ENGINE ACC. RUN WITH IGN. OFF (MAY BE SHORTED BRAKE DIODE). GMC Truck GMC MD Conventional 97.5" 1988

THE CURRENT CAPACITY OF THE HYDRAULIC BRAKE BOOSTER DIODE ASSEMBLY, WHICH CONTROLS CURRENT FLOW TO THE SENSOR ASSEMBLY, HAS BEEN INCREASED FROM 1 TO 3 AMPERES TO PROVIDE ADDITIONAL CIRCUIT CAPACITY. THE INCREASED CAPACITY ELIMINATES THE POSSIBILITY OF CIRCUIT OVERLOAD BY CURRENT SURGE WHEN THE BOOSTER RELAY IS ENERGIZED. THIS IMPROVEMENT WILL BE INCORPORATED INTO PRODUCTION ON APPROXIMATELY MAY 1, 1988.

AN OPEN DIODE MAY BE INDICATED IF THE BRAKE WARNING LAMP LOCATED ON THE INSTRUMENT PANEL FAILS TO LIGHT WHEN THE ENGINE CONTROL SWITCH (IGNITION SWITCH) IS IN THE "ON" POSITION AND THE PARKING BRAKE IS APPLIED. A SHORTED DIODE MAY BE INDICATED IF THE ENGINE CONTINUES TO RUN, OR ACCESSORIES CONTINUE TO OPERATE, WHEN THE ENGINE CONTROL SWITCH IS IN THE "OFF" POSITION AND THE HYDRAULIC FOUNDATION BRAKE IS APPLIED. SEE SECTION 5A-1 OF THE 1988 MEDIUM DUTY SERVICE MANUAL (X-8833) FOR ADDITIONAL INFORMATION REGARDING SYSTEM OPERATION AND DIAGNOSIS.

THE BRAKE BOOSTER DIODE ASSEMBLY IS A PART OF THE CAB BRAKE WIRING HARNESS LOCATED UNDER THE LEFT SIDE OF INSTRUMENT PANEL NEAR THE STEERING COLUMN. IF REPLACEMENT IS NECESSARY, FOLLOW THE BROWN WIRE ATTACHED TO THE PARKING BRAKE SWITCH BACK TO THE MAIN CAB BRAKE WIRING HARNESS WHERE THE DIODE ASSEMBLY IS CONNECTED TO A PINK WIRE (IGNITION) AND A TAN WIRE (RELAY) BY A CONNECTOR COVERED WITH ELECTRICAL TAPE. REMOVE THE TAPE IN ORDER TO REPLACE THE DIODE ASSEMBLY.

THE DIODE ASSEMBLY, PART NUMBER 12076433, IS AVAILABLE FROM GMSPO.

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.