The Sensor Module Function

The sensor module function




The SRS sensor module has an on board diagnostic (OBD) system, Volvo Diagnostics, to provide self diagnosis and other information, which can be read off using the Volvo Scan Tool (ST) connected to the data link connector (DLC).
When the ignition is switched on (ignition position II or engine on) the SRS on board diagnostic (OBD) system carries out an initial self test. After that, the SRS on board diagnostic (OBD) system goes into monitoring mode, carrying out continuous diagnostics on the system. The sensor module checks for internal faults in all SRS ignition cable outputs and in the output for the SRS warning lamp in the combination instrument.

Diagnostic Trouble Codes (DTCs)
If the on-board diagnostic (OBD) system detects a fault a diagnostic trouble code (DTC) is stored. At the same time as the first diagnostic trouble code (DTC) is stored, the values of a number of predetermined parameters are frozen. Only one set of frozen values can be stored at any one time.
When a fault is posted, a counter for that particular type of fault starts. A posted fault must persist for a set number of diagnostic cycles before the equivalent diagnostic trouble code (DTC) is stored. A diagnostic cycle lasts approx. one second. At the same time as the diagnostic trouble code (DTC) is stored, the SRS warning lamp lights.
The SRS warning lamp is lit for as long as the diagnostic trouble code persists. The diagnostic trouble code (DTC) remains stored in the sensor module memory until erased using the Volvo Scan Tool (ST). Before a diagnostic trouble code (DTC) can be erased the fault must be remedied and the diagnostic trouble code (DTC) must have been read off.
If the fault disappears (becomes intermittent) before the counter has reached the correct value, the on-board diagnostic (OBD) system senses this and resets the counter. The SRS warning lamp does not light.
When the ignition is switched off, all diagnostic trouble code (DTC) counters are reset, while the diagnostic trouble codes (DTCs) which were stored remain.
If a fault occurs in the airbag/seat belt tensioner circuits for more than ten diagnostic cycles (approx. 10 seconds) after the ignition has been switched on, the sensor module stores a diagnostic trouble code (DTC) and lights the SRS warning lamp.
If a fault in the SRS warning lamp circuit between the sensor module and combination instrument has occurred for more than 40 diagnostic cycles (approx. 40 seconds) after the ignition has been switched on, a diagnostic trouble code (DTC) is stored. The sensor module cannot light the SRS warning lamp because of the type of fault. In this case the SRS warning lamp is lit by a special electronic circuit in the combined instrument which lights the SRS warning lamp if the sensor module is faulty or if there is a break or short circuit in the supply in the cable between the sensor module and the combination instrument. The SRS warning lamp lights immediately this type of fault occurs. One exception is a fault such as a short circuit to ground in the SRS warning lamp cable (DTC SRS 127), which does not light the SRS warning lamp.
The on board diagnostic (OBD) system can store up to 7 diagnostic trouble codes (DTCs) at one time. If the number of diagnostic trouble codes (DTCs) posted exceeds 7 the later diagnostic trouble codes (DTCs) (from the eighth DTC) will not be stored. When stored diagnostic trouble codes (DTCs) are read off they are displayed in the order they were posted. There are 19 different SRS diagnostic trouble codes (DTCs).

Miscellaneous
Even if the sensor module detects a fault to one igniter, and stores a diagnostic trouble code (DTC), it will still try to activate all igniters in the event of a collision.
In the event of a collision in which the SRS system is activated, the sensor module stores all data about the system at the moment of the collision.
If supply voltage is too low or too high (less than 9 V and greater than 18.5 V) the indicator lamp will light at the same time as the sensor module goes into standby mode. As soon as the voltage is correct, the warning lamp goes out after 10 seconds and the system operates normally. The sensor module cannot store any of the 19 diagnostic trouble codes (DTCs) if supply voltage is too low or too high. There is no separate SRS diagnostic trouble code (DTC) for supply voltage too low or too high. If the supply voltage is too low or too high the Volvo Scan Tool (ST) cannot communicate with the sensor module.
If D+ voltage is too low <0.7 V, the SRS warning lamp will light when the ignition is in position II.