DIAGNOSIS FOR VACUUM ASSIST CRUISE CONTROL SYSTEM

THE PURPOSE OF THIS BULLETIN IS TO PROVIDE YOU WITH ADDITIONAL DIAGNOSIS INFORMATION ON VACUUM ASSISTED CRUISE CONTROL SYSTEMS. GENERAL OPERATION REFER TO FIGURE 1. DURING THE NORMAL VACUUM CONDITIONS OF LIGHT TO MODERATE LOAD, AIR FLOWS FROM PORT 'A' TO PORT 'C' TO THE 'INTAKE MANIFOLD'. THE SINGLE ARROWS IN FIGURE 1 SHOW THIS AIR FLOW. A SMALL AMOUNT OF AIR ALSO FLOWS FROM PORT 'A' TO PORT 'B' THROUGH THE 'CONNECTOR' AND TO THE 'VACUUM PUMP'. THE .035-INCH RESTRICTION IN THE HOSE CONNECTOR BETWEEN PORT 'B' AND THE 'VACUUM PUMP' KEEPS THE VOLUME OF AIR RELATIVELY SMALL. WHENEVER MANIFOLD VACUUM AT PORT 'C' DROPS TO A LEVEL LOWER THAN THE VACUUM AT PORT 'B' (PUMP OUTPUT), THE '3-PORT CHECK VALVE' CLOSES PORT 'C'. THE CHANGE IN VACUUM LEVEL AND CLOSING OF PORT 'C' IS CAUSED BY HEAVY LOAD OPERATION. UNDER HEAVY LOAD, THE SYSTEM WILL NOW OPERATE OFF THE VACUUM PUMP. AIR FLOW IS SHOWN BY THE DOUBLE ARROWS IN FIGURE 1. SYSTEM DIAGNOSTICS THE VACUUM ASSISTED SYSTEM DOES NOT REQUIRE PERIODIC MAINTENANCE. HOWEVER, THE FOLLOWING DIAGNOSTIC AND SERVICE PROCEDURES ARE APPLICABLE: A.IF THE 3-PORT CHECK VALVE IS BLOCKED CLOSED OR BLOCKED OPEN, THE SYSTEM WILL CRUISE BELOW ENGAGEMENT SPEED UNDER HEAVY LOAD OR SYSTEM SURGING WILL OCCUR. B.IF THE 3-PORT CHECK VALVE IS INCORRECTLY CONNECTED, THE CRUISE SYSTEM WILL EITHER BE INOPERATIVE OR WILL CRUISE BELOW ENGAGEMENT SPEED UNDER HEAVY LOAD. TO CORRECT CRUISE OPERATION BELOW ENGAGEMENT SET SPEED ON HEAVY LOADS CLEAN THE CHECK VALVE AND CONNECTIONS WITH MINERAL SPIRITS. (SEE FIGURE 2)...RECHECK BY BLOWING LOW PRESSURE AIR INTO PORT 'A' WHILE PORT 'B' IS PLUGGED AND THEN INTO PORT 'B' WHILE PORT 'A' IS PLUGGED. IN BOTH CASES, AIR SHOULD EXIT OUT OF PORT 'C'. TO CHECK FLOW IN THE OTHER DIRECTION BLOW AIR INTO PORT 'C'...NO AIR SHOULD EXIT OUT OF PORT 'B' OR PORT 'A'. CHECK THE 3-PORT VALVE FOR PROPER INSTALLATION. NOTICE THAT THE ARROWHEAD SHAPED PORTION OF THE 3-PORT CHECK VALVE MUST ALWAYS POINT TO THE INTAKE MANIFOLD (FIGURE 2).

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.