Analog inputs

The voltage interval 0-5 V from a temperature sensor or pressure sensor is converted in the control module into a digital value, often 0-255 (256 steps), which is equivalent to a byte.

If we know that the bit value is never less than 10 or more than 245 under normal operation, we can set a diagnostic trouble code if the value falls outside of this range.

See diagnostic trouble code P0117 for Trionic 8 by way of example.

In the example above, you can see that the diagnosis is unable to differentiate between interference and a short circuit to 5 V. The reason for this is that the input has a Pull-Up at 5 V, and this is equivalent to a bit value of 255. This is the greatest value the control module can read. Thus interference generates the same diagnostic trouble code as a short-circuit to 5 V or B+.

The A/D value has to be too high for 0.5 s in order to set the diagnostic trouble code. This is known as the filter time. The filter has to be in place to prevent short-term interference from mobile telephones or the ignition system, for example, from setting diagnostic trouble codes.

The diagnostic trouble code is set after 0.5 s. The MIL (Malfunction Indicator Lamp) or Check Engine, as it is also known, is activated only during the second run in a row where faults have occurred. If the fault is rectified without deleting the diagnostic trouble code, Check Engine will go out after the third fault-free run.

The control module uses a substitute value for coolant temperature which is based on the intake air temperature.

As long as the fault is active, a bus message is sent indicating that the cooling temperature value is implausible. This means that the receiving control module is able to undertake certain action, such as starting the radiator fan and resetting the temperature gauge.