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When ‘F.R.E.D.’ fails to communicate

What to do when the CAN can’t.
Tuesday, April 23, 2013 - 12:31
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Bus Differences
J1850 was applied to Ford vehicles in the mid 1990s in a two-wire protocol (technical term for network language) and assigned to pins 2 and 10 of the 16 pin OBDII J1962 compliant DLC. The two circuits make square waves that are mirror images of each other and typically are twisted pair wiring. Both the double wire application (as opposed to a single wire) and the twisted pair wiring add noise immunity (thereby increasing reliability) to this bus that Ford calls Standard Corporate Protocol (SCP). Each module has a pair of wires running to one of several harness splices. Pin 2 is labeled SCP + and pin 10 is labeled SCP - . 

Don’t look too hard at bus communications patterns. It’s either a square wave or it’s not.

If only one of the two bus wires is open or shorted, the bus might communicate one module to another as well as with your scanner. All bets for reliable messages, proper module operations and a U code free environment are off when this occurs, however. 

GM and Chrysler took a slightly different route to adhere to the OBDII complaint J1850 protocol with a single wire bus in Pin 2 of the DLC. GM termed their bus Class 2, while Chrysler used a bit of imagination and termed its very similar bus Programmable Controller Interface (PCI). Chrysler uses a wiring connection strategy of one wire per module terminating in a common shorting bar/connector called a Diagnostic Junction Port (DJP). This connection typically is located near the DLC in the area of the knee bolster under the steering column. The connection allows for removal of one or more PCI wires making fault isolation much easier. GM’s equivalent single wire J1850 bus (Class 2) has a varied wiring connection strategy depending on the model line. 

Some GM models use the single wire per module strategy where each wire connects to a single splice pack connector (called a star connector) in the same general area of the DLC. Termed the Star Configuration, the star connector usually is taped up in electrical tape near the DLC, but is easy to remove for fault isolation. Some models might have a second star connector on the opposite side of the IP area. 

Another wiring strategy GM used with its Class 2 bus was called the loop configuration. One these models each module had two Class 2 bus circuits looping them together. Fault isolation is more difficult on the loop configuration. To eliminate a module as being the source of a bus short to power or ground you must locate each module one by one in order to disconnect them individually from the entire circuit.  

However, with a loop configuration in the event of an open Class 2 bus circuit, bus communications from one module to the next will only cease to occur if there are two open circuit conditions. Some later GM Class 2 applications may use a combination of the loop and star configurations throughout the vehicle.

CAN – Controller Area Network
The CAN bus first appeared in European cars courtesy of its inventor Robert Bosch in the 1990s, but didn’t see the U.S. domestic market until around the 2003 model year. Gradually the American car makers along with the Asian car market adapted CAN on some models until the 2008 model year when federal regulations required every vehicle to be equipped with a high-speed CAN bus for powertrain and chassis modules. 

 

Individual networks may or may not be wired to the DLC.

For powertrain and chassis module communications the CAN bus will have two circuits similar to the Ford SCP J1850 bus and are pinned out in pins 6 (CAN High +) and 14 at the DLC (CAN High -). Depending on the domestic OEM and model year, the CAN equipped vehicle might have one or two other speeds of CAN used for other types of module communications such as body electronics and graphical displays not requiring as high of communication speeds as powertrain and chassis. Much in the same way you don’t require a cable modem for a simple text email, the Body Control Module (BCM) does not require high-speed dual wire CAN to pop the trunk or honk the horn when other modules are involved in such a process as is the case much of the time. 

Whether a J1850 bus or CAN bus, the network classifications are broken down into three speeds: Type A, (under 10Kbps), Type B (10-125 Kbps) and Type C. (125 Kbps and up). Pre-OBDII buses most often will be found in the Type A speed while OBDII J1850 buses will be Type B. GM calls its CAN GMLAN. Its dual wire bus is termed HS (High Speed) GMLAN used for powertrain and chassis communications (a Type C bus), and a single wire medium speed GMLAN bus (Type B) is reserved for other modules and functions on the vehicle. Ford simply calls its powertrain/chassis bus HS CAN (High Speed) and its Type B medium speed body electronic bus MS CAN (Medium Speed).

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