POOR PERFORMANCE/POWER LOSS KNOCK RETARD DIAGNOSIS

Models Affected: 1983 1/2 and 1982 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, previous models 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 AIDL scan tool was placed in the Knock Retard mode. The degree seen on the scan tool "O" (no retard) or any number between 0 and 19, the number seen would be the actual degrees of retard command as issued by the ECM.

1983 1/2 and 1984 models with ESC will no longer 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. The numbers seen on the scan tool (any number between 0 and 255) will stay at that number, 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 at 0, (0 and 255 may never be seen if the increasing counts are in large enough numbers, they will go right past them during the scan tool updates), if number changes (increasing) are occuring the timing is being retarded, (small increases, small degrees of retard); (large increases, large degrees of retard (19 DEG maximum).

A properly operating system indicates any number between 0 to 255 which remains steady, if the throttle is snapped open to WOT and any detonation occurs an increasing 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 other 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 can trigger the knock sensor to retard the timing.

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.