Sensor Module Diagnosis & Repair
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 correct 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 you try to activate the sensor by superlocking the vehicle.
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).
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.
If you have corrections, comments or suggestions, email us.
Page revised February 2, 2012