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Home >> Mercury >> 1994 >> Grand Marquis V8-281 4.6L SOHC >> Repair and Diagnosis >> Restraints and Safety Systems >> Air Bag Systems >> Testing and Inspection >> Diagnostic Trouble Code Tests and Associated Procedures >> Manufacturer Code Charts >> 24: Safing Sensor/Primary Crash Sensor Open >> Test Notes

Test Notes

Safing Sensor Output Feed/Return Circuit Open or Low Resistance in a Primary Crash Sensor

Normal 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 primary crash sensor should be 787 � 10 ohms.

The resistors inside the air bag diagnostic monitor are connected to Pins 11 and 12 and are equal in value. Note that Circuits 614 (GY/O) and 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 11, 8 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 ends 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 following 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 connection between Circuits 614 (GY/O) and 623 (P/W) inside the LH kick panel safing sensor is broken, then the resistor on Pin 12 is no longer in the diagnostic circuit. Both resistors are needed to pull up the diagnostic voltage to the correct value. In this situation, the resistor on Pin 11 is the only resistor inside the air bag diagnostic monitor pulling up the voltage. Therefore, the voltage on Pin 11 will be half of the normal expected voltage (since the total resistance now has doubled) and the voltage on Pin 12 will be about equal to the vehicle charging system voltage. The air bag diagnostic monitor does not measure the voltage on Pin 12, so the air bag diagnostic monitor bases its decision strictly on the voltage at Pin 11. If the voltage at Pin 11 is lower than it should be, the air bag diagnostic monitor will flash diagnostic trouble code 24. Another situation that can cause low voltage at Pin 11 is a drop in primary crash sensor resistance (resistance is too low). If this occurs, the voltage at Pin 11 will be pulled down to a lower than normal value because the parallel combination resistance of the primary crash sensors will be less than 393 ohms. In this situation, the voltage at Pins 11 and 12 will be identical, but the air bag diagnostic monitor does not measure the voltage at Pin 12. Therefore, a primary crash sensor with low resistance may cause low voltage at Pin 11 and the air bag diagnostic monitor will flash diagnostic trouble code 24 on the air bag warning indicator.





POSSIBLE CAUSES
Low voltage at Pin 11 and high voltage at Pin 12 can be caused by:

1. An open circuit or high resistance in the wiring harness in Circuit 614 (GY/O) or 623 (P/W).

2. An open circuit or high resistance inside the safing sensor across Circuit 614 (GY/O) and Circuit 623 (P/W).

Low voltage on both Pins 11 and 12 can be caused by:

3. Resistance to ground on Circuit 614 (GY/O) or 623 (P/W). Circuits 614 (GY/O) and 623 (P/W) should be open circuits to ground when the air bag diagnostic monitor is disconnected from the harness. Resistance to ground on these circuits can cause a drop in the diagnostic voltage on both circuits.

4. Low resistance in one or more of the primary crash sensors. If the resistance of one or more of the primary crash sensors is lower than normal, the voltage on Pin 11 will be pulled down too low.

5. A poor ground on Pin 3 of the air bag diagnostic monitor may cause voltage to appear on the air bag diagnostic monitor ground reference. Any voltage on the ground reference will cause the air bag diagnostic monitor to measure the voltage at Pin 11 as low, even though the voltage on Pin 11 with respect to sheet metal is normal.

6. Intermittent battery voltage at Pin 13 can cause the reference voltage inside the air bag diagnostic monitor to fluctuate and can cause diagnostic trouble code 24 in some circumstances.
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