, for revolutions per minute). Without the crank position signal, the engine control module (ECM) can’t detect where the cylinders are or how fast they’re moving.
If you have crankshaft sensor issues, the ECM can’t synchronize fuel injection, spark ignition (for gasoline engines) or control variable valve timing. Crankshaft sensor issues can cause a range of problems.
How CKP Sensors Work
There are two crankshaft sensor types, hall effect and inductive, which generate a signal based on magnetic fields. Both sensors look similar, but function slightly differently:
- Inductive CKP sensors have one or two wires and generate an analog signal.
- Hall effect CKP sensors have a three- or four-wire circuit and transmit a digital signal.
Regardless of type, the CKP sensor is mounted near a toothed reluctor ring or reluctor wheel.
This crankshaft position sensor mounts behind the harmonic balancer. There are many teeth on the reluctor ring, usually “missing” teeth corresponding to cylinder positions and cylinder number one’s top dead center (TDC). As each tooth of the reluctor ring passes the sensor, it briefly dampens its magnetic field.
There are a few things that can cause the crankshaft position sensor to fail, including damage, debris and faulty circuitry.
Even for modern electronics, the engine is a violent and destructive environment. Though built for this, most sensors eventually succumb to the ever-present heat and vibrations of the engine. Even tiny fluctuations in thermal expansion, or vibrations themselves, can weaken and break the internal wiring and circuits in CKP sensors. Bent, broken or worn reluctor ring teeth can also generate a weak or unstable signal, which the ECM will be unable to analyze.
Along the same lines, damaged metal parts can create debris in the form of metal filings or shavings, which the magnetic crankshaft position sensor can pick up. The CKP sensor works at a certain distance, accounting for the air gap from the reluctor ring, but captured metal shavings extend the magnetic field, closing the gap and leading to poor signal generation.
Finally, faulty circuits can cause CKP sensor failure. If the wires between the ECM and the CKP sensor are damaged, the ECM can’t recognize the signal. Any time you’re investigating crank sensor issues, it’s critical to verify the CKP circuit.
How to Diagnose a CKP Sensor Issue
If your CKP sensor is failing, the first thing you might notice is the MIL (malfunction indicator lamp), usually with CKP diagnostic trouble codes (DTC). Common CKP DTCs include: P0335-P0339 and P0385-P0389, crankshaft position sensor circuits “A” and “B.”
Hard-starting, start-stalling or no-start conditions are all more noticeable symptoms, but might not be accompanied by DTCs referring to crank sensor issues. A failing CKP sensor can cause hesitation and poor power upon acceleration. Also look out for a drop in fuel economy as the ECM compensates for a poor CKP signal, with or without the MIL.
These are the main ways crankshaft sensor issues present themselves, but it’s also good to know that most other speed sensors work similarly, such as wheel speed sensors. If you’re ever in doubt about the next step in the repair process, consult a trusted mechanic: Electrical matters are often better left to the professionals.
Check out all the relays, sensors and switches
Photo courtesy of Wikimedia Commons.
Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.