By Matt Dixon, assistant professor,
Southern Illinois University at Carbondale
Hall effect crankshaft position (CKP) and camshaft position (CMP) sensors are critical components of an engine management system.
The inputs they provide enable the powertrain control module (PCM) to determine engine speed and position including where a given cylinder is within the four-stroke cycle.
Such information is vital to command ignition coils and fuel injectors in proper time and sequence.
The data from these sensors also is utilized for other important functions including fuel metering, misfire detection, variable valve timing (VVT) control and more.
Although two-wire variable reluctance sensors producing an alternating current can still be found, the three-wire digital Hall effect sensor has become the most prevalent type on late-model vehicles.
Figures 1 and 2: Voltmeter monitoring the CMP sensor signal wire. The ignition is in the run position. As the metallic feeler passes under the sensor, signal voltage is pulled low by the sensor. When the feeler gauge is moved away, voltage remains at the 5 volts provided by the PCM.
Despite such importance, CKP and CMP sensor diagnostics are often misunderstood. This article will examine three-wire Hall effect CKP and CMP sensor operation, function and diagnostics.
Hall effect position sensors contain a magnet and electronic components, but, at a simple level, are switches. The switch is a transistor within the sensor.
The functions of the three wires are sensor supply voltage, signal voltage and ground. Unlike their two-wire counterparts, Hall effect sensors require external power and ground to function.
The transistor within the sensor connects or disconnects the signal circuit to ground. Voltage on the signal circuit is provided by the PCM utilizing five or 12 volts.
A small level of current is passed through a magnetic field within the sensor, which is altered by a revolving metallic tone ring.
The actual Hall effect is a change in voltage in relation to the change in magnetic field.
Hall effect voltage is processed using several electronic conditioning components to switch the transistor base. The result on the signal circuit is a digital high or low voltage signal.
While positioned over a metallic section of tone ring, the transistor is switched on, resulting in a low-voltage state. When over an air gap, the transistor is switched off, resulting in a high-voltage signal state.
A DVOM and a ferrous piece of metal such as a feeler gauge can be used to test basic functionality of a three-wire CKP or CMP sensor. See Figures 1 and 2.
The tone ring provides a metallic pattern of slots that rigidly connect to the crankshaft or camshaft(s).
The tone ring for the crankshaft can be an external plate located directly behind the harmonic balancer, be a part of the flexplate or flywheel, or bolted to the crankshaft internally.
Likewise, a camshaft tone ring can be placed and attached using different methods. Location and placement choices have pros and cons. For example, flexplates can crack around the center section often without the expected noise or other symptoms.
Such a crack can shift the outer section containing the tone ring slots. This has a dramatic impact on timing and results in noticeable driveability issues.
See Figure 3.
The trend over time has been an increasing number of slots in the tone ring pattern. Each slot provides an engine position pulse to the PCM. Additional slots provide improved timing precision and misfire detection. Often a CKP signature notch or groups of notches allow the PCM to quickly identify companion cylinders.
See Figure 4.
As the engine revolves, the CMP pattern enables the PCM to synchronize crank and camshafts and determine which cylinder is on which stroke.
Unique signature patterns afford some engines the capability to start even if a CKP or CMP sensor fails. Other engines will not start at all. If the engine does start on only one sensor, it may experience long crank time, reduced power output, lower rpm limits and an illuminated MIL.
Tone ring patterns can change between model years on the same engine.
Figures 5a and b: Be careful of changing patterns even on the same engine year to year. These are both Dodge 2.7L V6 CKP and CMP scope patterns. The top (a) was taken from a 2001 model and the bottom (b) from a 2008. Though the crank pattern is obviously different and perhaps easy to spot, take a look at the cam pattern. The top has a slot code pattern of 1-2-3-1-3-2 while the bottom is 1-3-1-2-3-2. This is important to consider during engine or head replacement using different parts.
See Figure 5 A.
This is important when considering installation of used or remanufactured engines or parts. This can be more difficult to visually catch than one might think. Incompatibility between CKP and CMP tone rings or the PCM family can result in a no start.
The number of CKP slots per unit of time provides the rpm value. Rpm value is used for many items beyond the tachometer and rev limiter, including fuel pump relay control strategy. If the rpm value is lost, the PCM is programmed to de-energize this relay.
Rpm is also an often-overlooked value in load calculation. Fuel injection systems determine airflow based off of either rpm and mass airflow signal or rpm and manifold absolute pressure values.
Correct air mass per unit of time is essential for accurate injector pulsewidth. Engine rpm can also be compared with transmission input shaft speed to verify torque converter lock up.
Crankshaft position is used for timing functions including injector firing. Port injection systems typically pulse injectors during the exhaust stroke. Gasoline direct injection systems pulse on the intake or compression stroke depending on operating mode.
Pulsing injectors on the wrong stroke can result in increased emissions and power loss. Base ignition timing and spark advance each depend on accurate position calculation.
An important spark advance input, the knock sensor, may only be monitored during certain degrees of crankshaft rotation. With camshaft phaser VVT, the CMP to CKP relationship is used to determine if advance or retard commands have been carried out.
A malfunction or slow operating system results in degrees of variance and a possible DTC. Crank position and acceleration is also used to detect misfire.
When each cylinder is “up to bat” on the power stroke, the PCM expects to see an acceleration in crankshaft speed. A lack of acceleration is counted as a “strike” or misfire. Enough misses in a group of revolutions result in a misfire code.
See Figure 6.
There is one new function to mention. Engine start-stop technology is appearing on conventional gasoline-powered vehicles to improve fuel efficiency. When the PCM determines conditions are suitable for automatic engine shutdown, the PCM closely monitors and logs the CKP pattern.
Crankshafts usually stop in one of a few places depending on number of cylinders. As the crankshaft comes to a rest there is no guarantee that it will only rotate in its normal direction. Up until now, it was unnecessary to ever think about monitoring for reverse rotation.
However, with automatic restart, it’s imperative to log exact crankshaft position for a rapid and seamless start. Both CKP and CMP patterns are utilized along with upgraded PCM software to accurately log shutdown crankshaft position.
PCM, CKP and CMP sensor diagnostics can be confusing. Unlike a typical five-volt engine coolant temperature sensor, CKP and CMP sensors utilize the ends of the voltage spectrum during normal operation.
There is no way to reserve a section for volts too low or too high types of failures. Instead, rationality is employed using a “tattletale” method. If either the CKP or a CMP sensor reports a toggling voltage pattern while the other(s) do not, the opposite sensor(s) are deemed to be inoperative.
P0335 no crank signal and P0340 no cam signal codes are set this way. Such rationality sounds simple enough but sometimes the PCM can be “tricked” into declaring the wrong failure. This is more likely during an intermittent failure. Failures such as P0339 intermittent crank signal failure can be downright perplexing.
Also, if neither CKP nor CMP sensors are functioning, it is possible to encounter a no start without any codes. It should be mentioned that CKP and CMP sensors often share a PCM supply voltage and a sensor ground.
See Figure 7.
A short in one sensor can bring down all sensors on a supply voltage circuit as can a sensor ground open. Monitoring key-on sensor supply voltage is a logical step during a no start. If sensor voltage is not detected, checks need to be repeated with different sensors disconnected.
Whether diagnosing a CKP or CMP sensor code, no start or other driveability issue, a two- or more channel oscilloscope is a powerful tool. Many scopes feature a recording function that is extremely helpful in catching glitches. One reason for this is the extremely large number of switches. If a CKP tone ring has 34 slots and the engine spins at 2500 rpm, then 85,000 slots pass by per minute. A glitch is sure to be felt in vehicle operation, but no other tool is likely to catch it.
The scope is also valuable in determining correct camshaft timing. Just a few degrees of CKP to CMP variance can result in codes and driveability issues. Without a known-good picture, it’s difficult to interpret the image with complete confidence.
Online resources such as the International Automotive Technicians Network (iATN.net) feature a waveform database that can be helpful. Deciding to tear into an engine for a suspected cracked flexplate or sheared off cam to sprocket dowel pin is easier done with a known-bad pattern.
While scope images can save time compared with component disassembly, scope hook up is best performed using the easiest access point. Some vehicles have a starter, manifold or other obstacle in the way of sensors. In such cases, the PCM is an easier access point.
See Figures 8 and 9.
Obtaining an accurate connector pin out is necessary to tap into a sensor signal at the PCM. Care needs to be taken with fragile connector covers and while backprobing the circuit. Terminal inspection and wiggle tests are legitimate, but collateral damage resulting from rough handling is best avoided.
Scan tools have mixed value for CKP/CMP sensors. CKP/CMP variance can be a helpful value in spotting timing chain stretch or related component wear. Many tools also offer a crank/cam relearn feature.
Though the specifics of this procedure can vary, it generally resets a correlation value in the PCM. Service procedures often call for a relearn after replacing sensors, timing chain/belts, tensioners or resetting cam timing.
The relearn procedure may be necessary for the misfire monitor and may require driving the vehicle.
Somewhat less helpful if not deceptive are datastream values such as CKP and CMP present/not present or SYNC true/false. I have experimented with intermittently interrupting and manipulating CKP/CMP signals while monitoring such PIDs. The scanner sometimes catches it. Scan tools convert serial data and, depending on the specific tool and number of PIDs being viewed, the update rate may not be nearly fast enough.
These sensors are normally very reliable, however, they do occasionally fail without good explanation. Heat, vibration and mechanical shock are plausible suspects for the sensor, while wiring issues, terminal spread and occasional PCM issues account for remaining circuitry.
Some sensors last hundreds of thousands of miles while some fail right out of the box.
When replacing a sensor, first be careful not to drop it as the magnet or internal electronics can be damaged. Also, follow instructions in regard to air gap. It is typically not adjustable, but be sure mounting surfaces are clean and fasteners are properly torqued.
Some sensors come with a sticker on the end that gets removed as the tone ring spins. I have tested increase of air gap using shims and found signal failure in as little as 0.100”. Without a doubt, CKP and CMP sensors collect vital information for the PCM.
When one or more fail to operate, your customer will know there’s a problem. As the big wheel keeps on spinning around, hopefully you’re ready to test these sensors to get the lowdown on why and keep customer satisfaction switched on high.