Supplemental Inflatable Restraint (SIR)

CIRCUIT DESCRIPTION
The DERM injects a current through the steering column from the "Force High" output, Terminal "A4," to the "Force Low" output, Terminal "A10." By monitoring the difference between the voltage at the "Sense High" input, Terminal "A7," and the "Sense Low" input, Terminal "A6," the DERM calculates the combined resistance of the Inflator Module, SIR Coil, harness wiring Circuits 347 and 348 and connector terminal contacts in the steering column.

TEST CONDITIONS
Tested once each ignition cycle during the "Steering Column Resistance Test" if:
- No faults are detected during "Turn-On."
- No faults are detected during "Continuous Monitoring" for one second.
- No crank signal present.
- "IGN 1" voltage is present.

FAILURE CONDITIONS
Code 21 will Set if the combined resistance of the inflator module, sir coil, harness wiring and connector terminal contacts in the steering column is greater than a specified value. (4.37 ohms).

ACTION TAKEN
DERM turns "On" the "Air Bag" telltale lamp.

CONDITIONS TO CLEAR
Cycle Code 21 will be cleared each time the ignition switch is turned "Off."

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test determines whether the fault is in the steering column or the diagnostic circuitry.
2. This test checks the integrity of the 430 ohm resistor and associated circuitry in the resistor module.
3. This test checks the integrity of the 49.9 ohm resistor, 100 ohm resistor and associated circuity in the resistor module.
4. This test checks the integrity of the two 2490 ohm resistors and associated circuity in the resistor module.
5. This test determines whether the steering column fault is due to the SIR Coil or the inflator module.
6. This test checks the integrity of Circuit 347A, Circuit 348B and the inflatable restraint wiring harness to SIR coil connection.
7. This test checks for a short from the "Force High" circuit to the "Sense High" circuit.
8. This test checks for a short from the "Sense High" circuit to B+.
9. This test determines whether the fault is in Circuit 347 or Circuit 348.