ECM CODE 33 MAP/BARO CLARIFICATION

WHEN DIAGNOSING A MAP AND BARO CORRELATION, SERVICEMEN SHOULD BE AWARE OF THE FOLLOWING: MAP/BARO CORRELATION IS CHECKED BY THE ECM WHENEVER THE IGNITION IS IN THE "RUN" POSITION AND THE ENGINE SPEED IS ZERO. IT CONTINUES TO SAMPLE EVERY 1.5 SECONDS AS LONG AS THESE CONDITIONS ARE MET. IF THERE IS A DIFFERENCE BETWEEN MAP AND BARO OF MORE THAN 5 KPA (BOTH SHOULD BE READING ATMOSPHERIC PRESSURE), THE ECM SHOULD RECORD A HARD CODE 33 WITH THE CHECK ENGINE LIGHT ILLUMINATED AFTER STARTING. UNDER THESE CONDITIONS, THE SYSTEM SHOULD BE IN "FAIL-SOFT" MODE. IF THE KEY IS NOT HELD IN THE "RUN" POSITION FOR AT LEAST 1.5 SECONDS BEFORE STARTING, THE CODE MAY NOT BE CAUGHT AND A HARD STARTING OR POOR PERFORMANCE CONDITION MAY BE EXPERIENCED. SOME MAP SENSORS MAY HAVE A SLOW COLD RESPONSE TIME (SLOW "WAKE-UP") ON COLD ENGINE STARTS. IF THE IGNITION IS HELD IN THE "RUN" POSITION FOR AT LEAST 1.5 SECONDS AND THEN STARTED BEFORE THE SENSOR "WAKES-UP" TO PASS THE CORRELATION TEST, THE ECM WILL SET A HARD CODE 33 AND RUN IN "FAIL-SOFT" MODE. IN ORDER TO AVOID POOR PERFORMANCE OR RUNNING IN "FAIL-SOFT" MODE UNTIL THE SLOW "WAKE-UP" SENSOR CAN BE REPLACED, THE KEY SHOULD BE HELD IN THE "RUN" POSITION FOR 10 SECONDS OR MORE BEFORE STARTING. THIS ALLOWS THE SENSOR MORE TIME TO RESPOND AND PASS THE CORRELATION TEST. THE RESPONSE TIME WILL BE LONGER AT LOWER AMBIENT TEMPERATURES. REMEMBER, AN INTERMITTENT CODE 33 WILL PROBABLY BE STORED IN MEMORY AFTER THIS PROCEDURE. A VEHICLE'S ABILITY TO GET THE MODULATED DISPLACEMENT TO SHIFT FROM 4 TO 6 OR 8 IS DIRECTLY AFFECTED BY MAP/BARO CORRELATION. CAR TO CAR MD OPERATION WILL VARY; HOWEVER, SHOULD A CUSTOMER EXPERIENCE A DIFFICULTY IN ACHIEVING A MODE SHIFT WITHOUT GOING TO WIDE OPEN THROTTLE, THE FAULT COULD LIE IN EITHER THE MAP OR BARO SENSORS. THE MODE CHANGES ARE COMMANDED BY THE ECM (IN THIRD GEAR) WHEN THE MAP VALUE APPROACHES THE BARO VALUE (5 KPA DIFFERENCE FOR A 4-6 CYLINDER CHANGE AND UNDER 4 KPA DIFFERENCE FOR A 6-8 CYLINDER CHANGE). THE WIDER THE THROTTLE OPENING, THE HIGHER THE MAP VALUE AND THE CLOSER THE MAP AND BARO VALUES ARE TO ONE ANOTHER. IF THE BARO SENSOR IS READING HIGH, THE ECM MAY NOT SEE A MAP VALUE CLOSE ENOUGH TO THE BARO VALUE TO CAUSE A MODE CHANGE. YOU MAY NOT SEE THIS IN PARAMETRICS BECAUSE 99 IS THE HIGHEST READING YOU CAN DISPLAY AND THAT IS A TYPICAL BARO VALUE. IF THE BARO VALUE IS READING 99 AND YOU SUSPECT IT MAY BE HIGHER, THE FOLLOWING FORMULA CAN BE USED TO CALCULATE THE ACTUAL PRESSURE READING. THIS FORMULA WILL WORK WITH BOTH THE BARO AND THE MAP SENSORS: (SIGNAL VOLTAGE (PIN A(-) TO PIN B(+)) REFERENCE VOLTAGE (PIN A(-) TO PIN C(+)) X 94.5) +14.3 = KPA THIS CONDITION COULD ALSO BE CAUSED BY A MAP SENSOR WHICH READS LOW OR RESPONDS TOO SLOWLY TO THROTTLE POSITION. IF THE MAP RESPONSE IS POOR (SLOW OR NEVER GETS WITHIN 4 KPA OF THE BARO VALUE), REPLACE THE MAP SENSOR. IF THE CONDITION IS CAUSED BY AN INACCURATE MAP SENSOR, THIS WILL MOST OFTEN BE ACCOMPANIED BY POOR ENGINE PERFORMANCE. DUE TO LARGER TOLERANCES AT COLD TEMPERATURES, THE ECM SETS A CODE 33 WITH A MAP/BARO DIFFERENCE OF 5 KPA. AT HIGHER TEMPERATURES (55F TO 125F), THE TOLERANCES ARE SMALLER AND WILL NOT BE CAUGHT BY THE ECM. AT THESE HIGHER TEMPERATURES, THE MAP/BARO DIFFERENCE SHOULD NOT EXCEED 2.5 KPA.

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.