Kohler Command Pro EFI ECH940 Service Manual page 38

EFI SYSTEM
Oxygen Sensor (O )
2
Components
Pin D
Cutaway Oxygen Sensor Components (O
C
A
B
A Protection Shield
C Lower Insulator
E Upper Insulator
High Temp
G
Water Seal
Temperature must be controlled very accurately and gas
constituents measured to a high degree of accuracy for
absolute sensor measurements. This requires laboratory
equipment to determine a good or bad sensor in fi eld.
Furthermore, as with most devices, intermittent problems
are diffi cult to diagnose. Still, with a good understanding
of system and sensor, it is possible to diagnose many
sensor problems in fi eld.
Using diagnostic software connected to ECU is a useful
technique for observing sensor performance. However,
user must understand that such software reads a signal
generated by ECU. If there is an ECU or wiring problem,
readings could be misinterpreted as a sensor problem.
Digital nature of signal to software means that it is not
reading continuous output of sensor. A voltmeter can
also be used as an eff ective tool in diagnosing sensors.
It is advisable to use an electronic meter such as a
digital voltmeter. Simple mechanical meters may place
a heavy electrical load on sensor and cause inaccurate
readings. Since resistance of sensor is highest at
low temperatures, such meters will cause largest
inaccuracies when sensor is in a cool exhaust.
38
Pin A
Pin B
Pin C
)
2
D
F
E
Planar Element
B
and Heater
Stainless Steel
D
Housing
Terminal Connection
F
to Element
Visual Inspection
1. Look for a damaged or disconnected sensor-to-
engine harness connection.
2. Look for damage to sensor lead wire or associated
engine wiring due to cutting, chaffi ng or melting on a
hot surface.
3. Disconnect sensor connector and look for corrosion
in connector.
4. Try reconnecting sensor and observe if problem has
cleared.
5. Correct any problems found during visual check.
Sensor Signal Observation
NOTE: Do not cut into or pierce sensor or engine
wiring to make this connection. Sensor
produces a very small signal. Corrosion or
damage to wiring could lead to an incorrect
signal because of repairs or contamination to
sensor.
G
1. Using a voltmeter, observe voltage between Pin C
and Pin D before engine is started. With key ON,
and sensor unplugged, voltage should read about
5.0 volts. With sensor attached, using diagnostic
software, voltage should read about 1.0 volt. This
voltage is generated by ECU. If it is not present,
there is a short in associated wiring and corrective
action should be taken. If voltage still is not present,
there is a problem with ECU or engine harness.
2. Reconnect sensor and start engine. Run engine at
suffi cient speed to bring sensor up to operating
temperature. Maintain for 1 to 2 minutes to ensure
that engine has gone closed loop. Once in closed
loop, sensor voltage should cycle between about
100 to 250 mv (low speed idle) and 700 to 900 mv
(high speed no load). If this cycling is not observed,
a determination must be made, if problem is with
engine or sensor.
3. Check engine harness for battery voltage on heater
circuit.
Removal Inspection
NOTE: Apply anti-seize compound only to threads.
Anti-seize compound will aff ect sensor
performance if it gets into lower shield of
sensor.
1. If sensor has heavy deposits on lower shield,
engine, oil, or fuel may be source.
2. If heavy carbon deposits are observed, incorrect
engine fuel control may be occurring.
3. If sensor is at room temperature, measure between
signal leads, black wire (Pin C) and grey wire (Pin D)
attached to sensor. If resistance is less than one
megohm, sensor has an internal short.
4. With sensor at room temperature measure heater
circuit resistance, purple wire (Pin A) and white wire
(Pin B), resistance should be 8.1-11.1 Ω.
5. If a damaged sensor is found, identify root cause,
which may be elsewhere in application. Refer to
Troubleshooting-Oxygen Sensor (O
KohlerEngines.com
) table.
2
62 690 13 Rev. A