|The following applies to recovering the OBD-I (On
Board Diagnostics) codes on the 1984 through 1993 model C4 Corvettes
via the 12 pin ALDL connector located under the dash just to the
right of the steering column.
|1994 and 1995 C4s also had the OBD-I system but used a 16 pin
access connector that would become standard for the OBD-II system
used on 1996 and later Corvettes. For 1994 and later models, you
will need to recover the codes using a special procedure involving
the speedometer and the odometer as explained in the service manual
or use a special diagnostic device known as a scan tool.
|The 1984 through 1996 Corvette has an On Board Diagnostic (OBD)
system which is part of the Electronic Control Module or ECM,
(also known as Powertrain Control Module or PCM in 1994 and later
Corvettes), plus a system of sensors located throughout the automobile.
The ECM/PCM gathers input from the sensors and continuously changes
the fuel/air mixture, timing and other engine and transmission
parameters so as to optimize the operation of the automobile for
the best compromise between performance, efficiency and exhaust
|Any time a sensor's output exceeds a Hi / Lo reference parameter
stored in a Programmable Read Only Memory (PROM) array in the
ECM / PCM module, an error code is set and retained in the ECM
/ PCM memory.
|This event causes the lighting of the Malfunction Indicator
Lamp (MIL) on the "tell tale" panel (more commonly known
as the "Check Engine" light) or the SYS (System) lamp
located on the instrument panel above the speedometer on later
|If the problem clears and remains cleared for 10 seconds, the
light will extinguish however, the error code is still stored
in the ECM / PCM memory.
|Regardless of whether the problem is constant or intermittent,
the error code can be recovered through a user friendly system
involving the MIL (Check Engine) or SYS lamp.
|To the immediate right of the steering column under the dash,
you will find a multiple pin electrical connector. This connector
is called the Assembly Line Data Link (ALDL).
|From 1984 until the end of the 1993 model year a 12 pin ALDL
was used. After that, a 16 pin ALDL connector was used. The 1994
and 1995 model year still used the OBD-I system even though they
have 16 pin connectors. The 16 pin connector in the 1996 C4 is
used for the much more complex OBD-II system and a scan tool is
required to discover the OBD-II system's secrets.
|The early ALDL connector has room for 12 pins however only 7
are populated and of those we are only interested in Pin "A"
and "B" for this procedure.