Test Notes

Normal Operation
The LH, RH and center radiator crash sensors all have internal resistors. The diagnostic monitor uses the resistor in the crash sensors in combination with the two resistors inside the diagnostic monitor to create a tightly controlled diagnostic voltage at pin 11 (GY/O wire). The crash sensors are tied together inside the diagnostic monitor at pins 17 (PK/O wire), 18 (PK/W wire) and 19 (W/Y wire). 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 vehicle is equipped with three crash sensors; the resistance of each sensor should be 1180 ohms (± 12 ohms).

The resistors inside the diagnostic monitor are connected to pins 11 (GY/O wire) and 12 (P/W wire) and are equal in value. Note that the GY/O and P/W wires are tied together at the joint connector. 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. Current flows through the driver air bag and into pin 10 (GY/W wire). Current then flows from pin 10 through the diode 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 Pins 11 and 12 Voltage Chart.

Pins 11 And 12 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 circuit between the GY/O and P/W wires inside the rear safing sensor is open, then pin 12 does not assist in the diagnostic voltage. 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 diagnostic monitor pulling up the voltage. Therefore, the voltage on pin 11 will be half of the normal expected voltage and the voltage on pin 12 will be approximately equal to the vehicle charging system voltage. The diagnostic monitor does not measure the voltage on pin 12, so the monitor bases its decision strictly on the voltage at pin 11. If the voltage at pin 11 is lower than it should be, the monitor will flash a code 24.

Another situation that can cause low voltage at pin 11 is a decrease in the 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. In this situation, the voltage at pins 11 and 12 will be identical. Therefore, a crash sensor with low resistance may cause low voltage at pin 11 and the monitor will flash a code 24 on the air bag 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 the GY/O or P/W wire.
2. An open circuit or high resistance inside the rear safing sensor across the GY/O and P/W wires.

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

3. Resistance to ground on the GY/O or P/W wire. The GY/O and P/W wires should be open circuits to ground when the 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 crash sensors. If the resistance of one or more of the crash sensors is lower than normal, the voltage on pin 11 will be pulled down too low.
5. Intermittent battery voltage at pin 13 can cause the reference voltage inside the diagnostic monitor to fluctuate and can cause a diagnostic trouble code 24 in some circumstances.
6. A poor ground on pin 3 of the diagnostic monitor may cause voltage to appear on the diagnostic monitor ground reference. Any voltage on the ground reference will cause the 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.