Theory of Operation
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
voltage 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 output 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 360º 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 pickups
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.
Component Testing
MAGNETIC SENSORS:
• The first step in testing a magnetic sensor is to scope
the signal output for proper amplitude and frequency. The quality of the
wave should be consistent as well.
• No sudden drops should be present. Remember that magnetic
sensors produce their own voltage. A low amplitude problem is either the
result of improper air gap or low starter cranking speed. It is even
possible for an ECM to put out a CRK or CAM code due to a bad starter or
battery. Always make sure that the starting and battery systems are up to
specs.
HALL EFFECT & OPTICAL SENSORS:
• Because of the similarities between these two types of
sensors the procedure to test them is much the same. Always check for proper
feed voltage, reference voltage and ground first. These particular voltages
vary between different manufacturers. The ECM provided ground is the second
point to check with this type of sensor. Once it is verified that power and
ground are present the sensor should toggle the reference voltage. Remember
that low cranking speed does not affect this type of sensor.
MAGNETO-RESISTIVE SENSORS:
•
These are basically tested in the same manner as the hall-effect type sensor
with an added difference, they do not toggle a reference voltage to ground.
Always check for supply voltage and ground. If these two are present and the
sensor is operational, there should be a square wave signal output.
As explained before, study the signal path from the sensor
to the ECM. And, always consider whatever stands between this sensor and the
ECM as possible suspects. A thorough understanding of the system being
worked on is of crucial importance to proper diagnostics. This article
simply tries to dissect and summarize the workings and procedures needed to
perform a proper diagnostics.
