Single Wire CAN Network Diagnosis
Part 5 of a 6 part series. Go here to access the full set.
Who Uses It:
SW CAN (Single Wire CAN) is a slower speed CAN network that was available to OEMs as a lower-cost alternative to non-Powertrain ECUs that would require Dual-Wire HS CAN (15765-4). There may be other OEMs that implemented it (feel free to chime in if you know) but all the SW CAN I've seen is Model Year 2005-later GM platforms where the Powertrain is DW High Speed. The implementation would traditionally be Body ECUs, but could include Chassis ECUs. In other words, the vehicle will have both DWCAN and SWCAN wired to the DLC, normally the BCM and OnStar ECUs acting as a Gateway.
How Its Networked:
Since the focus of this article is GM SWCAN, it will be wired to DLC Pin 1. The SWCAN bus is 5V DC, and should be diagnosed with a Digital Storage Oscilloscope, and not a DVOM.
A proper schematic and the RPO list will assist you in determining if the vehicle is equipped with any particular ECU. The GM bus does not contain a master list of how the vehicle is equipped so when you run the CAN bus check with the Tech 2 or GDS2 they will only report ECUs that actually respond to the network request.
Channel 1 lead goes to DLC pin 1, ground lead goes to DLC pin 4 or 5. DLC breakout box is preferred so you don't compromise (read: spread) the DLC pins. Set V/div to 2V DC, time can be set anywhere around 20 ms/div to get you going. SCOPE