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 in/out 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 Data Link Connector (DLC). 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 three different data bus systems. Depending upon model and equipment level, all three data bus systems are 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.
• 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 CAN bus. As each data packet has an identifier, 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 bus. These are integrated in the first module connected to the CAN bus and in the last module connected to the CAN bus 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 option for automatic disconnection of faulty modules from the data bus.

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

The ISO 9141 and the CAN bus are used in the Fiesta built from MY2002.25 and in the Fusion.

Due to the increased number of modules and the resulting ever-increasing data transmission, a second CAN bus (mid-speed CAN bus (MS CAN)) is used in the Fiesta and in the Fusion built from MY 2006. This operates at a lower speed and is mainly used for communication relating to the convenience electronics. A gateway is used in order to enable data exchange between the HS CAN bus and the MS CAN bus. The gateway serves as interface between the two CAN data bus systems and is installed in the electronic instrument cluster. The modules connected to the two CAN data bus systems depend upon the equipment level of the vehicle. One 120 Ohm terminating resistor of the HS CAN bus is installed in the PCM and in the electronic instrument cluster respectively. One 120 Ohm terminating resistor of the MS CAN bus is installed in the Generic Electronic Module (GEM) and in the electronic instrument cluster respectively. These terminating resistors are used for suppression of the data bus system. In order to be able to ensure correct functioning of the data bus system, the modules must always be connected with an integral terminating resistor.

The number of modules connected to the two data bus systems depend upon the equipment level of the vehicle.

CAN bus - vehicles built up to MY 2006