Summary
Diagnostic test modes (DTMs)Summary
The control module has a built-in diagnostics system (Volvo Diagnostics) which monitors itself and input and output signals continuously.
If the control module detects a fault, it posts a DTC and switches the ABS (TRACS) warning light on (there are exceptions to this, see D. DTC table Antilock Brake System (ABS)). At the same time as the DTC is posted, the value of three different parameters, which are also posted, is frozen. The control module can post up to 33 (TRACS 35) DTCs.
The most serious faults switch the warning lights on and cut out ABS, EBD and TRACS completely. Less serious faults switch the warning lights on and cut some functions out. When disconnected, the control module aborts or disables all functions (not the road speed signal); disabling prevents certain functions from being carried out.
If the faulty components only affect TRACS, TRACS alone is cut out. If the ABS light is on, this means the ABS system is not working normally and in some cases, EBD will not be working either.
With some DTCs, the control module runs a diagnostic check as soon as the ignition is switched on, which means that if the warning light stays on, the fault has persisted and ABS/EBD will not work. With other DTCs, the control module only runs a check when some function is activated, the car is being driven or if the road speed exceeds 20 km/h (12 mph). This means that the warning light may go out as normal when the ignition is switched on or when the engine is started but the fault may persist. Only when the warning light comes on will ABS/TRACS cut out.
Should a fault disappear for any reason once the DTC has been posted permanently in the control module, data on the fault will be retained in the control module.
For each DTC posted, there is a counter which counts the number of fault-free driving cycles since the DTC was posted (intermittent fault). A driving cycle is defined as the first time the car passes 20 km/h (12 mph) since the ignition was last switched on.
If the counter reads 0, this tells the control module that the fault is permanent. If the counter reading is greater than 0, the control module treats this as if the fault does not exist (intermittent fault). If the fault reoccurs, the counter is reset to 0. If the counter exceeds 63, more than 63 driving cycles without the fault reoccuring, the DTC is erased, the counter and the frozen value are reset and the warning light goes out.