Test Notes

Normal Operation
Each crash sensor has an internal resistor. The diagnostic monitor uses the resistors in the crash sensors in combination with the two resistors inside the diagnostic monitor to create a tightly controlled diagnostic voltage at Pin 11 (Circuit 614A (GY/O)). The crash sensors are tied together inside the diagnostic monitor at Pins 17 (Circuit 617 (PK/O)), 18 (Circuit 619 (PK/W)) and 19 (Circuit 621 (W/Y)). Therefore, the resistors in the sensors are connected in parallel. The parallel combination of all the sensors resistance should be equal to 393 ohms. The resistance of each sensor should be 1180 ohms (± 12 ohms).

The resistors inside the diagnostic monitor are connected to Pins 11 (Circuit 614A (GY/O)) and 12 (Circuit 623 (P/W)) and are equal in value. Note that the Circuit 614A (GY/O) and Circuit 623 (P/W) are tied together inside the rear 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. The air bag system is wired so that current will flow from both Pins 11 and 12 through the rear safing sensor and out to the driver side air bag and passenger side air bag. Current flows through the driver air bag and into Pin 10 (Circuit 615 (GY/W)). Current also flows through the passenger side air bag into Pin 9 (Circuit 616 (PK/O)). Current then flows from Pins 9 and 10 through the diodes inside the diagnostic monitor and out to the crash sensors through Pins 17,18 and 19. The current flows through each crash sensor resistor and stops 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 Diagnostic Pin Voltage Chart.

Diagnostic Pin Voltage Chart:

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 the 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 a code 41 to indicate high resistance (or an open circuit) in one of the 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 sensors do not normally have resistance across their contacts. The resistance across safing sensor contacts should be infinite (open). If resistance appears across the safing sensor(s) contacts, then current will flow from Pin 15 (Circuit 611 (W/O)) through the abnormal resistance across the safing sensor(s) contacts and into the Circuits 614 (GY/O), and 614A (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 a 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 crash sensors. Circuits 617 (PK/O), 619 (PK/W) or 621 (W/Y) may have an open circuit in the wiring.
2. An abnormally high resistance value across one or more of the crash sensors. One or more of the crash sensor resistors may have too high resistance or may be a 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 17,18 and 19 is too high with respect to the voltage on Pin 13.
4. Resistance across the safing sensors. The safing sensors are normally open switches with infinite resistance across their contacts. If the safing sensors are faulty and some resistance builds up across the open contacts, the voltage on Circuit 611 (W/O) will pull up the voltage on Circuit 614A (GY/O).