INFORMATION ON ENGINE OVERHEAT. GMC Truck GMC MD Conventional 97.5" 1990

SUBJECT: INFORMATION ON ENGINE OVERHEAT RESULTING FROM REPOSITIONING FAN IN SHROUD

MODELS: 1990-95 CHEVROLET AND GMC TRUCK LIGHT + MEDIUM DUTY TRUCKS EXCLUDING VANS AND SPORT UTILITIES

AUXILIARY ENGINE BELT-DRIVEN EQUIPMENT IS OFTEN ADDED TO COMMERCIALLY OWNED TRUCKS. MOST SERVICE PERSONNEL RECOGNIZE THE POTENTIAL EFFECT THIS CAN HAVE ON WATER PUMP BEARING LOADING. AN OFTEN OVERLOOKED CONCERN THAT CAN RESULT FROM INSTALLING AUXILIARY BELT DRIVEN EQUIPMENT IS THE LOSS OF SOME COOLING SYSTEM PERFORMANCE IF THE FAN IS REPOSITIONED FORE OR AFT FROM ITS ORIGINAL FACTORY INSTALLED POSITION WITHIN THE FAN SHROUD.

THE FORE/AFT LOCATION OF THE FAN WITHIN THE SHROUD HAS A DIRECT EFFECT ON HOW EFFICIENTLY AIR IS DRAWN THROUGH THE RADIATOR. A CHANGE IN THIS RELATIONSHIP WILL OFTEN RESULT IN A REPORT OF REDUCED COOLING SYSTEM PERFORMANCE AND POSSIBLE ENGINE OVERHEAT. DIAGNOSIS OF THIS TYPE CONCERN SHOULD START WITH SIMPLY ASKING THE OWNER IF THE PROBLEM WAS NOTED PRIOR TO THE INSTALLATION OF THE AUXILIARY EQUIPMENT. IF POSSIBLE, TEMPORARILY REMOVE THE ADD-ON EQUIPMENT AND INSTALL THE FAN TO ITS ORIGINAL LOCATION WITHIN THE SHROUD AND DETERMINE IF THIS CORRECTS THE COOLING PROBLEM. IN SOME CASES, A DIFFERENT OEM SHROUD AND/OR FAN MAY BE AVAILABLE THAT COMPENSATES FOR THE REVISED PLACEMENT OF THE FAN WITH THE ADDED EQUIPMENT.

STATIONARY OPERATION OF ENGINE OR TRANSMISSION DRIVEN EQUIPMENT SHOULD BE PRECEDED BY CHOCKING THE WHEELS AND RAISING THE HOOD (PARTICULARLY IN HOT WEATHER) TO ALLOW HOT AIR THAT COULD BE TRAPPED UNDER THE HOOD OF A RUNNING PARKED TRUCK TO ESCAPE.

FIGURES: 0

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.