By Karl Yamashita
March 4, 2003

A lot of installers, dealers and customers have asked many questions on Class 2 so I wrote this small article to clear some things up.

Introduction

What is Class 2 in GM vehicles that everyone is hearing about lately?
Class 2 or Class B is really a in-vehicle network SAE J1850 protocol that GM has been using for many years. It just a new data protocol that a majority of vehicle computer modules uses, which share the same single data wire through out the vehicle.
Recently GM has now included the driver’s door module, passenger’s door module, rear seat audio controller, vehicle communication interface module or OnStar®, steering wheel audio controls, rear seat entertainment (RSE), CD changer, radio, DVD/Video, amplifier and XM radio as part of Class 2. Since these new modules are part of the audio system and security system, you are now hearing more about Class 2.

Connected to this same data wire is the Body Control Module (BCM), Powertrain Control Module (PCM), Transfer Case Shift Control Module, Heater & A/C controls (HVAC), Inflatable Restraint Sensing and Diagnostic Module, Theft Deterrent Control Module, Memory Seat Module, Drivers Information Center (DIC), Key/Lights/Seatbelt Chimes (most vehicles), Instrument Panel Cluster (IPC), Suspension Control Module, Electronic Brake Control Module (EBCM) and other modules.

Frequently asked questions

Do all these modules communicate at the same time?
Yes they do. Each module on the bus has its own ID number and is sent out every two seconds as a state of health message (SOH) to let other modules know that it is present. Every module sends information to other modules when needed. When the ignition is first turned on, there could be as many as 100 or more messages sent on the data bus from various modules within 1 or 2 seconds.

How can all the modules communicate at the same time and not interfere with one another?
Well first, all the modules have a Data Link Controller (DLC) chip with and internal or external transceiver physically connected to the data bus. And second, each module is assigned a different level of priority. The DLC is the key to the Class 2 bus. It is designed to send and receive data depending on priority. When each modules sends information over the data bus, they also check to see if there is other data activity on the bus. If that module sees other data while it is sending data and it has low priority, it will stop sending and try again when the data bus is inactive.

What happens if one module goes bad or power is disconnected from it? Will it load down the data bus and disable crucial control modules like the PCM or BCM?
Again, the DLC chip or transceiver is the key. It is designed so that even if power is interrupted or it damaged, it will not affect the data bus. So lets say if the radio power was disconnected but it was still connected to the data bus, it will not interfere with any other control module’s communication on the data bus.

Does your product use the DLC chip?
Yes they do. We incorporate it in our products to insure your safety when you drive your vehicle. You may pay alot more for our products for Class 2, but is your life or family worth saving a few dollars?

Is it true that the factory radio is related to the airbags?
No. The factory has nothing to do with the airbags or any other modules. The airbags are controlled by the Inflatable Restraint Sensing and Diagnostic Module.

Why are the factory radios being relocated when installing an aftermarket stereo?
Some vehicles have chimes built into the radio. When ever the key is left in the key cylinder or parking lights left on, the radio sends audio chimes to the speakers or to amplifier (if equipped). If the radio is removed, there is no way for chimes to be heard. Seat belts are part of the chimes also, and if not retained could result in the driver not using the seatbelt. The retailer installing an aftermarket stereo could be liable for injuries if the driver was to get into an accident and not wearing a seatbelt.

Can using a test light on the data bus cause damage to modules on the bus?
No, the DLC chip or transceiver is short circuit protected to ground and to voltage spikes up to 40 volts. The worst thing that will happen is that modules will not be able to send SOH messages and data information. Basically the engine won't run unless the BCM, PCM and other modules can communicate to each other.

I've heard that one vehicle would not start if the factory radio was removed. I thought the radio has nothing to do with any engine control modules?
Currently we have not run into a vehicle that has this problem. We wonder if the installer had a wiring issue that may have disabled the data bus?

I tried to put a LOC at the headunit but the volume of the aftermarket amplifier does not vary with the radio volume. Why?
Some vehicles have fixed audio signal outputs going to the factory BOSE amplifier. The BOSE amp only accepts 2 channels of audio from the front outputs of the factory radio. The rear outputs of the factory radio go to the RSE. When you adjust the volume/fade/balance or even EQ settings, the factory radio sends data to the factory amplifier. The factory amplifier actually does all the fading/balancing and adjusts EQ settings directly.

All other information on the vehicles are confidential and cannot be disclosed.
If you would like more technicle information on Class 2, you may do a search on 'J1850 and/or Data Link Controller' on the web.