DRIVELINE VIBRATION/NOISE DIAG. (ELECT. VIB. ANALYZER)

SUBJECT: DRIVELINE VIBRATION AND NOISE

MODELS: 1985-92 M/L VANS

Various concerns related to vehicle vibration in 1985-92 M/L vans have been studied and analyzed. Contributors to vibration can come from the engine, driveline, or wheel/tire systems which makes an accurate diagnosis difficult.

To aid the service technician with a systematic approach to diagnosis and correction, GM service worked with Kent Moore to develop and release the Electronic Vibration Analyzer (EVA) (J 38792). By using EVA on a road test, the technician now has the ability to use the frequency of the vibration to identify the source of the vibration. This tool, along with a complete understanding of the STG Vibration Correction booklet 13002.02 dated August 1990, is necessary in determining the cause and correction of vibration conditions in a vehicle. GMC has released a video through the PPN program which gives an overall vibration diagnosis procedure for M/L vans. If you are not a PPN subscriber, this video ("Diagnosing Vibrations with the Electron- ic Vibration Analyzer") can be obtained by contacting your Zone Office.

GM service has made available a program for the dealers to address disturbances by installing "qualified components" into a problem vehicle. These qualified parts are:

a. Tires

b. Front propshaft (L vans)

c. Rear propshafts (M/L vans)

d. Tuned damper, rear axle pinion nose

These parts can only be obtained by establishing a case with GMC Truck Technical Assistance. To identify the correct system, it is important that the technician follow the flow chart contained in this bulletin (Figure 1 for M Van and Figure 2 for L Van) to obtain the necessary vibration frequencies prior to calling the Technical Assistance Engineer. GMC Truck Technical Assistance will then authorize shipment of the needed part(s) to your dealership.

SERVICE PROCEDURE

1. Road test the vehicle on a smooth surface (preferably asphalt). Use the Electronic Vibration Analyzer (EVA) (J 38792) to obtain the frequencies/RPMs at points of disturbance. A passenger should be present during the road test to read and record the information.

2. Use the recorded frequencies/RPMs from the road test to fill in the information on Figure 3.

3. Follow the EVA BALANCE PROCEDURES below to reduce the vibration complaint.

EVA BALANCE PROCEDURES

1. Raise the vehicle on a hoist and support it with safety stands. Support the rear axle at the outer most part of the axle tube. Keep the vehicle supported at curb height.

2. For reference, mark the propshaft at 90 degree intervals (four locations) with white paint or chalk. These marks will be used for the strobing function.

3. L-vans only: remove the front propshaft.

4. M/L vans: remove the rear tire/wheel assemblies and brake drums.

5. Inspect the propshaft and U-joints for undercoating. If undercoating is present, it must be removed.

6. Measure the runout on the rear propshaft and companion flange. There must be no more than 0.025 in. (0.63 mm) runout on the rear propshaft and no more than 0.015 in. (0.38 mm) runout on the companion flange.

7. Inspect the rear propshaft and U-joints for any possible damage.

8. If the runout IS UNACCEPTABLE AND/OR THERE IS DAMAGE, call GMC Truck Technical Assistance for further instructions.

If the runout IS ACCEPTABLE AND THERE IS NO DAMAGE:

a. Use EVA to system balance the rear propshaft to the rear axle by attaching the accelerometer to the bottom of the pinion nose with the word "UP" facing UP.

b. Clip the inductive pickup of an inductive type timing light to EVA's timing light loop and connect the timing light to a 12 volt source.

c. System balance the rear propshaft to the rear axle using the instructions contained in PPN's reference manual GMC TT-92-03-2 in section 5 "DRIVELINE VIBRATION ANALYSIS".

CAUTION:

DO NOT USE THE CRUISE CONTROL TO MAINTAIN A STEADY SPEED.

Important

DO NOT depress the brake pedal with the rear brake drums removed.

9. Road test the vehicle. If the vibration is not noticeable, the vehicle is repaired. If the vibration is still present, contact GMC Truck Technical Assistance for further instructions.

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Op. Description Labor Time --------- ----------- ---------- J9000 Road Test Actual Time up to 0.9 hrs.(*) T7338 Rear Propshaft 0.2 hrs. Runout Check

For proper claims documentation, refer to the Claims Processing Manual.

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.