Before beginning diagnosis, review the system Description and Operation in order
to familiarise yourself with the system function. Refer to
Rear Drive Axle Description and Operation
.
Symptom Identification
Important: Guide to typical rear axle noises, and common
sources that are often confused with rear axle noise
| • | Rear Drive Axle Gears - typically a whine type noise occurs with drivetrain
under load, and varies with vehicle and engine speed, and amount of throttle application.
Other typical type of noise is a clunk which is most pronounced when the foot is abruptly
lifted from the accelerator pedal, or when the pedal is abruptly depressed from its
free position while driving at relatively low speeds. |
| • | Rear Drive Axle Bearings - typically a grinding/growl type noise that
does not vary with vehicle speed, but does vary with drivetrain load. Will also be
apparent during coast, but will not if allowed to coast with transmission in neutral. |
| • | Transmission Gears - whine type noise that typically does not occur when
transmission gear is selected achieves a 1:1 ratio during driving. |
| • | Transmission Bearings - typically a grinding/growl type noise that does
not vary with vehicle speed, but does vary with drivetrain load. Will also be apparent
during coast, but will not if allowed to coast with transmission in neutral. |
| • | Tyres - typically a roar type noise, but can also present itself as a
whine that varies with inflation pressure, tread condition and road surfac. Does not
vary with vehicle speed, engine speed, or drivetrain load. |
| • | Engine - typically a whine type noise, that is apparent at certain engine
speeds regardless of whether driving vehicle or revving engine whilst vehicle is stationary. |
| • | Wheel Bearing or Wheel Drive Shafts - typically a grinding/growl type
noise and will be heard across entire vehicle speed range regardless of whether accelerating,
decelerating. If vehicle is allowed to coast with transmission in neutral, and noise
is still apparent then this is typically the cause. |
Classifying the Symptom
Important: Very often, rear drive axles are considered
noisy, when in reality, the noise is emanating from some other source, such as tyres,
front wheel bearings, rear wheel bearings, manual transmission rear bearings, engine
noises, muffler roar, automatic transmission or power steering pumps.
All rear drive axles have some slight humming noise. This will vary with the type
of body construction, load and tyre pressure.
Noise which emanates from any one of these can quite easily be confused with
a rear drive axle noise and unless a series of elimination tests are carried out to
definately confirm the real source of the noise, differential assemblies may be, and
often are replaced or repaired unnecessarily.
Differential symptoms can be classified into the following categories:
| • | Bearing related noises. |
Visual/Physical Inspection
| • | Inspect the system for aftermarket devices which could affect the operation
of the differential. |
| • | Inspect the easily accessible or visible system components for obvious
damage or conditions which could cause the symptom. |
| • | Check for the correct lubricant level and the correct specification. |
| • | Verify the exact operating conditions under which the concern exists.
Note factors such as vehicle speed, road conditions, ambient temperature, and other
specifics. |
| • | Compare the driving characteristics or sounds, if applicable, to a known
good vehicle and make sure you are not trying to correct a normal condition. |
System Testing
- Check the rear drive axle oil level.
- If oil level is correct then check oil condition.
| ⇒ | Drain and fill as required. |
- If oil is in good condition check for foreign particle contamination.
| ⇒ | Drain and fill as required. |
- If oil is not contaminated with foreign particles then check oil for water
contamination.
| ⇒ | Drain and fill as required. |
- If the oil is not contaminated with water, or if noise continues, then
road test the vehicle.
- Pre test: drive at low speed until thoroughly familiar with vehicle noises
by which time the rear drive axle should have reached normal operating tempreture.
- Determine driving conditions at which noise occurs: Accelerate with a
light throttle from a low speed (use highest gear possible so as to avoid gear changes
during the road test) to the maximum speed allowable (110km/h if possible). Release
foot from accelerator and allow vehicle to coast to a stop. Listen and note any noises/issues
heard/felt, and also record vehicle and engine speed, and whether accelerating or
coasting when the condition occurs.
- Eliminate transmission gear noise: Accelerate with a light throttle from
the lowest speed possible whilst maintaining a 1:1 direct ratio to maximum speed allowable
(110km/h if possible). Release foot from accelerator and coast to a stop. Listen and
note any noises/issues heard/felt, and also record vehicle and engine speed, and whether
accelerating or coasting when the condition occurs.
- Investigate if noise if responsive to drive train load: Accelerate with
a moderate throttle (load) from the lowest speed possible whilst maintaining a 1:1
direct ratio to a maximum speed allowable (110km/h if possible). Release foot from
accelerator and coast with the transmission in neutral (this removes load from the
rear drive axle) to a stop. Listen and note any noises/issues heard/felt, and also
record vehicle and engine speed, and whether accelerating or coasting when the condition
occurs.
- Investigate if noise is related to tyres: Drive at same conditions on
varying road surfaces, and safe to do so repeat with elevated tyre pressures.
- Investigate if noise is related to engine: Note the vehicle RPM and speed
where the condition occurs, when reaching the correct conditions, shift gears in order
to change the engine RPM. note if noise is still evident, if it is not then generally
not engine related.
- If road test confirms likely rear drive axle noise, then repair or replace
the unit as required.
Repair Instructions
