ISO15765-4 HSCAN Network bus communication diagnosis
Part 4 of a 6 part series. Go here to access the full set.
WHO USES IT:
ISO15765-4, dual-wire CAN, is mandatory on ALL North American vehicles sold from Model Year 2008 and forward for Powertrain communications. It was phased in by OEMs starting in MY2004, so refer the specific wiring schematic and the DLC connector. If there are pins in DLC cavities 6 & 14 after MY2003 it is wired up for HSCAN.
HOW IT IS NETWORKED:
DLC Pins 6 & 14 are assigned to HS-CAN. Similar to J1850PWM, it is a differential bus. You have a CAN + and CAN - for the bus to properly communicate. The bus operates at roughly 2.5V DC and is pulled up and down on the respective bus line. The bus is communicating whenever at least one ECU is active, so key ON is not required, nor is scan tool connectivity required to have bus activity.
You will want to use a DSO (Digital Storage Oscilloscope) to properly diagnose any CAN bus issues, but you can start with your DVOM. The CAN bus has two 120 ohm terminating resistors in the circuit. These terminating resistors are in place to dampen bus ringing, which occurs when the communication message ends. Will the bus communicate without them? Perhaps, but you will get data corruption as the bus ECUs will misinterpret the message packets.
With the key OFF and OUT, measure the resistance at DLC pins 6 & 14. The value should always be between 58 and 62 ohms. Period. When one of the resistors is blown, you will have somewhere north of 118 ohms. Stop there, pull a schematic and find the location of both terminating resistors. Sometimes they are internal to an ECU, sometimes they are external in the harness. The schematic will guide you. Examples below for GM, Ford and Nissan.
Note on Nissan: they do not list locations on the schematic, but they normally put them in "bookends", namely the ECM and IPDM.
Channel 1 lead goes to DLC pin 6, channel 2 lead goes to DLC pin 14. 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. This is a high-speed signal, so you'll get lots of activity. You are less concerned about the individual packets and more concerned about the bigger picture. When the bus is behaving badly, you will see it on the scope very quickly!
The HS CAN busses that are being wired to other DLC pins (GM Chassis Expansion 12/13, Ford/FCA IHS, etc) diagnosis is identical, as the bus architecture is the same. Also, many platforms from 2004 to 2009 have HSCAN and legacy protocols (J1850PWM, J1850VPW, ISO9141-2, J2610, etc) wired up simultaneously. Use the schematics!
If the vehicle is equipped with a Central Gateway ECU (like an FCA TIPM) keep in mind the DLC is not on the bus, but wired to the CGW only. To test the bus, you will need to go on the other side of the gateway ECU.
Great writeup Bob, Some notes on this from a Mercedes perspective. Of course as you stated the location and configuration of terminating resistors will vary by manufacturer and vehicle, in Mercedes terminating resistors are used in both modules and junction blocks, but to my knowledge never hardwired to the harness itself. In general a star topology is used with junctions at the centers of the