FALSE CODE,PLUG,FOUL,HARSH SHIFT - NEED PROM CHANGE

VEHICLES AFFECTED: 1985 Oldsmobile And Buick 'C' Models Equipped With 3.8L Engine (VIN Code 3)

PROM CHANGE TO CORRECT FALSE CODE 34, 42 & 43

Code 34 is the Mass Airflow Sensor malfunction code. The Mass Airflow Sensor supplies a variable frequency signal to the ECM. Code 34 used to be set if the ECM did not see a pulse from the MAS in any 250 msec time period. Also, once the code was set, the 'Service Engine Soon' light would remain on until the ignition was turned off. Some large electical transients (engagement of the starter motor) could cause the MAS to shut down and not supply any signal to the ECM for approximately one to two seconds. After that time period the MAS would recover, however by then the malfunction code had been set and the SES light would remain on until the ignition was turned off. A change was implemented whereby the code is not set unless the ECM has not received a signal within any four second time period, thus the code is not set if the MAS goes away for a couple of seconds. Another change was added which allows the SES light to turn off if the KAS starts operating properly.

Code 43 is the Electronic Spark Control (ESC) malfunction code. With the engine running, if the ignition is turned off and then the engine restarted within four seconds it is possible to set a false Code 43. Increasing the diagnostic timer from two seconds to four seconds corrects this condition.

Code 42 is the EST/BYPASS circuit monitor. When the engine is running the EST line is supposed to toggle (switch on and off) at a rate equal to the reference pulses. If the ECM sees four reference pulses without detecting any EST pulses, then the ECM forces Bypass spark which in turn sets Code 42. Under some conditions the EST line was not toggling. This forced Bypass Spark and Code 42 to be set. A change has been made to prevent Code 42 under these conditions.

PLUG FOULING IN COLD WEATHER

A new fuel recalibration change has been developed to prevent spark Plug fuel fouling during cold weather starts.

HARSH 4-3 SHIFTS

Another change included in this PROM is a recalibration to prevent harsh 4-3 downshifts, this change ensures that the torgue converter is disengaged prior to the 4-3 downshift.

The above conditions can be corrected by ordering a new PROM, part number 16047215 from PARTS Department and installing as per the 1985 Chassis Service Manual.

VIN BREAK POINTS

Model Plant VIN ----- ----- --- Oldsmobile Orion F4328962 Oldsmobile Wentzville F1326511 Buick Wentzville F1402088

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.