Sprinter Electrical Systems

With most domestic vans and light trucks, the level of connectivity and complication is minimal compared to a Sprinter. The key to proper service and repair of this vehicle is the CAN - the Controller Area Network. Also called the CAN-Bus, it is a vehicle standard designed to allow all control units to communicate with each other and share data over a simple wired network.

The Sprinter is 100 percent Mercedes-Benz, not a Dodge/Freightliner chassis with a Mercedes engine. As such, the Sprinter is not an a la carte vehicle that has components picked from multiple suppliers like many domestic trucks. It has all Mercedes components, designed to work together at the factory.

Consequently, the Sprinter does not diagnose like Dodge, Isuzu or Ford vehicles. Mercedes vehicles log a lot of fault codes and event codes for most operational faults. Being able to read and understand fault codes on all control units is vital to diagnose CAN-Bus problems.

To better understand the fundamentals of the Sprinter's electrical system, Fleet Maintenance Magazine editor David A. Kolman visited with Eric J. Ord, a highly regarded independent source of Sprinter technical information. Ord has developed three separate scan tools for the Sprinter, one of which was delivered to all the locations in the U.S. and Canada for the world's largest fleet operator. Over the years, Ord has conducted more than 100 training courses on the Sprinter and has written numerous technical materials on Sprinter diagnosis and repair.

Ord is managing director of Megara LLC. With offices in the U.S. and Europe, the company is involved in projects with several commercial and municipal fleets on Sprinter vehicles, as well as other projects for U.S. and foreign military customers with Mercedes-Benz vehicles.

NUMEROUS CONTROLS

For most domestic vans and light duty trucks, the CAN-Bus traditionally has limited application, usually with the few sensor values shared between the engine, transmission and instrument cluster, Ord explains. With Sprinter, there are a lot of control units and sensors that, share information and transmit signals.

On the average model year 2002 to 2006 Sprinters, there are generally around 11 or more individual control units on one CAN-Bus network, he says. For 2007 to 2010 models, there can be more than 29 control units on three networks.

"Without a CAN-Bus network, a Sprinter would have either miles of wire or many duplicate sensors or both," he says. "The cost of all of this copper would add thousands of dollars to the vehicle price and hundreds of pounds of weight.

"The Sprinter is a Mercedes and Mercedes parts tend to be expensive. If the approach to solving a problem is just to replace parts until the problem goes away, that will put you upside down on the vehicle quickly."

CAN BEGINNINGS

The vehicle CAN-Bus concept was pioneered in Germany by Robert Bosch GmbH in the 1980s. Daimler-Benz AG was one of the first to adopt CAN-Bus for vehicles. In 1991, Mercedes-Benz started using a CAN-Bus for communications between electronic control modules (ECM) in passenger cars on the W140 S-Class 600SEL.

For commercial vehicles, the CAN-Bus has been implemented since the 1990s in two forms: a two-wire CAN for vans and light commercial vehicles, and a four-wire CAN for trucks and heavy equipment. The difference between the two CANs is double redundancy of signals versus single redundancy. Both systems use a twisted pair cabling to minimize interference.

When Mercedes designed the Sprinter back in the 1990s, Ord says the company saw the need to have all of the control units and sensor values shared over an automotive/industrial standard Bus system. The earliest European Sprinters had a very simple CAN with very few components linked. For North American Sprinter models, starting in 2002, CAN-Bus is used for all main systems and controls for powertrain, interior, chassis, etc.

There are other types of automotive networks which use fiber optic cables to broadcast complex network messages using light pulses, notes Ord. For 2007 to 2010 Sprinters, the MOST (Media Oriented Systems Transport) fiber optic network is used in some vehicles for in-vehicle entertainment, communications and navigation systems.

For 2002 to 2005 Sprinters, there is only one network (see Diagram 1): chassis/powertrain CAN (CAN C low speed). For 2007 to 2010 Sprinters, there are four (see Diagram 2): powertrain/chassis M-CAN (ISO CAN C high speed); interior (body) I-CAN (ISO CAN B low speed); diagnosis D-CAN (high speed proprietary MB protocol); and Digital Media Bus for radio/navigation system (MOST).

The networks connect to each other through gateway modules, he says. These gateway control units serve the same purpose as a network hub switch on an office computer network for different rooms or groups. The gateway control units, which also serve as a sort of translator from one type of network to the other, are arranged as follows:

• The instrument cluster (IC) is the interface between M-CAN and I-CAN.
• The electronic ignition switch (EIS) is the interface between M-CAN and I-CAN and between D-CAN and OBD2 diagnostic connector.
• The radio (COMMAND Audio Gateway) is the interface between I-CAN and MOST.

CAN OVERVIEW

"To begin with, it is important to know that the CAN Data Bus is a bi-directional Bus, that is, each connected control unit is able to send and receive messages," says Ord.

The CAN Data-Bus consists of a twisted-pair cable that is similar to network cables used for computer networks or telecommunications. All control units (computers) that are in the vehicle are connected to these cables. CAN cables are connected together at junction boxes or Z-splices, also called the J-Splice in some factory literature because Mercedes uses a "J" for junction to describe the location. Splices are soldered connections.

CAN-Bus cables express specific voltages when they are active and when they are idle. Each CAN cable is either CAN H, for high, or CAN L, for low. With CAN H cables, the voltage rises when CAN is active; with CAN L cables, the voltage drops when the CAN is active. Depending on the vehicle, type of CAN and speed inherent, voltage levels are a fixed value when sending or receiving.

Data is transmitted over both cables with the Bus levels being a mirror image of each other. In other words, if the level on one cable is 0, the other cable transmits the level 1 and vice versa, as a replacement for on/off signals from analog components in older systems.

"CAN-Bus cables are a twisted pair design. If troubleshooting wiring problems and a pair of cables are found twisted to each other, you may have just found the CAN-Bus network cables. The twisted pair wire concept is used for two reasons: for voltage fault identification and as a redundant safety concept. If a voltage peak occurs on just one line due to interference problems, the receivers can identify this as a fault and ignore the voltage peak.

"The twin cable design is also handy if a short circuit or interruption occurs on one of the two CAN lines. Software in the control units can reroute messages by switching to a single line. The damaged data line is shut down with CAN message speed slowed, but not stopped. Software on networked control units manages how and when the control units on CAN can send and receive messages."

DATA FLOW

The data flow path is important to understand for effective diagnosis, Ord points out. The 2002 to 2006 Sprinter CAN is set up in order of data flow. The engine control module is the first ECU, the immobilizer control unit (referred to as WSP for Chrysler; SKREEM by Mercedes-Benz) is the last. The CAN terminates at the ECM (CDI/CR control unit) and at the immobilizer control unit with a 120O resistor. In the 2007 to 2010 Sprinter, data flow is different as there are several networks and interfaces between the networks.

Each control unit sends and receives the messages, but only some will use them. Messages are sent along the whole CAN-Bus to all ECUs that are connected, explains Ord. The messages are disregarded and just sent along if it has no function for that ECU. The message will continue until it reaches the intended recipient for processing into a function or change.

In practical terms, a message from the crank sensor sends "engine speed" to the ECM. That message will be sent by the ECM along the CAN-Bus so "engine speed" can be accepted by the automatic transmission, instrument cluster, brake and traction system and other control units that have a need to process the message (see Diagram 3).

"The same message would just pass through a control unit that could not use the data and continue to pass along through the network unit it reached the endpoint terminating control unit."

CAN messages are a piece of data that contains seven parts, says Ord, and the CAN protocol allows up to 2,048 different message types. Each message is 8 bytes of data. This 8-byte message is also called the CAN Data Telegram. Each part of the message is important and provides the necessary information so that the content of the message can be received and used by the applicable control unit.

Within the message contents, the CAN Data Telegram shows which sensor sent it, the priority, the intended recipient (single ECU address or multiple ECU addresses), the subject (status of component X), the content of the message (actual data bytes), acknowledgment of receipt and end comment. As these messages can relate to status or operation, the contents can vary within the standards.

CAN-Bus software and hardware create an efficient network post office for messages. "To avoid confusion and a loss of data, each message can only be sent when the CAN Data Bus is free," Ord explains. "If several control units simultaneously start to send a message, a procedure called arbitration initiates. With arbitration, the network determines which message has the highest priority. Priorities are set due to the specific function initiated by the message."

By way of example, a message that contains "driver airbags deployed" has higher priority than "blower fan level." If a control unit's message is halted during arbitration, the sender will automatically repeat the message transmission as soon as the CAN network is free again.

"CAN messages are not just to broadcast status automatically," says Ord. "Some messages are requests for data from one control unit to another. The air conditioner/heater control unit may send a request message to the engine control unit for 'coolant temperature' to avoid overheating and the appropriate control unit then reacts to the request. Since 'coolant temperature' is used by several control units that continuously monitor this CAN message in real time, if the engine control unit is busy, another could send the coolant temperature status to the air conditioner/heater."

If there are any network problems or missing data, both the sending control unit and any receiving control units on the CAN-Bus will log either fault codes or CAN event codes.

PRACTICAL TROUBLESHOOTING

When control units and components are networked, a different approach is necessary to diagnose problems. Common faults can include drivability issues (low power, transmission in limp-home mode, etc.), convenience issues (no a/c, poor heat output, etc.) or operational issues (power windows not functioning, doors will not lock, etc.). When a drivability or operational problem is being addressed on a vehicle with CAN-Bus, limited knowledge of how CAN works can lead to a long and difficult process that may or may not yield any results.

Because the Sprinter is a fairly complex vehicle that has a lot of electronic control units, Mercedes engineers built in a very comprehensive fault logging capability in each control unit, Ord says. For many control units, there are hundreds of possible fault codes that provide a solid roadmap to pinpoint the cause of many types of vehicle problems. "Most CAN-Bus network problems are identified by the control units through fault codes; this is the first place to start the diagnostic process."

Understanding fault codes, also known as diagnostic trouble codes (DTC), "is the most important first step in identifying where a CAN-Bus network problem has occurred," Ord emphasizes. "It is critical that suitable scan tools be used for testing - ones that will actually recognize all of the control units, not just engine and transmission.

"CAN networking is one of the reasons that generic OBD2 scanners and multi-make scan tools are practically useless on Sprinters. They don't allow diagnosis of all the control units, so faults can't be easily pinpointed in a whole system, and keep in mind, all control units are on CAN."

An enhanced feature that is available on 2007 to 2010 model Sprinters is CAN Event Codes. These specifically relate to CAN network message traffic. CAN Event Codes provide very specific information relating to the CAN message, sender and anticipated recipient. "That makes troubleshooting easier because it breaks the possible causes down to a few likely suspects."

MORE INFORMATION

Additional information on CAN-Bus can be found in Daimler and Dodge service literature and through some aftermarket sources such as Robert Bosch publications. A complete guide to fault codes for Sprinters, plus helpful troubleshooting information, can be found in The Complete Sprinter Fault Code Guide - North American Edition, written by Ord. The format of the book, soon to be available on Amazon.com, is not diagnostic tool specific so it can be used by any technician, no matter the types of OEM and/or aftermarket scan tools.

"With the right tools and techniques, electrical issues on Sprinters can be easily overcome and problems solved efficiently," Ord concludes. "With the wrong approach, money is wasted on parts thrown into a vehicle until eventually the right thing is replaced."

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