SRS Airbag 6.2, Description of Functions

The sensor module has twin electronic deceleration sensor elements connected to one microprocessor which calculates the deceleration of the car. Results from the two sensor elements are compared with each other and with default parameters stored in the sensor module memory, in order to eliminate incorrect calculations.

If both results show that parameters have been exceeded, a violent frontal collision is indicated, and the sensor module closes the seat belt tensioner, steering wheel airbag and passenger airbag circuits simultaneously. A current passes through the igniters activating the gas generators which inflate the airbags and tension the seat belts.

Airbag modules and pyrotechnical seat belt tensioners are activated simultaneously by the sensor module.

The entire SRS system is continuously monitored by the microprocessor in the sensor module. Should a system fault occur the sensor module stores information about its type and location. This facilitates fault-tracing.

The SRS on-board diagnostic (OBD) system is described in the following section "SRS Airbag 6.2 on-board diagnostic (OBD) system".

The SRS sensor module and other SRS components are described above in section "SRS Airbag 6.2, components".

SRS Airbag 6.2, on-board diagnostic (OBD) system
General

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 remains. The diagnostic trouble code (DTC) remains stored in the sensor module memory until cleared using the Volvo Scan Tool (ST). Before a diagnostic trouble code (DTC) can be cleared 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 SF15 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 SF15 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 SF15 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 SF15 warning lamp lights immediately this type of fault occurs. One exception is a fault such as a short circuit to ground in the SF15 warning lamp cable (DTC SRS-127), which does not light the SF15 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 (<9 V and >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 SF15 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.

Reading off Volvo Diagnostics

Information about faults and other information can be read off using the Volvo Scan Tool (ST) connected to the data link connector (DLC).

- Connecting the Volvo Scan Tool (ST) is described in section CA.

The Volvo Scan Tool (ST) communicates with the sensor module through a standardized communications network. The Volvo Scan Tool (ST) manual will be distributed in the fourth quarter of 1995.

The data link connector (DLC) is located in the center console by the cigarette lighter tray. The data link connector (DLC) is common to Volvo Diagnostics and the statutory diagnostic system OBD II, On-Board Diagnostics II (OBD II only certain markets, USA/CDN).

To activate Volvo Diagnostics for the SRS system:
- ignition switched on
- supply voltage normal

Read off diagnostic trouble code (DTC)
Diagnostic trouble codes (DTCs), status, counters and frozen values can be read off using this function

See chapter C for a list of diagnostic trouble codes (DTCs) and fault tracing using diagnostic trouble codes (DTCs).

Diagnostic trouble codes (DTCs) can be posted for the following faults in airbag/seat belt tensioner circuits:
- short-circuit.
- open-circuit.
- short-circuit to ground.
- short-circuit to supply voltage
- high capacitance.
- low capacitance.

Diagnostic trouble codes (DTCs) can be posted for the following faults in the SRS warning lamp circuit:
- open-circuit or short-circuit to ground
- short-circuit to supply voltage

Parameter values (status or frozen values) are posted for the following:
- steering wheel module circuit.
- passenger module circuit.
- left seat belt tensioner circuit.
- right seat belt tensioner circuit.
- SRS warning lamp circuit.
- back-up power.
- voltage level X-supply.
- voltage level D+.
- internal fault in the SRS sensor module (in EEPROM, RAM and ROM for example).

Clear diagnostic trouble code (DTC)

Stored diagnostic trouble codes (DTCs) and frozen values can be cleared using this function.

Diagnostic trouble codes (DTCs) cannot be cleared until they have been read oft at least once and the cause of the fault has been remedied.