So what the heck is a class 2 serial data bus anyway?

Many moons ago as vehicles started getting more complicated with more features (especially computers and modules), wiring and connectors started to become a major issue. Wiring is expensive, adds weight, is one of the top reliability problems and is costly to install. In addition, every variation in features and configuration required a unique wiring harness. The company that I worked for teamed up with Chrysler and developed one of the first serial communications protocols in the industry for vehicles and established the J1850 standard. Our company designed and manufactured the transceiver chips. This new concept addressed many of these challenges by creating a system that can allow many of the vehicles computers and modules to communicate on a single (Chrysler) or dual (Ford) wires. You route this bus to all participating modules and computers and they place their traffic (requests for data and transmission of data) on this single bus in the form of serial digital data (a stream of variable square waves) and all the other modules look for and read their relevant traffic/data. Its kind of like a bunch of people talking at a party in a single room and your spouse askes you a question and you automatically hear and respond while ignoring most of the other traffic. Or at least you will if you know what's good for you. Lol.

Many more standards evolved in the next couple of decades and today one of the most common is the European developed CAN protocol. That said the serial communication protocol for our Blazers is that original J1850 standard that I mentioned earlier. So how does it work?

If you look at a set of wiring diagrams for your blazer you will see a page called Class 2 data or serial data. Every module that communicates on this bus is shown on that wiring page. All those single wires that facilitate this single wire data bus all meet at a circuit symbol in the middle of the diagram. That is called a comb, a metal comb like insert that is pushed into a socket under the left side of the dash and when inserted parallels all the modules on that bus. How do you know if the bus is working properly? Not easily as DIY mechanics with our simple scanners and lack of oscilloscopes but if you had the correct equipment you would see this stream of square waves on the bus. The main failure of the class 2 bus is either a generic wiring fault or a module has died and in doing so corrupts the data bus so that nothing can communicate properly. When this is suspected the play is to locate that socket and remove the comb insert. Than you add modules onto the buss until the bus crashes and then you have found the offending module.

There is a lot more to the Class 2 bus but this will get you started understanding its function and how to diagnose the most common failure. Here is a good video explaining the system.


https://www.google.com/search?q=j185...id:Rwsfe4KT330


George

 

Great info! Thanks!