Camshaft Position Sensor (CMP) and Top Dead Center (TDC) Sensor
SPECIAL NOTES: Note that the information provided in this guide is of a generic nature, and is intended for informational purposes only. However, since the basic operating principles of any given engine sensor is largely similar across all makes and models, it is possible to apply the information provided here to a large range of applications. Nonetheless, be aware that neither similarities in operation, appearance, or location, nor effects on engine operation when any given sensor fails is guaranteed, and it is therefore recommended that the relevant technical manual be consulted for details on the location, manufacturer specific diagnostic information, replacement procedures, and other technical information pertaining to the affected application. END OF SPECIAL NOTES.
What does the Camshaft Position Sensor (CMP)/ Top Dead Center (TDC) Sensor do?
NOTE: On most applications, the Camshaft Position (CMP) sensor, and the Top Dead Centre (TDC) sensor is the same thing known by two different names.
Also known as, “cylinder identification sensors”, or less commonly as “phase detectors”, the primary function of the CMP is to determine which cylinder should next be supplied with fuel. In practice, the CMP sensor provides the PCM (Powertrain Control Module) with data about the engine’s firing order, but note that the PCM always calculates fuel delivery times with reference to when cylinder #1 is approaching TDC.
However, on some applications, the PCM does not need to identify cylinder #1 or the firing order, since this information is obtained from a sensor that identifies the position of the crankshaft and/or other rotating components relative to the TDC position of cylinder #1.
Why is a Camshaft Position Sensor (CMP) / Top Dead Center (TDC) Sensor needed?
For a modern engine to operate smoothly and efficiently, the engine management system needs to initiate, monitor, control, and regulate several processes concurrently. Typically, these processes include control and regulation of the ignition timing, fuel injection timing, and control of the injector pulse width, valve/camshaft timing, and others, including calculating the best time to purge the EVAP system.
In practice, while many sensors contribute to the overall engine management strategy at any given moment, using one sensor, in this case the CMP sensor to provide primary input data against which all other inputs can be measured, provides a simple, cost effective, and reliable way to ensure efficient operation of the engine at all times.
How does a Camshaft Position Sensor (CMP) /Top Dead Center (TDC) Sensor work?
There are three types on CMP sensors in use today, all of which we will briefly discuss below-
Hall Effect sensors
This is the most common type of CPM sensors in use today. On older applications that still use distributors, the sensor is located in the distributor, while on more modern applications, the sensor is located close to the camshaft(s).
In distributor-based systems, the sensor is located on one side of a perforated rotating screen that separates the sensor from a magnet. As the screen rotates, the perforation allows the sensor and the magnet to interact, with the interaction producing a magnetic field that is converted into a magnified electrical pulse. This pulse occurs every time the perforation in the screen passes between the sensor and the magnet, but is always relative to the position of cylinder #1, which represents the input data the PCM uses to calculate an appropriate fuel delivery strategy.
On distributor-less systems the electrical pulse is produced in the same way, but in these systems, the rotating screen is replaced by a device fixed onto the camshaft that allows a pulsing signal to be created as the camshaft rotates.
AC output sensors
These sensors produce an AC (Alternating Current) signal, which is produced by an exciter coil that is fed with a high frequency current (typically, between 150 and 2 500 cycles p/sec) by the PCM. When a slot in the camshaft passes the coil as the camshaft rotates, the slot inductance of the coil changes, which produces a an AC current that serves to indicate the position of cylinder #1 relative to TDC. This type of CMP sensor is commonly used on Opel/Vauxhall ECOTEC engines.
Where is the Camshaft Position Sensor (CMP) / Top Dead Center (TDC) Sensor located on the engine?
On distributor-less systems, the CMP sensor is most commonly located in or on the valve cover, and in such a way that it is in close proximity to the reluctor device on the camshaft. Note that on engines with multiple camshafts, each camshaft may be provided with its own CMP sensor.
On distributor-based ignition systems, the CMP sensor is most commonly located inside the distributor, and removal of the distributor cap is required to gain access to the CMP sensor.
What Does the Camshaft Position Sensor (CMP) / Top Dead Center (TDC) look like?
The image above shows an example of a typical Camshaft Position Sensor, such as might be found on almost any distributor-less engine. Note that in most cases of engines with separate inlet and exhaust camshafts, the sensors are identical in appearance and electrical specifications.
Note though that in a few cases the inlet/exhaust CMP sensors may not be identical; neither in appearance, nor in terms of its internal resistance and/or other electrical specifications. Therefore, it is important always to refer to the manual for the affected application to identify CMP (and, for that matter, all other engine sensors) correctly to avoid misdiagnoses and additional damage to the application’s electrical system.
Possible symptoms of a bad Camshaft Position Sensor (CMP) /Top Dead Center (TDC) Sensor
NOTE: Note that regardless of the type of CMP sensor used, the signal a CMP sensor generates must be in phase with input data from the CKP (Crankshaft Position Sensor) on applications that use CMP sensors to determine the position of cylinder #1.
In this regard it is important to note that improperly installed timing belts, or excessive wear/stretching of steel timing chains is the most common mechanical cause of phase differences. It should also be noted that excessive endplay in the camshafts of some late model Honda applications is a major cause of the CMP sensors on these applications generating inaccurate, implausible, or intermittent signals.
Nonetheless, some common symptoms of failed or failing CMP sensors could include the following-
- Illuminated “CKECK ENGINE” light
- Fuel consumption may increase significantly
- A no-start, or hard starting condition may be present
- Idling may be rough, erratic, or the idling speed may fluctuate wildly
- Engine may misfire on one or more cylinders during operation; in most cases, the misfiring cylinders will be identified by dedicated misfire related codes
- Engine may stall frequently, or unexpectedly
- Depending on the nature of the problem, there may be varying degrees of power loss present
- In severe cases, the PCM may initiate a failsafe or limp mode that will persist until the problem is corrected
Note that in addition to one or more one or manufacturer specific codes, one or more of the following generic OBD II codes may also be present-
- P0340 – Camshaft Position Sensor Circuit Malfunction
- P0341 – Camshaft Position Sensor Circuit Range/Performance
- P0342 – Camshaft Position Sensor Circuit Low Input
- P0343 – Camshaft Position Sensor Circuit High Input
- P0344 – Camshaft Position Sensor Circuit Intermittent
- P0345 – Camshaft Position Sensor A Circuit (Bank 2)
- P0346 – Camshaft Position Sensor A Circuit Range/Performance (Bank 2)
- P0347 – Camshaft Position Sensor A Circuit Low Input (Bank 2)
- P0348 – Camshaft Position Sensor A Circuit High Input (Bank 2)
- P0349 – Camshaft Position Sensor A Circuit Intermittent (Bank 2)
- P0365 – Camshaft Position Sensor B Circuit (Bank 2)
- P0366 – Camshaft Position Sensor B Circuit Range/Performance (Bank 2)
- P0367 – Camshaft Position Sensor B Circuit Low Input (Bank 2)
- P0368 – Camshaft Position Sensor B Circuit High Input (Bank 2)
- P0369 – Camshaft Position Sensor B Circuit Intermittent (Bank 2
- P0390 – Camshaft Position Sensor B Circuit (Bank 2)
- P0391 – Camshaft Position Sensor B Circuit Range/Performance (Bank 2)
- P0392 – Camshaft Position Sensor B Circuit Low Input (Bank 2)
- P0393 – Camshaft Position Sensor B Circuit High Input (Bank 2)
- P0394 – Camshaft Position Sensor B Circuit Intermittent (Bank 2)
- P0395 – Camshaft Position Sensor B Circuit High Input (Bank 2)
- P0396 – Camshaft Position Sensor B Circuit Intermittent (Bank 2)
How to test the Camshaft Position Sensor (CMP) / Top Dead Center (TDC) Sensor
Unlike most other engine sensors, CMP sensors often last the life of the vehicle, and most CMP sensor related codes are caused by wiring issues. Therefore, any diagnostic procedure that involves CMP sensors must start with a thorough visual inspection of all associated wiring and connectors. Look for the following-
- Corroded, burnt, shorted, damaged, or disconnected wiring and/or connectors; make repairs as required
- Perform resistance, continuity, reference voltage (where applicable), and ground integrity tests on all associated wiring and connectors. Replace or repair wiring as required to ensure that all electrical values fall within the manufacturer’s specified values.
NOTE: Due to the large number of design specifications of CMP sensors across all manufacturers, it is not possible to provide accurate diagnostic data/procedures for all, or even most applications in this brief guide. Note that the most reliable test results are only obtainable through the use of an oscilloscope, or with a high-end, dealer grade scan tool that can function as an oscilloscope., and then only if suitable reference data in the form of an applicable waveform library is available. If suitable diagnostic equipment and/or reference data is not available, the wiser option would be to refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
However, be aware that while it is possible to perform some basic testing of CMP sensors on a DIY basis, this guide can only provide a few generic tests with a digital multimeter that may or may not reveal the root cause of the problem. Here is what to look for-
Check inductive sensor resistance
On inductive CMP sensors, the internal resistance should be in the 200-Ohm to 900-Ohm range, but note that the obtained reading should be checked against reliable reference data that applies to the affected application. There is no single resistance value that applies to all inductive CMP sensors.
Check Hall Effect sensor output
Connect the positive probe of the multimeter to the signal circuit of the sensor, and the negative probe to a suitable ground. With the engine idling, the displayed voltage should be an average of about 2.5 volts, while the duty cycle (“ON” time) should be about 50% or so.
AC output sensors
Note that these sensors can generally not be tested with a multimeter, due to the nature of the signals they produce. The only reliable way to test these sensors is with an oscilloscope, or sometimes a dual-channel oscilloscope to check phase synchronicity between the CMP and CKP sensors.
How to replace the Camshaft Position Sensor (CMP) / Top Dead Center (TDC) Sensor
In the vast majority of cases, replacement of the CMP sensor(s) will follow this general pattern-
- Ensure that the engine is cold to prevent sustaining burns and scalds
- Locate the sensor(s) on the valve cover
- Disconnect the wiring
- Remove the single retaining screw/bolt
- Extract the sensor, and insert the replacement
- Insert, and tighten the retaining screw/bolt but do NOT over tighten the screw/bolt to prevent stripping the soft threads in the valve cover
- Reconnect wiring, and test drive the vehicle to verify that the problem had been resolved
NOTE: In most cases, CMP sensors are located where they are in direct contact with engine oil, which is why these sensors are provided with oil seals, and usually in the form of a rubber O-ring. To prevent oil leaks after a CMP sensor replacement, be sure to replace the O-ring or other required oil seals as well.