Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
97
only, and the circuit grounds through the sensor hous-
ing. With these, a loose mounting or dirt and corrosion
on the sensor threads creates resistance on the ground
circuit. With a two wire HO2S, one wire carries the sig-
nal and the other grounds the sensor through the PCM.
A heated oxygen sensor (HO2S) has a three or four wire
circuit: three if the sensor and heater have a common
ground and four if each has a separate ground circuit.
A simple procedure for HO2S diagnosis is to remember
the phrase “lean and low.” When the exhaust oxygen
content is high, there is somewhat less difference be-
tween the oxygen in the air and the oxygen in the ex-
haust. Therefore, the sensor will produce a lower volt-
age (100mv to 420mv), which indicates a lean condition
to the PCM.
When exhaust oxygen content is low, there is a large dif-
ference between the oxygen in the air and the
oxygen in the exhaust. Therefore, the sensor will
produce a higher voltage (460mv to 900mv), which
indicates a rich condition to the PCM.
The sensor continually switches between high and low
voltage, signaling the PCM to maintain the ideal air-fuel
ratio. An HO2S voltage of 440 to 550mv is equivalent to
about a 14.7 to 1 air-fuel ratio. An HO2S must be at
572
o
F (300
o
C) before it generates a valid signal.
An HO2S produces a low voltage, low current analog
signal that is very susceptible to induced voltage inter-
ference. Since signal voltage is rapidly changing, it is
difficult to get an accurate reading on a DMM. Most
control systems provide HO
2
S information on the data
stream, but this is not always a reliable diagnostic aid.
The best way to evaluate HO
2
S operation is to monitor
circuit activity with a lab scope.
When viewing an HO
2
S trace on a lab scope look for
even amplitude with rounded peaks; the rising slope
tends to be slightly steeper than the trailing slope. Fre-
quency of the signal increases with engine speed. The
mid-point of the wave moves up on the screen when the
mixture is rich and down when it is lean. Some noise on
the trace is normal and is exaggerated if scope leads are
too close to secondary ignition leads. A short or clipped
peak can indicate a cylinder firing, EGR flow, or other
problem that affects combustion, figure 5-12.
Position Sensor
On an automotive control system, position information
is generally provided to the PCM by a potentiometer
sensor. A potentiometer is a variable resistor that
provides an analog voltage signal to the PCM based on
the motion or position of the monitored component.
Most automotive systems use a potentiometer as a TP
sensor, to provide a driver demand signal based on
accelerator pedal position. Systems may also use a po-
tentiometer to measure EGR valve opening, canister
purge valve condition, and vane airflow sensor
position information to the PCM. A potentiometer
requires three wires:
• Reference
• Signal
• Ground
All potentiometers work in a similar fashion to convert
mechanical movement into a variable voltage. The volt-
age supplies input information to the PCM as an analog
signal. The PCM applies a reference voltage, usually 5
volts, to one end of a variable resistor. A terminal at the
opposite end of the resistor connects to the PCM
ground path. The third terminal, which attaches to a
movable wiper that sweeps across the resistor, sends
the variable signal voltage back to the PCM. Internal
PCM circuits digitize the analog potentiometer signal for
processing.
Signal voltage is high or low, depending on whether the
movable wiper is near the supply end or the ground end
of the resistor, figure 5-13. Most automotive poten-
tiometer sensors transmit a low signal voltage when the
mechanical device they attach to is at rest. For exam-
ple, when the throttle or EGR valve is closed, the po-
tentiometer signal voltage is at its lowest, which is
slightly above zero. As the linkage moves to its fully
open position, signal voltage rises to its highest, which
is slightly below reference voltage.
Check potentiometer operation by connecting a DMM
to the signal and ground wires and operating the device
through its full range of travel, figure 5-14. Look for cor-
rect minimum and maximum voltages and a smooth
transition as the device moves from closed to open. Sig-
nal voltage that is too high in the closed position indi-
cates either a ground circuit problem or an incorrectly
adjusted sensor.
A common potentiometer failure is signal dropout,
where the signal is lost as the wiper moves through a
worn area on the resistor contact. Although difficult to
isolate with a DMM, signal dropout is easily spotted
when performing a sweep test with a lab scope, figure
5-15. A regular, reoccurring dropout indicates wear,
while an irregular dropout indicates problems on the
power, or reference voltage, side of the circuit. This can
be caused by a circuit or PCM failure, troubleshoot the
circuit to find the source of the problem.
Pressure Sensor
Automotive control systems measure MAP or baromet-
ric pressure (BARO) with a pressure-sensing device to
determine the load on the engine. These sensors gen-
Summary of Contents for ASE-A8
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