Test Notes
Normal OperationEach primary crash sensor has an internal resistor. The diagnostic monitor uses the resistor in the primary crash sensors in combination with the two resistors inside the 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 diagnostic monitor at pins 17 (Circuit 617, PK/O) and 18 (Circuit 619, PK/W). Therefore, the resistors in the sensors are connected in parallel. The parallel combination of all the sensor resistors should be equal to 393 ohms. The resistance of each sensor should be 787 ohms ± 10 ohms.
The resistors inside the diagnostic monitor are connected to pins 11 (Circuit 614, GY/O) and 12 (Circuit 623, P/W) and are equal in value. Note that Circuits 614 and 623 are tied together inside the safing sensor. Therefore, the two resistors inside the diagnostic monitor are connected in parallel and will function the same as one resistor of half the original value. Current flows from the ignition voltage within the diagnostic monitor, through the resistors, out to Circuits 614 (GY/O) and 623 (P1W) on pins 11 and 12, through the 5-wire safing sensor and out to the driver side air bag. Current flows through the driver side air bag and into pin 10 (Circuit 615, GY/W). Current then flows from pin 10 through the diode inside the diagnostic monitor and out to the primary crash sensors through pins 17,18 and 19. The current flows through each primary crash sensor resistor and terminates at the case ground of each 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 below. The diagnostic monitor measures the vehicle charging system voltage at pin 13 (battery input). By measuring the voltage at pin 13, the 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 crash sensors has increased in value, then the equivalent resistance of the crash sensors will be greater than 393 ohms. When crash sensor resistance increases, there is less overall resistance pulling the diagnostic voltage down. Therefore, the voltage at pin 11 will increase and the diagnostic monitor will flash 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 safing sensor contacts. Safing sensor(s) do not normally have resistance across their contacts. If resistance appears across the safing sensor contacts, then current will flow from pin 15 (Circuit 611, W/O) through the abnormal resistance across the 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 diagnostic monitor will flash 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 diagnostic monitor and the primary crash sensors. Circuit 617 (PK/O) or 619 (PK/W) 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.
3. An intermittent open between the positive terminal of the battery and pin 13 may cause the diagnostic monitor to determine that voltage on pins 17 and 18 is too high with respect to the voltage on pin 13.
4. Resistance across the safing sensor. The safing sensor is a normally open switch with infinite resistance across its contacts. If the safing sensor is faulty 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).