For 1990-2009 cars

# POOR PERFORMANCE/POWER LOSS KNOCK RETARD DIAGNOSIS

#### VEHICLES AFFECTED: 1983 1/2 and 1984 Models Equipped With Electronic Spark Control (Knock Retard)

Due to assembly line testing procedures, ECM data stream changes were made to measure signals from the knock sensor to the ECM. Prior to this change, data strean provided direct degrees of retard as transmitted from the ECM.

In order that meaningful diagnosis can be made to determine if the engine ESC system is operating properly, the following explanation is being provided.

1981, 1982 and early 1983 with ESC would provide the direct degree of retard taking place when the ALDL scan tool was placed in the Knock Retard mode. The reading on the scan tool would be any number from 0 to 19. The number would be the actual degrees of retard command as issued by the ECM.

1983 1/2 and 1984 models with ESC do not indicate direct degrees of retard on the scan tool. The numbers seen when the knock retard mode or spark mode is selected will be any number from 0 and 255. A fixed number indicates no retard is taking place. If retard is taking place, the numbers will continually increase up to 255 and 'roll over' to start again. Small increments of increase indicate small degrees of retard whereas large increments indicate large degrees of retard (up to 19 degrees).

A properly operating system indicates any reading from 0 to 255 which remains steady at steady throttle opening and if the throttle is snapped to WOT with any detonation occurring, an incremental 1 or 2 count increase will be displayed indicating that some retard took place.

If an ESC malfunction, disconnect, or electrical interference is being picked up on the sensor, large count increases will be seen even at idle. If engine or transmission noises are picked up by the sensor, count increases will also be seen indicating that spark is being retarded; Examples, rocker arm, rods, bearings, carbon, transmission gear or front pump noises.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.