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

MODELS/YEARS: 1985 NINETY-EIGHT WITH 3.8L V-6 ENGINE

PROM CHANGE TO CORRECT FALSE CODE 34,42 & 43

Code 34 is the Mass Air Flow (MAF) sensor malfunction code. The Mass Air Flow 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 MAF sensor in any 250 millisecond time period. Also, once the code was set, the "Check Engine" light would remain on until the ignition was turned off. Some large electrical transients (engagement of the starter motor) could cause the MAF sensor to shut down and not supply any signal to the ECM for approximately one to two seconds. After that time period the MAF sensor would recover; however, by then the malfunction code had been set and the "Check Engine" 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 MAF sensor signal goes away for a couple of seconds. Another change was added which allows the "Check Engine" light to turn off if the MAF sensor starts operating properly.

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.

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.

PLUG FOULING IN COLD WEATHER

A new fuel recalibration change has been developed to prevent spark plug fouling during cold weather starts. Some vehicies may require this new PROM to correct this condition.

HARSH 4-3 DOWNSHIFTS

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

The above conditions can be corrected by ordering a new PROM, part number 16047216 from WDDGM and installing as per the applicable chassis service manual.

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.