A crank sensor is a component used in an internal combustion engine to
monitor the position or rotational speed of the crankshaft. This
information is used by engine management systems to control ignition
system timing and other engine parameters. Before electronic crank
sensors were available, the distributor would have to be manually
adjusted to a timing mark on the engine.

The crank sensor can be used in combination with a similar camshaft
position sensor to monitor the relationship between the pistons and
valves in the engine, which is particularly important in engines with
variable valve timing. It is also commonly the primary source for the
measurement of engine speed in revolutions per minute.

Crank sensors in engines usually consist of magnets and an inductive
coil, or they may be based on magnetically triggered Hall effect
semiconductor devices. Common mounting locations include the main crank
pulley, the flywheel, and occasionally on the crankshaft itself. This
sensor is the most important sensor in modern day engines. When it
fails, there is a small chance the engine will start (engine will likely
cut out after a few minutes of driving) but it mostly will not start.

Some engines, such as GM's Premium V family, use crank position sensors
which read a reluctor ring integral to the harmonic balancer. This is a
much more accurate method of determining the position of the crankshaft,
and allows the computer to determine within a few degrees the exact
position of the crankshaft (and thereby all connected components) at any
given time.

Another type of crank sensor is used on bicycles to monitor the position
of the crankset, usually for the cadence readout of a cyclocomputer.

CAM and CRK Sensor Repair Technician Observation

The crank (CRK) sensor signal is probably the most important signal in a
modern automotive engine control system. This signal provides the ECM
with crankshaft speed and position, as well as a cylinder # 1 reference
point. There are various names given to this signal. The distributor
reference, CRK signal, CAS, PIP, etc (depending on the manufacturer).
The way this signal reaches the ECM will affect the approach that is
taken to a proper diagnostic procedure. By analyzing the signal path to
the ECM using a wiring diagram and an oscilloscope, the correct
diagnostics determination can be made.

The CAM sensor signal is found on systems with sequential fuel
injection, in which the ECM triggers the injectors independently instead
of in group mode as in older systems. The CAM signal is also called CID,
TDC, etc, depending on the manufacturer. The CAM sensor provides the ECM
with camshaft position so that it can determine the correct injection
and ignition sequence. Some systems (with distributors) do not need the
CAM sensor to start the vehicle, and can simply start in non-sequential
mode. However COP and most DIS systems do need the CAM sensor so that
the ECM can determine the position of cylinder # 1 TDC on compression
stroke and fire the correct coil.

The relationship between CAM and CRK signal is very important for proper
ignition sequencing to occur. A stretched or jumped timing belt/chain
will create severe engine performance problems on DIS/COP systems, since
the ECM doesn't know when to trigger the coils. On other systems the ECM
will shut down ignition entirely if it sees a discrepancy between these
two signals.

CAM and CRK sensors come in four different varieties: MAGNETIC, HALL
EFFECT, OPTICAL AND MAGNETO-RESISTIVE.

The magnetic sensor actually produces its own signal. It is in essence a
small generator. A coil winding inside the sensor picks up the magnetic
fluctuations from the vibration damper or the flywheel (or both in some
cases). A toothed reluctor wheel on either the damper or flywheel
induces a voltage signal to the sensor. Magnetic sensors work on the
principle of induction, which states that a metal object or magnet when
placed across a coil winding will induce a current on that coil.
Magnetic sensors are heavily dependant on the air gap between the sensor
and reluctor wheel, and on the speed of rotation. The air gap has to be
set as close as possible without touching the reluctor, and the engine
cranking rotational speed has to be fast enough to produce the right
signal amplitude. It is common to see vehicles that will not start due
to a defective starter that is cranking the engine slower than normal .
Systems that employ a magnetic sensor also have a threshold voltage,
which is the v oltage value at which the signal is first recognized by
the ECM. Most distributor pick-up coils are of the magnetic type
although hall effect distributors pick-ups are also found on some
systems.

Once the signal reaches this pre-programmed voltage the ECM recognizes
the signal and will act upon it (pulse the injector, etc). Magnetic
sensors are usually shielded or with its wires twisted to prevent
electromagnetic interference. On some systems the ECM provides a small
bias voltage for diagnostics purposes. If the ECM sees a problem with
this bias voltage, it will set a code for either a shorted or open
circuit. Special attention should be paid to the polarity of these
sensors. They are polarity sensitive. If for whatever reason the
polarity (wires) is inverted, the vehicle will not perform properly or
will not run at all.

The hall effect sensor requires its own voltage and contains a switching
transistor within the sensor casing. This type of sensor needs a voltage
supply, reference voltage and a ground to operate. Transistors are
electronic switches that turn ON or OFF when a current is applied to one
of its three leads (Base lead). The sensing semi-conductor device or
miniature coil in a hall-effect sensor is tied to the base lead of this
internal transistor.

When the triggering mechanism (reluctor wheel) comes close to the hall
effect sensor the magnetic lines cut across the sensing semiconductor
device, which triggers the small internal transistor. This internal
transistor then toggles the reference signal between ground and
reference voltage. Hall effect sensor outputs a square wave signal
simply because all they do is toggle their reference voltage to ground.
In essence they are magnetic sensors, with an added internal switching
transistor so that the sensed signal goes to the base lead of the
internal transistor to trigger it instead of straight to the ECM, like a
regular magnetic type sensor. Some hall effect sensors actually employ
their own permanent magnet within its casing. This variation uses a
shutter type triggering wheel that breaks across the magnetic field. The
momentary interruption of this magnetic field is what triggers the base
of its internal transistor. Regardless of what hall effect sensor
variation used, they all out put a square wave. Hall effect sensors are
not affected by slow engine cranking speeds. They will simply toggle the
reference voltage to ground, regardless of cracking speed.

The optical sensor uses a principle somewhat similar to the hall effect
sensor, but instead uses light as its triggering method. Optical sensors
are light activated devices. These sensors use an LED (light emitting
diode) as their light source, and a phototransistor as their triggering
component. Optical sensors always have a shutter disk with small holes.
Due to the more sensitive nature of the phototransistor, these holes are
fairly small and can detect tiny amounts of engine speed fluctuations.
Optical sensors are much more exacting in their operation and are able
to detect very small engine variation problems much faster than any of
the other two of sensor variants. Optical sensors also put out a square
wave. They need a supply voltage and ground to feed the LED light source
and phototransistors, as well as a reference voltage. The shutter wheel
passes between the LED and the phototransistor; and as this shutter
wheel turns, it momentarily breaks the light beam emitted by the LED.
This light beam breaking action is detected by the photo-transistor,
which instead of having a base lead has a small lens or eye that is
always looking for the light source. The action of the shutter wheel
breaking the light source also triggers the phototransistor, which in
turns toggles the reference voltage to ground. Optical sensors may also
have two LED light sources. One for the 360 § of crank rotation and the
other with 4-6-8 slots to denote each cylinder position depending on the
amount of cylinders on the engine. It is fairly common to see dirt and
oil contaminate the small holes on the optical triggering wheel and
cause erratic or no signal output at all. Neither optical or hall effect
sensors are affected by slow engine cranking speeds.

The newer styled magneto-resistive sensor is yet another derivative of
the hall-effect sensor. This sensor also puts out a square wave, but
with one fundamental difference. Magneto-resistive sensors DO NOT ground
their reference voltage. They are constructed with two internal sensing
pick-up devices one besides the other. When the reluctor wheel tooth
comes into proximity with the sensor, the first of the two sensing
pick-up devices will trigger the base of the transistor and toggle the
output signal high (i.e. 5 volts). A split second later, the second of
the two sensing pick-ups will then toggle the output signal low (0
volts) or ground. This sensor uses the leading and trailing edges of the
reluctor tooth to output a square wave. The leading tooth edge toggles
the sensor high and the trailing edge toggles it low. The output is a
regular square wave. Magneto-resistive sensors are also not affected by
slow engine cranking speeds.

CONDITIONS THAT AFFECT OPERATION

The following conditions should be used as guidelines affecting all CAM
& CRK sensors mentioned here. It is always important to determine the
specific vehicle operation before making a diagnostics decision. Keep in
mind that the way the CAM or CRK signal reaches the ECM will determine
the diagnostic route to follow. These signals will either go to the
ignition module first then to the ECM or just straight to the ECM. If a
CAM or CRK code is set, careful consideration should be given to the
particular vehicle strategy. A signal that first goes to the ICM and is
not reaching the ECM could be due to it being shorted/open circuited at
the ICM. Furthermore, on most of the sensor-ICM-ECM type of systems the
actual hall effect voltage reference is provided by the ICM itself.
These smart ICMs make all the decisions after processing the actual
CAM/CRK signal and only then send a reference position signal to the
ECM. A quick glance at the wiring diagram should be the first step.
Learn and study the particular system before attempting to perform a
diagnostic.

Magnetic sensor signal output strength (amplitude) is very dependant on
the air gap between it and the triggering mechanism (reluctor wheel),
and also the speed of engine rotation. The air gap usually comes out of
adjustment over time due to engine vibration. Although the air gap on
most magnetic sensors is not adjustable, dirt and metal filing tend to
stick to the tip of the sensor and cause air gap sensing problems. A
simple cleaning sometimes fixes the problem. Engine cranking speed is
greatly affected by battery and starter condition. A slow cranking speed
problem might make the vehicle not start at all. The lower cranking
speed will also lower the sensor's signal amplitude. Internal sensor
coil condition is also a main cause of magnetic sensor failure. Water
and moisture get into the casing and corrodes the sensor's internal
coil.

Hall effect sensors are fairly unaffected by engine cranking speed
problems. They can still output a square wave even if the engine is
turned by hand. Air gap and dirty sensors is also a main problem for
hall effect sensor, as well as internal degradation due to corrosion and
vibration.

Optical sensors main ailment is dirt in the optical shutter wheel. Since
these sensors are much more sensitive, anything that interferes with the
light beam will also affect the output signal. These sensors are not
affected by low cranking speeds and they have no air gap to contend
with. However, a warped optical shutter wheel may also render the sensor
useless.

Magneto-resistive sensors are not affected by slow engine cranking
speeds either. They are however extremely sensitive to signal noise
created by out-of-adjustment air gaps and dirty sensor tip. Because of
their signal noise sensitivity, clean sensor tips are a must with these
sensors.