CRANKSHAFT/CAMSHAFT POSITION SENSORS
The advent of distributorless ignition systems has added yet
another family of sensors to the inventory of underhood
electronics, the crankshaft (CKP) and camshaft (CMP) position
sensors. These sensors serve essentially the same purpose as
the ignition pickup and trigger wheel in an electronic distributor,
the only difference being that the basic timing signal is read off
the crankshaft or harmonic balancer instead of the distributor
shaft. This eliminates ignition timing variations that can result
from wear and backlash in the timing chain and distributor gear.
It also does away with timing adjustments (or misadjustments as the
case may be).
On 1996 vehicles with Onboard Diagnostics II (OBD II), the
crankshaft position sensor is also used to detect variations in
crank speed caused by ignition misfire. If the computer senses
enough of these, it will illuminate or flash the Check Engine or
Service Engine Soon light to signal the driver he has a problem.
General Motors uses a variety of crankshaft position sensors.
One is a Hall effect crank position sensor that reads a notched
metal "interrupter" ring on the back of the harmonic balancer.
This was first used on the early 3.8L V6 Buick Sequential Fuel
Injection (SFI) engines (and turbos) with distributorless
Computer Controlled Coil Ignition (C3I). The crank position
sensor provides an on-off signal to the ECM that the ECM uses to
monitor engine rpm and crank position. The system also uses a
separate cam position sensor in place of the original distributor
to inform the ECM about valve timing. This enables the ECM to
determine the correct firing sequence which it then uses to
control both injector and ignition timing. Ford uses a similar
setup on its 5.0L V8 with distributorless ignition.
Another type of crankshaft position sensor GM uses is the
"combination sensor" which you will find mounted on the front of
the 3.0L and 3300 V6. GM calls it a combination sensor because
the crank position sensor contains a pair of Hall effect switches
that generate two separate signals. There are two notched
interrupter rings on the back of the harmonic balancer. One ring
has three notches which causes one of the Hall effect switches to
generate three crank position signals every revolution. The
other ring has only one notch, which causes the other Hall effect
switch to generate a single "sync-pulse" signal that the ECM uses
to calculate rpm and ignition timing.
Another variation of the combination sensor is the "fast
start" system used on GM's 3800 engine. A pair of Hall effect
switches are mounted by the crank pulley while a cam sensor is
mounted over the timing gear. One crank signal generates 3
pulses per revolution while the other generates 18. This allows
the coil module to "sync" with the engine more quickly so the
engine will start almost instantly.
The third type of crankshaft position sensor GM uses is a
magnetic pickup that reads slots machined in a "reluctor" ring in
the center of the crankshaft. This setup is used with the Direct
Ignition Systems (DIS) on the 2.0L, 2.5L and 2.8L engines, and
the Integrated Distributorless Ignition (IDI) on the 2.3L Quad 4.
The crank reluctor ring has six equally spaced slots 60 degrees
apart. A seventh slot is spaced 10 degrees from one of the
others so the crank sensor will generate an extra "sync-pulse"
every revolution. The ECM then uses the information to calculate
proper ignition and injector timing. This type of sensor must be
carefully positioned so the air gap is within .050 in. of the
crankshaft reluctor ring.
MAGNETIC CRANK POSITION SENSOR CHECKS
Whether a crankshaft position sensor is the magnetic type or
a hall effect switch, most problems can be traced to faults in
the wiring harness. A disruption of the sensor supply voltage,
ground or return circuits can cause a loss of the all-important
timing signal resulting in an engine that cranks but won't start.
When troubleshooting a suspected crankshaft position sensor
problem, you must follow the diagnostic flow charts in the manual
to isolate the faulty component -- otherwise there's no way to
know if the problem is in the ignition module, computer, wiring
harness or sensor.
On older GM applications, a trouble Code 12 while cranking would
indicate no reference signal being generated. On older Ford
applications, a Code 14 would indicate a problem with the crank
position sensor signal, which Ford calls a "PIP" (Profile
Ignition Pick-up) signal.
On OBD II vehicles, a faulty crankshaft position sensor may
set a code P0338 or P0339. A faulty camshaft position sensor
may set a code from P0340 to P0349.
Magnetic sensors can be checked by unplugging the electrical
connector and checking resistance between the sensor's two
terminals. On the 2.3L Quad 4, for example, the sensor should
read between 500 and 900 ohms. On a Saturn 1.9L engine (which
has a high failure rate of crankshaft sensors), the resistance
specification is 700 to 900 ohms. Always look up the exact
specifications for the vehicle because the specifications will
vary from one application to another.
Testing the resistance of a magnetic cranksahft position sensor
with ah ohmmeter will tell you if the sensor is within
specifications. If it reads open or is shorted, it must be
An "intermittent cranks but won't start because of no spark" problem
may occur if a magnetic crankshaft position sensor changes resistance
excessively when it heats up and cools down. Some increase in
resistance is normal when it heats up. But if the resistance suddenly
jumps or drops outside of specifications, the sensor is bad and
must be replaced.
The sensor can be HOT tested by removing it and immersing the magnetic
tip in a pan of hot water. Watch the resistance as it heats up. Then
remove the sensor and watch the resistance as it cools down. DO NOT
attempt to heat the sensor with a propane torch or open flame as doing
so will damage the sensor.
A magnetic crank position sensor should also produce an
alternating current when the engine is cranked so a voltage
output check is another test that can be performed. With the
sensor connected, backprobe the connector and read the sensor's
output voltage while cranking the engine. If you see at least
20 mV on the AC scale, the sensor is producing a good signal.
If you have a scope, you can also observe the sensor's waveform.
It should produce an alternating current signal that changes
frequency and amplitude with engine rpm.
HALL EFFECT CRANK POSITION SENSOR CHECKS
Hall effect crankshaft position sensors typically have three
terminals; one for current feed, one for ground and one for the
output signal. The sensor must have voltage and ground to
produce a signal, so check these terminals first with an analog
voltmeter. Sensor output can be checked by disconnecting the
coil and cranking the engine to see if the sensor produces a
voltage signal. The voltmeter needle should jump each time a
shutter blade passes through the Hall effect switch. If observed
on an oscilloscope, you should see a square wave form.
If your diagnosis reveals a faulty crank sensor, the only
option is to replace it. With Hall effect sensors, the sensor
must be properly aligned with the interrupter ring to generate a
clean signal. Any rubbing or interference could cause idle
problems as well as sensor damage. Magnetic crankshaft position
sensors must be installed with the proper air gap, which is
usually within .050 in. of the reluctor wheel on the crankshaft.
CAMSHAFT POSITION SENSORS
On many engines with distributorless ignition systems and
sequential fuel injection, a camshaft position sensor is used to
keep the engine's control module informed about the relative
position of the crankshaft. By monitoring cam position (which
allows the control module to determine when the intake and
exhaust valves are opening and closing), the control module can
use the cam position sensor's input along with that from the
crankshaft position sensor to determine which cylinder in the
engine's firing sequence is approaching top dead center. This
information is then used by the engine control module to
synchronize the pulsing of sequential fuel injectors so they
match the firing order of the engine. On some applications,
input from the camshaft position sensor is also required for
The camshaft position sensor may be magnetic or Hall effect,
and mounted on the timing cover over the camshaft gear, on the
end of the cylinder head in an overhead cam application, or in a
special housing that replaces the distributor (in the case of
some of the GM applications). Operation and diagnosis is
essentially the same as that for a crankshaft position sensor.
On engines with Variable Valve Timing (VVT), a camshaft position
sensor may also be used to monitor the relative position of the
cam to determine if cam timing is advanced or retarded. The PCM
uses this information to monitor the operation of the VVT system.
The relative timing of the overhead cam(s) may be changed at
higher rpm to increase power.