J1850VPW network bus communication diagnosis
PART 1 of a 6 part series. Go here to access the full set.
WHO USES IT:
J1850VPW (Variable Pulse Width) is a communication bus used by both General Motors and the company formerly known as Chrysler/DaimlerChrysler. Common implementations would be found between model years … for GM and … for Chrysler. GM calls their bus CLASS 2, Chrysler calls it PCI, but its the same thing from a diagnostic perspective.
HOW IT IS NETWORKED:
J1850VPW is wired to DLC pin 2, and is a 7 volt DC digital square wave on GM and 7.5V square wave on Chrysler. Both are variable pulse width with network arbitration. You do NOT want to diagnose this with a DVOM, as you need to see the on and off thresholds to make sure they are reaching logic ON and logic OFF. The bus will be always be active when the ignition is in the ON/RUN position, and most will also be active regardless of ignition status if the BCM is awake. Scan tool request is NOT necessary to verify bus activity!
Refer to the OEM wiring schematic for the appropriate model as most will have some form of splice pack where you can access many ECUs in one spot. GM typically labels them as SP201, SP202, SP300, etc, which indicates where in the vehicle they are located.
On a GM product, the PCM or the BCM can provide the bias voltage, so if you are disconnecting the splice pack to isolate ECUs you will need one of those connected to get bus activity. The Tech2 also has a very useful Class 2 message monitor feature which shows you active ECUs in real time,
Channel 1 lead goes to DLC pin 2, ground channel to 4 or 5. DLC breakout box preferred so you don't compromise (read: spread) the DLC pins. Set V/div to 1V DC, time can be set anywhere around 1ms/div to get you going.