Ford Workshop Service and Repair Manuals

General

In a communications network (data bus system), various modules of different systems are connected to one another via one or several lines.

The sole purpose of the data bus system is the transmission of data between the connected modules themselves, as well as between the connected modules and the Worldwide Diagnostic System (WDS).

In a data bus system, complete data blocks are transmitted instead of single on/off pulses. In addition to the actual information, these data blocks also contain data regarding the address of the module to be addressed, the size of the data block and information for monitoring the content of each individual data block.

Data bus systems offer various advantages:

• Simplified data transmission between the modules due to a standardized protocol
• Fewer sensors and connectors
• Improved diagnostic options
• Lower costs

The WDS is connected to the various bus systems and to the power supply via the standard 16-pin (DLC)Data Link Connector. The signal for the module programming is also transmitted via the DLC.

If, in a data bus system, there is a break in one or both lines or a short to ground or short to voltage is present, then communication between the modules and with the WDS is faulty or is no longer possible at all.

In order to be able to establish communication with one another, the modules of the individual systems must use the same language. This language is called a protocol.

At present, Ford uses four different data bus systems. Depending upon model and equipment level, all four data bus systems may be used. Each of these data bus systems has its own protocol.

Data bus systems:

• Standard Corporate Protocol (SCP) bus. This consists of two twisted wires. It is used for communication between the Powertrain Control Module (PCM) and the WDS via the DLC. Depending upon engine version and year of manufacture, a third wire (ACP bus) is used for programming the PCM. This bus is only used in conjunction with the SCP bus.
• International Organization for Standardization ISO 9141 bus. This consists of a single wire and is used exclusively for communication between the modules and the WDS. The fault memories of the various modules are read out via the ISO 9141 bus.
• The Local Interconnect Network (LIN) bus is a standard especially for the cost-efficient communication between intelligent sensors and actuators in motor vehicles. Local Interconnect Network (LIN) is used in every situation where the band width and versatility of CAN is not needed. The LIN specification comprises the LIN protocol, a standard format for describing a complete LIN and the interface between a LIN and the application. A LIN comprises a LIN master and one or several LIN slaves. The LINnetwork utilizes the master/slave principle for the purpose of bus access control. This has the significant advantage that few resources (CPU performance, ROM, RAM) are required for bus management in the slave module. The master is implemented in a control module or a gateway which has the necessary resources. All communication is initiated by the master. Consequently, a message always consists of a header, which is generated by the master, and a response from the slave. The data transfer rate is in the region of up to 20 Kbit/s. The LIN master knows the time sequence of all data which are to be transmitted. These data are transmitted by the corresponding LIN slaves (e.g. ultrasonic sensors) when requested to do so by the LIN master. LIN is a single-wire bus, i.e. the data is transferred in the cable via one wire. Usually the same cable is also used to provide the supply voltage. The ground connection of the supply voltage also acts as the ground connection of the data transmission. No terminating resistors are used in the LIN.
• Controller Area Network (CAN) bus. This consists of two twisted wires and operates serially (data is transmitted sequentially). It is used for communication between the modules themselves and between the modules and the WDS. The modules are connected to the data bus in parallel. New modules can be incorporated easily, without modifying the other wiring or modules. The transmitted data is received by every module connected to the Controller Area Network (CAN). As each data packet has an identifier (label), in which the priority of the message is determined as well as the content identification, each module can detect whether or not the data is relevant for its own information processing. This enables several modules to be addressed with a particular data packet and supplied with data simultaneously. For this purpose, it is ensured that important data (for example from the Anti-lock Brake System (ABS)) is transmitted first. The other modules are only able to submit their data to the data bus after the high-priority messages have been received.

In order to guarantee a high degree of error protection, two 120 Ohm terminating resistors are installed in the CAN. These are integrated in the first module connected to the CAN and in the last module connected to the CAN respectively and are used for suppression as well as the elimination of voltage peaks. In order to ensure correct functioning of the data bus system, the modules must always be connected with an integral terminating resistor.

The advantages of the CAN bus are:

• Minimization of wiring requirements.
• High degree of error protection (fault / fail-proof).
• Robustness.
• Good extendibility.
• Prioritization of messages.
• Inexpensive.
• Automatic repetition of faulty messages.
• Independent system monitoring and the ability to automatically disconnect faulty modules from the data bus.

In vehicles built from MY 2003.75, an additional second CAN is used depending upon vehicle model. The only significant difference is a lower transmission rate and at present it is mainly used for the convenience electronics. In order to be able to differentiate between individual CAN systems, the CAN with the high transmission rate is designated as high-speed (HS) CAN and the CAN with the lower transmission rate as mid-speed (MS) CAN. As in all CAN systems, two 120 Ohm terminating resistors are also installed in the MS CAN in order to increase the error protection. In order to enable communication between the modules on the HS CAN and the modules on the MS CAN, one module is connected to both data bus systems. The connection of both data bus systems is designated as a gateway. In this gateway, the received data is converted to the transmission rate required for the relevant data bus and is transmitted. This ensures an optimal distribution of information between both data bus systems.

Network components

ISO 9141 bus