GMC chervolet camaro 1999 Service Manual page 51

0-48 Vib.ration Diagnosis. and Correction
Inspect the tire and wheel assemblies for the following
conditions:
• Unusual wear such as cupping, flat spots, and
heel-and-toe wear
(These conditions can cause tire growl, howl,
slapping noises, and vibrations throughout
the vehicle.)
• Proper inflation
• Bulges in the s.idewalls
(Do not confuse bulges with normal ply splices,
commonly seen as indentations in the sidewall).
• Bent
rilT)
flanges
By inspecting these characteristics of the tire and
wheel assemblies, you may discover the cause of the
vibration. The inspection will also provide assurance
that the vehicle is safe for road testing.
Road Test
Purpose of Road Testing
The purpose of road testing is to duplicate the
vibration complaint and to find any operating conditions
that change or eliminate the vibration. Most
importantly, road testing will determine whether the
vibration is related to the engine speed or to
the road speed.
In order to complete a quick and accurate road test,
install an engine tachometer (such as a scan tool) and
the EVA in the vehicle. Place the EV A vibration
sensor in a location where the customer's concern can
be felt.
Which Component Group is Causing tt,e .
Vibration.
After you have related a vibration to either the engine
or to the vehicle-speed, break the vibration down
further in order to fit into one of the following groups of
rotating components:
• The engine, the clutch disc (manual transmission),
the propeller shaft (within the driveline support
assembly), the transmission flexplate (automatic
transmission), and the transmission torque
converter ·
• The transmission output shaft and the rear axle
differential pinion (mounted in the rear of the
differential)
• The tires, the wheels, the hubs, and the rotors
These 3 groups represent the major areas that can
produce vibration complaints. The components in each
group are related to each other because the
components are either bolted or splined together. This
means that each group of components rotates at
the exact same speed.
These categories can be broken down further in order
to identify the exact component responsible for the
disturbance. The emphasis is on testing (and
more testing) in order to pinpoint the source and to
eliminate unnecessary parts replacement.
Perform a road test for ALL vibration complaints
unless the disturbance occurs only with the vehicle at
a standstill.
General Information
Types of Road Test Procedures
Caution: Refer to Road Test Caution in Cautions
and Notices.
Important: Before performing any road test, inspect
the tires and wheels. Refer to Tire and Wheel
Inspection
The following road test procedures are the most
informative and the most used:
• Slow acceleration test
• Neutral coast-down test
• · Downshift test
• Neutral run-up test
• Brake torque test
• Steering input test
• Standing start acceleration test (launch shudder)
These tests will help to pinpoint the vibration. Perform
all.of the tests on a smooth and level road.
Slow Acceleration Test
This test will identify those conditions which are
related to the engine-speed or to the vehicle-speed.
Additional tests may be necessary in order to
determine the. exact cause of the vibration.
Caution: Refer to Road Test Caution in Ca1Jtions
and Notices.
1. On a smooth, level road, slowly accelerate up to
highway speed.
2. Look for disturbances that match the customer's
description.
3. Observe the following reading~ where the
disturbance occurs:
• The vehicle speed, km/h (mph)
• The engine speed (RPM)
• The frequency (if possible)
Now perform the neutral coast-down test and the
downshift test.
Neutral Coast-Down Test
Caution: Refer to Road Test Caution in Cautions
and Notices.
1. On a smooth level road, accelerate to a speed
slightly higher than the speed at which the
vibration occurs.
2. Shift the vehicle into NEUTRAL gear and coast
down through the vibration range.
Observe whether the vibration is present in
NEUTRAL gear.
If the vibration still occurs in NEUTRAL gear, then the
vibration is definitely sensitive to vehicle-speed. At
this point, the following components have been
eliminated as a cause of the vibration:
• The engine
• The clutch disc (manual transmission)
• The transmission flywheel (automatic
transmission)
• The torque converter
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