J1850PWM Network bus communication diagnosis
Part 2 of a 6 part series Go here to access the full set.
WHO USES IT:
J1850PWM (Pulse Width Modulated) is a communication bus used by Ford and it's sister companies (Mazda, Jaguar) from model years 1995 to around 2006. Ford's internal name is SCP, or Standard Corporate Protocol.
HOW IT IS NETWORKED:
J1850PWM is wired to DLC pins 2 & 10, and is a 5V DC differential bus. There is a positive and negative line and they are inverted mirror images of one another. The bus will continue to communicate if one of the lines is down, and multiple ECUs will generally set SCP+ or SCP- DTCs, indicating a bus fault on the respective line.
You do NOT want to diagnose this with a DVOM, as you need to see the on/off thresholds to make sure they are reaching logic ON and logic OFF. In addition (and this is a deviation from J1850PWM), the 2 channel DVOM will allow to you compare the bus signals to verify differential integrity.
The bus will be active when the ignition is in the ON/RUN position and most likely will be active regardless of ignition status if the BCM/GEM/SJB is awake. Scan tool request is NOT necessary to verify bus activity!
Ford IDS software has a network bus check feature that is good, not great. When you run this routine, the software will electrically (dynamically) test the bus continuity. Chances are you will already know there is an issue because of the DTCs that are set and communication status with the scan tool.
Channel 1 lead goes to DLC pin 2. Channel 2 lead goes to DLC pin 10. Ground lead goes to either 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 1 ms/div to get you going.
The Ford PTS service material provides excellent resources on how the network is configured. In the Workshop tab, section 4-18 is soup to nuts on Network Communication. In the Wiring tab, section 14-1 contains all the network schematics. Be advised: most Ford platforms wired with SCP also could have ISO9141-2 (K-Line) on DLC pin 7, and/or UBP (UART-Based Protocol) on DLC pin 3. This is why the schematic will be your BFF.