Test Notes
Primary Crash Sensor Circuits High Resistance or OpenNORMAL OPERATION
Each primary crash sensor has an internal resistor. The air bag diagnostic monitor uses the resistor in the primary crash sensors in combination with the two resistors inside the air bag diagnostic monitor to create a tightly controlled diagnostic voltage at Pin 11 (Circuit 614, GY/O). The primary crash sensors are tied together inside the air bag diagnostic monitor at Pins 17 and 19. Therefore, the resistors in the sensors are connected in parallel. The parallel combination of both sensor resistors should be equal to 393 ohms. The resistance of each sensor is 787 � 10 ohms.
The resistors inside the air bag diagnostic monitor are connected to Pins 11 and 8, and 12 (Circuit 623, P/W) and are equal in value. Note that Circuits 614 (GY/O) and Circuit 623 (P/W) are tied together inside the safing sensor. Therefore, the two resistors inside the air bag diagnostic monitor are connected in parallel and will function the same as one resistor of half the original value. Current flows from Pins 1 and 6 through the resistors, out to Circuits 614 (GY/O) and 623 (P/W) on Pins 8, 11 and 12, through the LH kick panel safing sensor and out to the driver side air bag module and passenger side air bag module. Current flows through the driver side air bag module and into Pin 10 (Circuit 615, GY/W) and through the passenger side air bag module and into Pin 9 (Circuit 616 PK/BK). Current then flows from Pins 9 and 10 through the diodes inside the air bag diagnostic monitor and out to the primary crash sensors through Pins 17 and 19. The current flows through each primary crash sensor resistor and terminates at the case ground of each primary crash sensor. Pin 11 is the midpoint of the resistor network and voltage at Pin 11 will change with vehicle charging system voltage. The expected voltage at Pin 11 is shown in the table. The air bag diagnostic monitor measures the vehicle charging system voltage at Pin 13 (battery input). By measuring the voltage at Pin 13, the air bag diagnostic monitor can accurately predict what the voltage at Pin 11 should be in a normal functioning system.
If the resistance of one or more of the primary crash sensors has increased in value, then the parallel equivalent resistance of the primary crash sensors will be greater than 393 ohms. When primary crash sensor resistance increases the voltage at Pin 11 will increase and the air bag diagnostic monitor will flash diagnostic trouble code 41 to indicate high resistance (or an open circuit) in one of the primary crash sensor circuits. Another situation that may cause high voltage on Pin 11 is if resistance builds up across the normally open LH kick panel safing sensor contacts. The LH kick panel safing sensor does not normally have resistance across its contacts. If resistance appears across the LH kick panel safing sensor contacts, then current will flow from Pin 15 (Circuit 611, W/O) through the abnormal resistance across the LH kick panel safing sensor contacts and into Circuit 614 (GY/O). Since the voltage at Pin 15 is battery voltage or higher, this will also cause an increase in voltage at Pin 11 and the air bag diagnostic monitor will flash diagnostic trouble code 41.
POSSIBLE CAUSES
Higher than normal voltage on Pin 11 can be caused by:
1. An open in one or more of the circuits between the air bag diagnostic monitor and the primary crash sensors. Circuits 617 (PK/O) or 621 (W/Y) may have an open circuit in the wiring.
2. An abnormally high resistance value across one or more of the primary crash sensors. One or more of the primary crash sensor resistors may have too high resistance or may be completely open circuit. 787 � 10 ohms is normal.
3. Resistance across the LH kick panel safing sensor. The LH kick panel safing sensor is a normally open switch with infinite resistance across its contacts. If the safing sensor is damaged or worn and some resistance builds up across the open contacts, the voltage on Circuit 611 (W/O) will pull up the voltage on Circuit 614 (GY/O).
4. An intermittent open between the positive terminal of the battery and Pin 13 (Circuit 937,R/W) may cause the air bag diagnostic monitor to determine that voltage on Pins 17 and 19 is too high with respect to the voltage on Pin 13. Check fuse 5 and power-up connections.