CPU (Central Processing Unit)

The control module microprocessor (CPU) is the unit in the control module that does the work. The CPU performs a calculation each time the internal clock ticks. Which calculation is to be done is determined by the program stored in ROM (Read Only Memory).

A CPU with 8, 16 or 32 bits is used in cars. In a 16-bit computer, 2 bytes are handled simultaneously with each clock pulse.

The CPU of a control module in the car is normally activated when the ignition is switched ON. The control module can also be activated through actuation of a button on a panel, a door being opened or the remote receiver getting a radio signal. Control modules connected to the I-bus can be activated by a voltage pulse. This pulse is close to B+ and is sent by the control module that is doing the "waking", after which it sends its message.

The CPU may sometimes freeze up because it receives unreasonable instructions. This could be an interruption or a combination of input data that was not foreseen when the program was developed. An example of unreasonable instructions is that the control module receives input data that it is to both lock and unlock the central locking system at the same time. The cause could be a cable that got caught in a door.

A frozen CPU is indicated by lack of communication between the control module and the diagnostic tool (despite all conditions being fulfilled), that the CPU continues performing the last instructions, pulling the fuel pump relay, holding a sensor at 2200 RPM or continually activates a lamp.

The easiest way to reactivate the CPU is to briefly disconnect the battery cable or interrupt the power supply to the control module some other way. It is not enough to switch off the ignition if the control module has +30 feed as it is the CPU that determines shut-off.