Nox sensor / diagnostic impacts
Nox sensor / diagnostic impacts
Mobile Selective Catalytic Reduction
Compressed
Air supply
DEF tank
Temperature Sensors
N2 + H2O
Injection
nozzle
High Nox
Decomposition Area
Exhaust
J1939 Nox
Sensor
SCR Brick
in Tailpipe area
NOx Sensor monitor
Component Location:
The aftertreatment outlet NOx
sensor location can vary
depending on engine
application. It is usually located
in the exhaust system in the
acoustic section of the EGP.
The sensor can be damaged
by water.
The Nitrogen Oxides (NOx) sensor is a “smart device” consisting of three
components. i.e, NOx sensor, Connecting cable, NOx sensor Control Module.
It’s used to measure the outlet NOx emissions from the engine. It receives
and sends information to the Engine ECM via the J1939 datalink. The NOx
sensor performs its own Internal diagnostics and reports malfunctions back to
the Engine ECM using J1939. The assembly is a single part, the sub
components cannot be replaced individually!
NOx sensor functionality.
At key on the NOx sensor heats up to 100C.
It then waits for a “Dew Point” signal from the engine ECM.
“Dew Point” is the point after which there is no moisture or
condensation in the exhaust that would cause the NOX sensor to
fail. The dew point is currently set to 120C. This measurement is
taken from the outlet EGP temperature sensor.
When the dew point signal is received from the Engine ECM the
sensor then heats up to 800C. Note: Water contact at this point
will damage the sensor.
It then starts to take NOx measurements.
NOX information is broadcasted on the J1939 to the engine ECM
This information is acted on every time the NOx monitor runs.
NOx sensor modes
“Un-Powered Mode”
In this condition, voltage supply to the
device is OFF. This is the normal state
of the sensor when the vehicle is in an
OFF condition. There is no output from
24v
the sensor.
CAN
Powered - Sensor Inactive
Power is supplied to the sensor via the
vehicle keyswitch. The sensor enters the
pre-heat phase, to remove any moisture
which may be present. This preheat phase
lasts for approximately 60 seconds.
Powered - Sensor Active
On receipt of the ‘Dewpoint’ message the sensor
element is heated to approximately 800C. Once the
sensor has reached the necessary temperature the
measured NOx and oxygen concentrations are
broadcast by the sensor.
NOx Sensor
Pin 1 - Can +
Pin 2 - Can -
Cummins Part Number - 4954222
Pin 3 - Battery -
- 4984577
Pin 4 - Ignition + 20 amp fuse
NOx Sensor Wiring.
NOx Sensor mounting
The NOx sensor Electronic Control Unit (ECU), must be mounted in a
location close enough to the sensor element to ensure adequate stress
relief of the connected cable, but sufficiently protected from excessive
temperatures.
Proper operation of the sensor requires that fresh air be able to pass to the
sensor element through the air permeable membrane in the ECU. To
ensure that this membrane is not obstructed or blocked by standing
moisture or other debris, the ECU must be mounted within the constraints
shown in the drawing below.
Painting and vibration
Painting/Coating
The NOx Sensor Electronic Control Unit is equipped with a
Gortex vent which allows for the transfer of fresh air into the sensor as a
reference gas. It is important to ensure that this vent is not obstructed by
any paint or coatings applied to the vehicle chassis after sensor
installation.
If painting or coating is required, the vent shall be protected by masking
during the process, with the masking material removed before sensor
operation.
Vibration Levels
The Nox sensor probe and associated electronic control unit
must be protected from excessive vibration levels. The sensor probe is
installed in the exhaust pipe work or inside the EGP, and is designed to
withstand vibration levels under normal operating conditions. .
The Nox sensor control module is usually mounted on the chassis or
superstructure.
Any cables should not be “tight” or under tension and allow for
movement.
NOX Sensor location vehicle
NOx Monitor Operation - OBD 1.5 engine
Conditions for the NOx monitor to run:
Dosing Rate to be between
800 to 3800 ml/h 6cyl engine.
600 to 3800 ml/h 4cy engine.
Engine Speed to be between
1400 - 1700 rpm 6 cylinder engine.
1400 - 1750 rpm 4 cylinder engine.
NOx Out sensor giving a stable reading
Fueling stable for 15 seconds
The un-intrusive stage will take two readings then work out the average,
if the average reading is within specification then the monitor will have
passed, if it is outside specification then the intrusive part of the algorithm
will run.
NOx sample 2
NOx sample 1
NOx
If measured NOx average (two samples) is within
a tolerance of the estimated level, then test
passed. If not then intrusive part runs.
NOx
Drive Cycle
comparison
NOx Monitor Operation - Intrusive
The intrusive part of the algorithm uses these conditions to run:
Engine speed over
• 1500 rpm 6 cylinder engine.
• 1500 rpm 4 cylinder engine.
Estimated engine NOx needs to be between 800 to 1800 ppm, this
will mean that load will need to be applied to the engine.
Doser to be in dosing state
NOx Out sensor giving a stable reading
The system will monitor for 15 seconds at these conditions and take a
reading, it will then switch off dosing and wait another 20 seconds and
take a second reading.
If the engine goes out of these conditions then the intrusive part will abort
and attempt to restart.
NOx sample 2.
Taken after
dosing is
disabled
NOx
NOx sample 1.
Dosing active
Intrusive Part - NOx increase
The level of NOx will increase when dosing is disabled.
If the difference between these readings is above a
predetermined limit then the monitor will pass.
NOx
Drive Cycle
comparison
NOx Monitor Operation - Intrusive
Once completed the monitor will then compare the two readings:
If the difference between the two readings is too low the monitor will fail
for the first time, if the monitor has a second failure fc(s) 2772 / 2773 will
log and the MIL will illuminate.
If the difference between these readings is above the predetermined limit
then the monitor will pass.
NOx Monitor Operation - Intrusive
Note: The Intrusive part will only run if the first part of the NOx monitor
fails, if the monitor passes then the intrusive part will not run.
To remove the derate the monitor will have to run and pass.
The correlation between % load and Dosing for estimated NOx ppm are
as follows:
Greater than 50% load will give a dosing rate of over:
•
800ml/h 6 cylinder engine.
•
600ml/h 4 cylinder engine.
Greater than 60% - 6 cylinder / 45% - 4 cylinder load will give an
estimated NOx output of over 800ppm
NOx Monitor Operation - OBD 2 engine
The conditions and operation of the NOx monitor have been
revised: The exact conditions to run the Nox count vary per
application and power rating.
The Nox monitor will now operate once per drive cycle. It will take
24 samples and calculate a Nox average, if this average is within
specification then the count has passed, if not the Nox count has
failed.
If the Nox count fails on a given drive cycle, an active, pending
MIL off code is stored. If the Nox count then fails on a second
consecutive test, the code will be active, confirmed, MIL ON.
If the test passes the pending code is erased.
NOx sample
NOx
If measured NOx average (24 samples) is within a
tolerance of the estimated level, then “Nox count”
passed. If not “Nox count” failed.
2 consecutive Nox count fails will give MIL ON
NOx
Drive Cycle
comparison
New INSITE™ 7.2 Aftertreatment NOx Reset
Test Information
New INSITE™ 7.2 electronic service tool Aftertreatment High NOx Reset Test
will help clear OBD malfunction indicator lamps (MIL) for Fault Codes 1694,
2772 and 2773.
About:
The Aftertreatment High NO`x Reset Test is used to turn off a malfunction
indicator lamp (MIL), once the issue causing the fault has been corrected. The
faults that cause the MIL lamp can not be reset by INSITE™ electronic service
tool.
Lets look at this on Insite !
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