Ultrasonic
Sensor Module Diagnosis & Repair
Introduction
If the "Alarm Fault" message is displayed on the Message Center, the
most likely cause is malfunction of the ultrasonic sensor module
mounted on the headliner (photo at right).
This device monitors the
interior of the
vehicle for intruders when the vehicle is in the "superlocked"
state. Replacement of the
sensor is
simple and, if you are lucky, will solve the problem. A Technical
Service Bulletin was issued on the subject, stating the problem is most
commonly found on 1998 models (Vin WA...). After this an improved
version of the sensor was issued.
Photo at right:
Ultrasonic motion sensor mounted near driver's head on headliner above
"B" pillar.
Self-Diagnostic Process Causing "ALARM FAULT" Message
The BECM rather cleverly does a check on the sensor every
time
you switch the engine off and get out. The BECM knows when you remove
the key from the ignition and when you open and close the driver's door
-- it assumes this means the driver has got out. It then checks that
the correcct signals from teh sensor are received, detecting the
movement of the driver as he or she gets out. If the sensor does not
work 5
consecutive times, the BECM it generates the "Alarm Fault" message the
next time youu try to activate the sensor by superlocking the
vehicle.
Diagnostic Procedure
Persuade an unsuspecting volunteer to sit inside the vehicle while you
remote superlock it from the outside to
activate the sensor. Get your victim to move around -- this should set
the alarm off.
If the alarm goes off
but the fault message persists, the problem is most likely poor
connections to the ultrasound sensor. Check the connections to the
sensor and (at the other end) to the BECM (Connector C362, a black 16
pin connector on the front of the BECM uinder the right front
seat). Pin 6 is the signal from the sensor telling the BECM a movement
has been detected, and pin 15 supplies power to the sensor. If you are
diagnosing system operation, it is useful to know one detail about the
signal between ultrasonic module and BeCM:
When the module detects movement, it grounds the signal wire 4 times.
Photo at right:
BECM removed from vehicle, showing connector C362 second from right on
lower row of connectors on front of unit.
It might also be worth checking the ground connection for the
circuit, which is E367, located in the left side of the luggage
compartment behind the trim panel.
If the alarm does not go off,
tell your volunteer to wave a hand in front of sensor. If it
then activates, the problem is most likely a bad sensor with
insufficient sensitivity.
Replacement of Sensor Module
Replacement of the sensor module is fortunately straightforward, and is
described uin the "electrical" section of the shop manual. To remove it
you just pull the unit out of the headliner (it is held in place by a
spring clip at each end), and disconnect the multiplug on the back of
it. Replacement is the reverse of removal.
Repairing the Original Module
If you are handy with electronics, an alert reader from Finland reports
that you can probably repair it yourself. He observes: "Instead of
replacing the whole expensive ultrasonic module, I managed
to fix mine just by replacing the ultrasonic receiver transducer inside
the module
(see the picture pelow). It costs just few euros and should not be
too hard to find." It is a standard 40 kHz ultrasonic transducer. A
faulty receiver or transmitter transducer
can be identified
with a good multimeter: capacitance of a working transducer should be
around 1500-2000 pF. The photo below illustrates the module's circuit
board and the ultrasonic receiver transducer itself (red arrow).
Parts Needed
The part required for replacement of the entire module is: Ultrasonic
sensor YWC103640LUM. These parts have
a date stamped on them; if dated
1-10-98 or later it
is the improved version. If you are getting a used sensor as a
replacement, you will be safer if it is from a 1999 or later
vehicle.
