14 Air Repair OBDII Review Volume 2 Number 1
OBDII Readiness Case Study
From the April 2005 issue of Air Repair.
By Angelo Vitullo, Emissions Program Instructor,
Automotive Technology Department; St. Louis Community College,
Forest Park Campus
The owner of a 1998 Chrysler Cirrus, with a 2.5L engine
and A/T, SMPFI, and odometer at 91,000 miles informed
a service manager of an OBDII emissions test reject
because of monitors. The inspection reports showed the
service manager that the vehicle had been rejected at the
test station seven times over a five-month period. Setting
monitors during normal driving wasn’t an issue since a
cross-country trip was taken during that five-month
period. The repair facility scan tools also confirmed the
test station monitor status report. Many unsuccessful
things were tried, including the installation of a
remanufactured PCM, and the technician driving the
vehicle according to drive trace procedures also accom-plished
nothing. Finally the vehicle was returned with
instructions to “drive the car for a few days.” The
customer departed more frustrated than ever.
Once I got involved at Outreach’s request, I con-tacted
the repair facility and the customer directly to get
the information I needed to repair the vehicle. Using my
scan tool to check the OBDII system, the only monitor
that was complete was the HO2S monitor. This monitor
is enabled at idle after a cold start with engine temperature
below 127 degrees F and battery temperature within +/-
27 degrees of engine temperature. I wondered why that
monitor ran and no others did?
When eyeballing
long lists of live
sensor data, it is
easy to overlook
key pieces of
information while
trying to rationalize
every value
displayed. But I did notice the Ambient/Battery Tempera-ture
Sensor was sending an erroneous very cold signal of
-7.6 degrees F. On a 70 degree F day, this was a red flag.
By researching the “enabling criteria” necessary to
run monitors for the EGR, HOS2 and CAT monitors, I
found they all require a minimum ambient temperature of
19 degrees F.
Also, the OBDII Misfire Counter was not function-ing.
This vehicle, as with most others, must “learn” the
specific electrical
characteristics of
the crank sensor,
the crank sensor
air gap, the ma-chining
character-istics
of the
crankshaft tone
wheel, and so on. Chrysler calls this learning process the
Adaptive Numerator. There is a drive cycle consisting of
a series of vehicle decelerations required to perform this
learning process for this particular vehicle. Some brands
of vehicles can perform this learn function in the shop
bay. Only research will reveal the proper procedure for
the subject vehicle in your shop. Without guidance, we
are lost. The scan tool readout stated the vehicle did not
complete the Adaptive Learn procedure, hence the misfire
counter, a continuous component monitor (CCM) was
disabled. In researching the enabling criteria for the
Misfire Monitor, I learned it also would be disabled due to
the ambient temperature requirement of 19 degrees.
While researching the wiring diagrams and compo-nent
locations guide, I saw there was a temperature
measurement thermistor located behind the left headlamp
housing. It was a three- wire sensor called the Battery
Temperature Sensor. The calibration error had suspended
the running of all monitors except the
HO2S Heater Monitor. Why? Because that monitor
will run as long as there is a cold start and battery
temperature is within +/- 27 degrees of engine tempera-ture.
In other words if the engine coolant temperature is
18 degrees or less during a cold start the HO2S Heater
Monitor will run. Knowing a prior repair shop replaced
the PCM in vain, I double checked the part number to
confirm it was correct.
I was confident the problem was the Ambient/Battery
Temperature Sensor. After a wiring check I replaced the
sensor and, using the scan tool, recorded a realistic
temperature value. The software engineers did not
program this sensor for rationality code setting capability.
The next goal was to run all the diagnostic monitors
(OBDII Drive Cycle) to confirm there will be no system
failures and resulting MIL Lamp Illumination. The vehicle
ran all the monitors and passed. The customer was
ecstatic that the problem was fixed.
OBDII MON
DISABLED STATUS
OBDII MON DISABLED LoTemp : YES
OBDII MON DISABLED LoBatt : NO
OBDII MON DISABLED LoBaro: NO
SENSORS
CURRENT ADAP CELL ID: O
ENG COO LANT TEMP DEG: 73.4F
ENG COO LANT TEMP VOLT: 2.59V
INTAKE AIR TEMP DEG: 68.0F
Continued on page 15.
