Diagnostic Functions: Overview

Overview

General
The control module has an integrated diagnostics system (the Volvo On-board Diagnostics (OBD)). The control module continuously monitors itself and the input and output signals in the system. If the control module detects a fault it stores a diagnostic trouble code (DTC) and the check and warning lamps begin to flash. When a diagnostic trouble code (DTC) is stored, the system registers and freezes the value of 3 different parameters. These are stored together with the diagnostic trouble code (DTC). The control module can store up to 7 diagnostic trouble codes (DTCs). If the total number of diagnostic trouble codes (DTCs) exceeds 7, the four first diagnostic trouble codes (DTCs) will always be saved, while the last three will be the latestdiagnostic trouble codes (DTCs) to be stored.
When a transmission fault is registered by the control module (permanent fault), an emergency program is activated to deal with the fault. The transmission control module (TCM) takes corrective action to protect the transmission and maintain as much driveability as possible. There are different emergency programs depending on the type of fault and how serious it is:
- Emergency mode I
- Emergency mode II
- Emergency mode III
- Limp home mode
Emergency mode I is activated for minor faults. Limp-home mode is activated for the most serious faults. For a more detailed description of the emergency programs, see Emergency programs, AW50-42 Emergency Programs In The Event Of A Fault.
If the fault disappears (is intermittent), the transmission control module (TCM) returns to normal operation the next time the ignition is switched on. The indicator and warning lamps will stop flashing after the ignition is switched on and off a certain number of times. The ignition may have to be switched on up to five times, depending on the diagnostic trouble code (DTC) that has been stored.
If a fault disappears after a diagnostic trouble code (DTC) has been permanently stored in the control module, information about the fault remains stored in the control module.
For each diagnostic trouble code (DTC) stored, a counter records the total number "fault-free" operating cycles since the diagnostic trouble code (DTC) was stored. When the counter is at 0, the control module treats the malfunction as a permanent fault. If the counter value is greater than 0, the transmission control module (TCM) interprets it as an intermittent fault. If the fault recurs, the counter is reset to 0.