What Does the Crankshaft position sensor (CKP) Do?
On all applications, the Crankshaft Position Sensor has three, and in some cases, four functions, these functions being-
- to provide the PCM (Powertrain Control Module) with input data on the position of the piston in cylinder #1 during engine cranking to calculate an appropriate ignition timing strategy during engine start-up
- to provide the PCM with continuous ignition trigger signals when the engine is running for the PCM to make appropriate adaptations to the ignition timing as and when required
- to monitor the crankshaft speed during engine operation as a means to detect misfires
- to provide a fixed reference point against which to check and correlate the positions of one or more camshafts on engines that have two or more camshafts
Why is the Crankshaft position sensor (CKP) Needed?
In practice, most modern engines start on cylinder #1, when the piston in that cylinder is approaching the top of the compression stroke. However, a gasoline engine will only start and run if both the ignition spark and fuel are present in the cylinder at exactly the right moment, both relative to each other, and the position of the piston.
Since the PCM cannot monitor the position of piston #1 directly, the Crankshaft Position Sensor generates electrical signals relative to a fixed reference point, which is in its turn, relative to the position of piston #1. However, since internal combustion engines have to rotate twice to complete one engine cycle, the PCM “counts” this fixed reference point only once during every 720-degree engine cycle.
Moreover, the position of piston #1 is also relative to the position of the camshaft, to ensure that when the piston is approaching the top of its compression stroke, all the valves are either closing or are already closed to ensure that proper cylinder compression is present during the combustion process.
As a practical matter, though, the correlation between the positions of the crankshaft and one or more camshafts is established and maintained via either a timing belt or a timing chain, and the PCM monitors this correlation continuously via input signals from both the Crankshaft Position Sensor and one or more Camshaft Position Sensors.
During engine cranking, the PCM compares input signals from the Crankshaft Position Sensor with input signals from one or more Camshaft Position Sensors to verify the correlation between the positions of the crankshaft and the camshafts. It is perhaps worth noting at this point that while the PCM uses input data from the Crankshaft Position Sensor to calculate an appropriate ignition timing strategy, the PCM uses input data from the Camshaft Position Sensor(s) to calculate an appropriate fuel injection timing strategy.
If the PCM recognizes and verifies that inputs from both the Crankshaft and Camshaft Position Sensors are valid, it will complete all implicated starting circuits, and the engine will start and run. However, in cases where the input data from the Crankshaft and Camshaft Position Sensors do not match exactly, the engine might still start and run, but the engine will likely run roughly, or it may not run at idle speed. In these cases, the PCM might also set one or more trouble codes, and possibly illuminate a warning light on the dashboard.
Lastly, the PCM also uses input data from the Crankshaft Position sensor to detect misfires via small changes in the crankshaft’s rotational speed that are caused by imbalances in torque outputs by misfiring cylinders.
How Does the Crankshaft position sensor (CKP) Work?
This image shows a typical Crankshaft Position Sensor (green arrow) mounted relative to the reluctor wheel (red arrow), which is required to make the sensor work.
In practice, the sensor generates a small magnetic field when the teeth on the reluctor wheel rotate past the tip of the sensor. This magnetic field is then converted into an electrical signal, which the PCM uses as a trigger to initiate the process of producing an ignition spark.
In this example, the red arrow points to a gap in the teeth, which is referenced to the position of piston #1. Since this gap is wider than the distance between any two other teeth, it produces a “longer” signal, which the PCM recognizes as a fixed reference that serves as the starting point of any ignition timing strategy, regardless of the engine speed. Nonetheless, while the PCM can advance or retard the ignition timing based on operating conditions such as the engine speed and engine load, these adaptations occur in the PCMs internal circuits. As a point of interest, this is why engines that do not use Stop-Start technology always start on cylinder #1.
Note, though, that the reluctor wheel has more teeth than the number of cylinders that any engine can contain. This is true of all engines, and the reason for this is that the number of teeth (on the reluctor) divided by the number of cylinders improves the resolution of the total number of signals produced during a 720-degree engine cycle. The improved resolution makes it possible to make smaller adaptations to the ignition timing more often, as opposed to large adjustments that not affect efficient engine operation, but also increase both fuel consumption and exhaust emissions.
Where is the Crankshaft position sensor (CKP) Located on the Engine?
This image shows the typical location of the Crankshaft Position sensor relative to the reluctor ring on the harmonic balancer, aka, the crankshaft pulley at the front of the engine.
Note that while this is the preferred location for most vehicle makes and models, the Crankshaft Position Sensor can also be located on the side of the engine block, where it acts on a reluctor wheel that is incorporated into the crankshaft. This is a common location on many GM and related vehicle brands.
In still other cases, the Crankshaft Position Sensor is located between the engine and the transmission, where it acts on a reluctor wheel that is incorporated into the flywheel or flexplate.
In most cases, though, it is relatively easy to access the Crankshaft Position Sensor, although we do recommend that you research the actual location of the sensor on your particular vehicle to ensure that you do not test or replace the wrong sensor.
What Does the Crankshaft position sensor (CKP) Look Like?
Although most, if not all Crankshaft Position sensors follow this general pattern, the actual appearance of some sensors can vary greatly from this example.
Note though that it is critically important to replace a Crankshaft Position sensor only with its exact equivalent since the air gap, or clearance between the tip of the sensor and the teeth on the reluctor ring largely determines how well (or otherwise) the sensor works.
Therefore, while two sensors might appear to be identical in all respects, the replacement sensor might be a tiny fraction of an inch longer or shorter than the original. If the sensor is too long, the sensor’s tip may contact the teeth on the reluctor ring, which will destroy the sensor. If the sensor is too short, the clearance between the sensor’s tip and the teeth on the reluctor may be too big for the sensor to generate a signal.
Note also that while most Crankshaft Position Sensors have only two wires, some sensors have three wires, one of which carries a 5-volt reference voltage. The two types of sensors are NOT interchangeable, so be sure to check the number of pins in the electrical connector before you purchase a replacement sensor.
What are the Symptoms that the Crankshaft position sensor (CKP) is Bad?
The most common symptoms of a bad or failing Crankshaft Position Sensor are much the same across all applications, and could include one or more of the following-
- Stored trouble codes and an illuminated warning light
- Depending on both the application and the nature of the problem, multiple fault codes may be present
- The engine may stall unexpectedly, repeatedly, or intermittently. This is common on sensors that fail when the engine reaches a certain temperature
- The engine may be hard to start, or a no-start condition may be present
Note that there is a subset of symptoms that could result from damage to the reluctor ring. These could include one or more of the following-
- Illuminated CHECK ENGINE light and multiple stored trouble codes that relate to ignition timing and/or Camshaft/Crankshaft correlation issues. Note that this typically happens when the reluctor ring is not securely locked in place on the crankshaft, or when the wrong reluctor ring is installed during engine repairs or rebuilds
- Misfires caused by damaged, broken, or clogged teeth on the reluctor ring. Note that while bad or failing Crankshaft Position Sensors could produce misfires under some conditions, this is rare
How do you test the Crankshaft position sensor (CKP)?
While Crankshaft Position Sensors can and do fail, it is more common for its associated wiring to develop defects, faults, and malfunctions that affect the sensor’s operation. Nonetheless, there are a few simple tests you can do to determine if the sensor has failed, or not, as the case may be. Here is what you can do-
Use a scan tool
If you have access to a scan tool, connect it to the vehicle, and scan the vehicle for fault codes. Crankshaft Position Sensor issues will always set fault codes, so if you find codes relating to the Crankshaft Position Sensor, you can take the presence of relevant fault codes as evidence that the sensor is bad- assuming of course, that there is no damage to the sensors’ wiring and/or connector, which will also set fault codes.
The most common Crankshaft Position Sensor codes include the following, but note that all of the below codes can also be set by faults and/or defects in the Crankshaft Position Sensor’s wiring-
- P0385 – Crankshaft Position Sensor B Circuit Malfunction
- P0386 – Crankshaft Position Sensor B Circuit Range/Performance
- P0387 – Crankshaft Position Sensor B Circuit Low Input
- P0388 – Crankshaft Position Sensor B Circuit High Input
- P0389 – Crankshaft Position Sensor B Circuit Intermittent
- P0315 – Crankshaft Position System Variation Not Learned
If you do not have access to a scan tool, you can use a digital multimeter to test the sensor’s internal resistance. Simply place the test probes on the terminals in the connector, and take a resistance reading. If you have zero resistance, there is a short circuit in the sensor, while infinite resistance means that there is an open circuit in the sensor. In both cases, the sensor is bad and must be replaced.
Check the condition of the reluctor ring
If the reluctor ring is accessible, inspect it for broken, damaged, or clogged teeth. In some cases, such as when the teeth are clogged with mud, dirt, or ice, the sensor will almost certainly not work. Replacing or cleaning the reluctor ring will usually restore the sensors’ function.
Check the sensor’s output voltage
If the sensor is accessible, use a multimeter to measure the sensors’ output on the disconnected connector when the engine is cranking. On most applications, the sensor will output about 200 millivolts AC, but this may vary between different makes and models. Compare the actual reading you obtain with reliable service information to make sure you do not replace a serviceable sensor.
How do you replace the Crankshaft position sensor (CKP)?
On almost all applications, replacing a Crankshaft Position Sensor is a simple, straightforward procedure. Simply locate the sensor, disconnect the electrical connector, and remove the retaining bolt. In most cases, the sensor is secured with only one small bolt, although in some instances, there may be two small bolts.
Pull the old sensor out of its position, insert the new sensor, replace and tighten the retaining bolt(s), and reconnect the electrical connector, but be sure to verify that the connector locks securely into place.
Note though that on some applications, the Crankshaft Position Sensor is located inside, or behind a set of covers that protect the timing belt. On these applications, it may be necessary to remove major engine components that could include (among others) the following-
- radiator hoses
- drive belt (serpentine belt) and belt-tensioning devices
- power steering pump and/or power steering hoses
- A/C compressor and/or A/C hoses
In these cases, we recommend that you seek professional assistance with diagnosing and/or replacement of a suspect Crankshaft Position Sensor to prevent causing inadvertent damage to one or more engine components and/or wiring harnesses that may not be related to the Crankshaft Position Sensor.