Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
• Inspect the engine control module (ECM) grounds
for being clean, tight, and in the proper locations.
• Remove and inspect the air filter element for dirt or
for restrictions.
• Inspect the air intake ducts for being collapsed,
damaged areas, looseness, improper installation, or
leaking especially between the MAF sensor and the
throttle body.
• Inspect the Transaxle Range Switch input with the
vehicle in drive and the gear selector in drive or
overdrive.
INPUT SENSORS
An assortment of sensing devices used throughout the
vehicle electronically transmit operating information to
the PCM. Sensor signals can describe the state, tem-
perature, position, or operating status of the system or
component they monitor. Automotive sensors are clas-
sified as:
• Speed sensor
• Oxygen sensor
• Position sensor
• Pressure sensor
• Temperature sensor
The crankshaft position (CKP) signal is the primary input
and the foundation upon which all other PCM functions
are based. Manufacturers refer to this input as the dis-
tributor, tach, engine speed, RPM, or reference signal,
depending upon the system. The purpose of this signal
is to let the PCM know the engine is turning over and
how fast it is turning. If there is no signal, the PCM will
not initiate ignition or fuel delivery. An erratic signal on a
running engine results in incorrect operation of the igni-
tion system and the fuel injectors. Systems use Hall-ef-
fect sensors, magnetic pickups, and optical sensors to
provide a CKP signal.
Different electronic control systems assign different pri-
orities to the other sensors once the engine is running.
Often, a system can operate without any noticeable dri-
veability problems when only a few of the high priority
sensors are functioning. Begin troubleshooting by
checking the signal of these high-priority sensors. Typ-
ically, the list includes the throttle position (TPS), mani-
fold absolute pressure (MAP), or mass airflow (MAF),
and engine coolant temperature (ECT) sensors. These
components, as well as other sensing devices in the
system, fit one of the categories mentioned above.
Regardless of how a sensor is used, you need to know
the following to analyze its performance:
• What type of signal it transmits
• How it provides information to the PCM
• What its specific application is
• How it connects to the circuitry
The type of signal can be analog or digital, DC or AC.
The signal may provide information to the PCM as a
variable voltage, a digital step signal, or a pulse train.
The specific application is what the unit is sensing. A
wiring schematic will show how the unit connects to the
circuitry.
Speed Sensor
Vehicles use a number of speed sensors. Speed sen-
sors can monitor the crankshaft, camshaft, distributor
shaft, transmission components, driveshaft, axle shafts,
or wheels to determine how fast they are rotating.
There are three basic designs for automotive speed sen-
sors:
• Magnetic pickup
• Hall-effect switch
• Optical sensor
A magnetic pickup is an analog device that produces an
AC voltage, while Hall-effect switches and optical sen-
sors are digital and produce a DC signal. For all types,
the PCM uses the frequency of the signal to determine
the speed of the sensed component. Refer back to
Chapter Two for further details of the operation of these
sensors.
Oxygen Sensor
As discussed in Chapter Three a heated oxygen sensor
(HO2S) is an analog voltage generating device that pro-
duces a linear voltage signal based on the oxygen con-
tent of the exhaust, figure 5-11. An HO2S provides feed-
back information to the PCM on how well the electronic
control system is responding to changing demands for
fuel.
Although there are several HO2S types, most are gal-
vanic batteries. The HO2S produces a signal whose fre-
quency, amplitude, voltage levels, duty cycle, and peak-
to-peak voltages provide combustion efficiency
information to the PCM, which is used to regulate fuel
injector pulse width. An OBD II system uses two sen-
sors, an upstream HO2S before the catalytic converter
and downstream HO2S after it. The PCM can gauge
converter operating efficiency by comparing the two
signals. The following discussion applies to upstream
units.
An HO2S can have a one, two, three, or four wires link-
ing it to the PCM. Single wire circuits deliver a signal
96
Summary of Contents for ASE-A8
Page 2: ......