Cavalier/Z24 L4-138 2.3L DOHC VIN D MFI (1995)
CLASS II SERIAL DATA
U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the "Class
II" communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the
transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other
words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue.
The device with the lower priority message must wait. The most significant result of this regulation is that it provides scan tool manufacturers with
the capability of accessing data from any make or model vehicle sold in the United States.
The data displayed on the Tech 1, Tech 2 or other scan tool will appear the same, with some exceptions. Some scan tools will only be able to
display certain vehicle parameters as values that are a coded representation of the true or actual value. For more information on this system of
coding, refer to decimal/binary/hexadecimal conversions table. On GM automobiles, the Tech 1 and Tech 2 display the actual values for vehicle
parameters. It will not be necessary to perform any conversions from coded values to actual values.
DECIMAL/BINARY/HEXADECIMAL CONVERSIONS
Beginning in 1996, Federal Regulations require that all auto manufacturers selling vehicles in the United States provide scan tool manufacturers
with software information to display vehicle operating parameters. All scan tool manufacturers will display a variety of vehicle information which
will aid in repairing the vehicle. Some scan tools will display encoded messages which will aid in determining the nature of the concern. This
method of encoding involves the use of a two additional numbering systems: Binary and Hexadecimal.
The binary number system has a base of two numbers. Each digit is either a 0 or a 1. A binary number is an eight digit number and is read from
right to left. Each digit has a position number with the farthest right being the 0 position and the farthest left being the 7 position.
The 0 position, when displayed by a 1, indicates 1 in decimal. Each position to the left is double the previous position and added to any other
position values marked as a 1. Refer to the Decimal/Hexadecimal/Binary conversion table when using scan tools that display encoded messages.
The hexadecimal system is composed of 16 different alpha numeric characters. The alpha numeric characters used are numbers 0 through 9 and
letters A through F. Refer to the Decimal/Hexadecimal/Binary conversion table when using scan tools that display encoded messages.
The hexadecimal system is the most natural and common approach for scan tool manufacturers to display data represented by binary numbers and
digital code.
