If you’ve ever tinkered with older GM vehicles (roughly mid-1990s to early 2000s), you might have encountered the term "Class 2" communication or J1850 VPW. This was General Motors’ protocol for in-vehicle networking before the CAN bus took over popularity. For hobbyists working on diagnostics, instrument clusters, or engine swaps, understanding J1850 VPW can be incredibly useful.
What is J1850 VPW?
J1850 is an SAE standard for automotive communication, and GM used VPW (Variable Pulse Width) as the modulation method. Unlike PWM (Pulse Width Modulation, used by Ford in J1850), this protocol is single-wire. J1850 PWM is a differential pair. GM called its implementation "Class 2", which ran at 10.4 kbps—slow by today’s standards but sufficient for its time. Most systems also supported “4x mode” which was 41.6 kbps. J1850VPW allowed modules like the PCM (Powertrain Control Module), BCM (Body Control Module), and instrument cluster to share data.
How Does VPW Work?
- Single-Wire Bus: Unlike CAN (which uses two wires), VPW communicates over a single wire, often grey in GM vehicles.
- Message Structure: Messages include a header, data bytes, and a checksum.
- Varying pulse widths: '1' bit -HIGH 64uS, '0' bit -HIGH 128uS, Start of Frame - HIGH 200uS
Where Was J1850 VPW Used?
GM implemented Class 2 from the early 90s to the early to mid-2000s. Think like GMT800 (Silverado) or 4th gen Camaro. It was eventually phased out in favor of CAN bus around the mid-2000s. GM used single and 2-wire can once replacing J1850 VPW.
Why Should Hobbyists Care?
If you’re working on an older GM vehicle, understanding VPW helps with:
- Diagnostics: Use a scan tool that supports VPW (like an older OBD2 reader).
- Retrofitting: Swapping clusters or modules from VPW-era vehicles.
- Reverse Engineering: Logging and decoding VPW messages for custom projects.
How to Interface with VPW?
You have a few options to interface with J1850VPW:
- Commercial off-the-shelf ELM327-based devices can be an easy and tempting solution. The drawback here is varying degrees of quality and documentation. OBDLINK has worked for me in the past.
- OEM and aftermarket tools are excellent for troubleshooting, but cost is the drawback.
- OBDX-pro tools are not ELM327s, and he offers a range of them. Great for troubleshooting or programming.
- My J1850VPW Transceiver. It's suitable for making custom circuits and such. It's not a troubleshooting tool like the one mentioned above. Module Using DLC or J1850VPW-Transceiver
Final Thoughts
While J1850 VPW is outdated, it’s still relevant for classic GM enthusiasts and tinkerers. If you’re diving into automotive hacking, learning VPW is a great way to understand early vehicle networking before moving on to CAN bus. I have a ton of information about it on my YouTube channel, so take some time and check it out if you found this interesting.