A/C - Moan/Vibration Explainations
GMC NUMBER: 92-1B-121GROUP: Air Conditioning
DATE: August 1992
CORPORATE NUMBER: 821202R
SUBJECT:
AIR CONDITIONING NOISE/VIBRATION
MODELS:
1987-92 S/T TRUCKS WITH 2.8L ENGINE AND AIR CONDITIONING
THIS BULLETIN CANCELS AND REPLACES GMC TRUCK SERVICE BULLETIN 88-T-152 (CORPORATE NUMBER 821202R), DATED AUGUST 1988 TO UPDATE THE SERVICE PROCEDURE AND MODEL YEARS. ALL COPIES OF 88-T-152 SHOULD BE DESTROYED.
Some 1987-92 S/T Trucks equipped with 2.8L engines may exhibit a moan noise or vibration when the air conditioning compressor is engaged. Some customers may express concern that this noise/vibration is a result of a malfunctioning A/C compressor
The A/C compressor is a high pressure pump that transfers refrigerant through the system. The normal pumping action of this compressor causes pulsations that are transferred and amplified through various engine and body components resulting in the moan/vibration experienced by the customer. Higher ambient temperatures increase the pressure in the system resulting in stronger pulsations and increased noise/vibrations. Replacement of the A/C compressor will not eliminate this condition. This noise/vibration does not affect the efficiency or durability of the A/C system.
If the service writer's observation of the A/C system indicates that the system is operating as designed provide the customer the "A/C System Operation Description" provided with this bulletin.
In the event that some customers, who operate their vehicle under high ambient conditions for extended periods of time, continue to find this noise/vibration to be objectionable, even after reviewing the "A/C System Operation Description", a modification is available that will dampen the pulsations emitted by the A/C compressor
The installation of a brace kit (P/N 10115893), a high inertia pulley (P/N 6552544), and a Kevlar (tm) serpentine belt (P/N 10189266) may be required depending on the model year of the vehicle.
The brace kit was factory installed on all 1990 and later vehicles with the 2.8L engine and air conditioning. Vehicles built prior to 1990 will require brace kit installation.
The Keviar (tm) serpentine belt was installed on 1992 vehicles with the 2.8L engine and air conditioning beginning with the VIN breakpoints listed below:
Moraine GMC N2512007
Pontiac West GMC N0507209
Shreveport GMC N8509971
Vehicles built prior to these breakpoints will require the Kevlar (tm) belt.
SERVICE PROCEDURE
Brace Kit Installation
1. Verify that Brace Kit has not been installed previously. (Fig 1)
2. Follow installation instructions provided with Brace Kit (P/N 10115393).
High Inertia Pulley (P/N 6552544)/Kevlar(tm) Belt (P/N 10189266) Installation
NOTICE:
Removal of the A/C compressor and system discharge is not required to make this modification.
1. Remove upper fan shroud to obtain clearance to remove compressor clutch plate hub and rotor.
2. Remove serpentine belt.
3. Follow instructions for clutch plate hub and rotor/pulley replacement in section 1B3 of the Unit Repair Manual.
4. Install Kevlar (tm) belt (P/N 10189266) if vehicle built prior to the supplied breakpoints.
5. Reinstall upper fan shroud.
SERVICE PARTS INFORMATION
Parts are available. In case of limited inventory, parts will be placed on 400 control to waive VIP surcharges. Only verifiable emergency orders will be accepted. GMSPO will make every effort to obtain parts. However, the part will be shipped premium transportation at the dealer's expense and all other order types will be placed on backorder until the 400 control is removed.
WARRANTY INFORMATION
For vehicles repaired under warranty use, see chart.
A/C SYSTEM OPERATION
Air conditioning is accomplished through a heat exchange process which incorporates an engine powered pump/compressor.
The A/C compressor is a high pressure pump which forces refrigerant through the system. The compressor has rotating vanes which capture the gaseous freon and compresses it into a condenser. The rotating vanes give pressure surges or pulsations which result in a resonating noise that is transmitted through the engine and frame into the vehicle.
This noise is often called AIR CONDITIONING MOAN and is a normal vehicle condition. Higher ambient temperatures increase the system pressures resulting in an increased intensity of the pulsations and the accompanying moan. The durability of the system is not affected by this condition.
Replacement of the air conditioning compressor will not correct this normal condition but it will cause you the inconvenience of having your vehicle out of service.