REAR BRAKE NOISE/MOAN/HOWL UPON BRAKE APPLICATIONS
SUBJECT: REAR BRAKE NOISE/HOWL/MOAN
MODELS: 1988-90 C3 AND K3 LIGHT DUTY TRUCKS WITH SINGLE REAR WHEELS
Some owners of 1988-90 C3 and K3 light duty trucks may experience a moan or howl noise upon brake applications. The noise may be more noticeable at low speeds nearing the end of the stop.
The noise is caused by normal brake drum vibrations causing the rear brake backing plates to oscillate at their resonant frequency. The noise will not damage any of the vehicle components.
To correct this condition the rear axle assemblies on C3 and K3 trucks have been modified to change the resonant frequency.
VEHICLES INVOLVED
Vehicles built after the following breakpoints have the new axles installed in assembly:
VIN Plant
GMC Truck 1GTGK34N8LE548173 Pontiac East Chevrolet 1GCGK34N6LE244095 Assembly GMC Truck 2GTGK39N6L1561496 Oshawa Truck Chevrolet 2GCGC39N6L1252916 Assembly
Vehicles from Fort Wayne Assembly are not affected by this change as Fort Wayne does not build the C3 or K3 series vehicles.
SERVICE PROCEDURE:
To correct this condition in service a tuned dampener assembly has been developed. When installed at the rear leaf spring "U" - Bolts the dampener will oscillate 180 degrees out of phase with the resonant frequency effectively eliminating the generation of the noise.
SERVICE PROCEDURE (Figure 1):
1. Raise the vehicle on a hoist and support the rear axle independently to relieve the tension on the leaf springs.
2. Remove existing nuts and washers from "U" - Bolts.
3. Remove existing anchor plates.
4. Remove existing "U" - Bolts and existing spacers.
5. Clean anchor plates, spacers and axle mating surfaces with a wire brush.
6. Install spacers and replacement "U" - Bolts (1).
7. Install anchor plates.
8. Install Replacement washers (2) and free running replacement nuts (3).
9. Torque nuts to 20 N.m (1 5 lbs. ft.), then tighten them in a diagonal sequence to a torque of 160 N-m (1 18 lbs. ft.).
10. Install dampener assembly (4) with the dampening arm toward the propshaft (Figure 1).
11. Install replacement washers (2) and prevailing torque nuts (5).
12. Torque nuts to 20 N.m (1 5 lbs. ft.), then tighten them in a diagonal sequence to a torque of 160 N-m (1 18 lbs. ft.).
13. Remove axle support and lower hoist.
14. Road test vehicle to verify repair.
SERVICE PARTS INFORMATION
Part Number Description -------- -------------------------------- 15659640 Tuned dampener service kit-1 kit required per vehicle
Parts are expected to be available on April 15, 1991. Until then normal part orders will not be accepted by GMSPO. Only verifiable emergency VIP orders will be accepted. GMSPO will make every effort to obtain parts. All parts will be placed on 400 control to waive VIP surcharges. However, the parts will be shipped premium transportation at dealer's expense. All other order types will be canceled as incorrectly ordered while 400 control is in place.
WARRANTY INFORMATION
For vehicles repaired under warranty use: Labor Operation: T7146 Labor Time: 0.9 hr.
NOTE: Labor Operation is coded to base vehicle coverage in the warranty system.
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.
