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TSB 03-05-99

Axle Diagnosis

Page 1 - Verify the Condition

Visit Geno's Garage for Truck accessories.

Note: See also MasterTech issue #10 October 1999 for more tips

Date: Sep. 24,1999

models: 1999 - 2000 WJ Grand Cherokee

Discussion:

This bulletin involves the diagnosis of the vehicle axle.  

Symptoms:

A sound may appear to be coming from the general area of the vehicle axle(s). The sound may be described as a growl, a howl, or a whine. They may occur at different vehicle or engine speeds. They may also occur during different axle loading conditions, i.e. light driving, heavy driving, float, cruise, or coast.  

Diagnosis:

A correct and thorough diagnosis of the axle condition is perhaps the most important tool that the technician has in effecting a successful axle service. Because the transmission of an axle-like sound may be due to vehicle components other than the axle, a good diagnostic procedure is necessary in determining if the condition can be corrected by service to the internal axle components. Proper diagnosis may lead the technician to vehicle components other than the axle.

  A systematic diagnostic approach must be used to diagnosis, analyze, and repair any axle sound. The systematic diagnostic approach involves six steps: verify the condition, verify related symptoms, analyze the symptoms, isolate the condition, repair the condition, and verify the service.

VERIFY THE CONDITION:

1).    Road test the vehicle with the customer to verify the driving environment and whether the condition is present. If a road test with the customer is not possible, then contact the customer and service writer. Obtain a detailed description of the condition and record this information. The customer should be questioned in detail to assist the technician in determining the exact driving conditions and environment that the customer states the condition is present. If no sound is present during your road test, then return the vehicle to the service advisor or customer. If an axle-like sound is present, then continue with your diagnosis.

Questions to ask the vehicle customer:
a). Condition description: a sound (a high pitched whine, a low frequency growl, or a cyclic howl), a vibration, or other related symptoms.
b). Condition of occurrence: is it constant or intermittent.
c). Vehicle description: engine speed, vehicle speed, load, temperature, maneuver.
d). Environment: ambient temperature, wet or dry road, smooth or rough road.

2).    Verify that all tires are inflated properly, are not damaged, are designed for the vehicle, are the correct size, and all four tires are the same. Inspect the wheels in like manner.
3).    A thorough road test of the vehicle axle will involve different drive modes and is most effective when the axles are at normal operating temperatures. While performing the road test, evaluate the vehicle using the following subjective rating system and sound description type. Enter your subjective rating along with the respective type of sound in the corresponding drive mode box.

NOTE: THE VEHICLE SHOULD BE DRIVEN IN A SAFE AREA AND IN A SAFE MANNER WHILE PERFORMING THESE DIFFERENT DRIVE MODES. OBSERVE ALL VEHICLE SPEED LIMITS.

Subjective Rating System:

• 10 = Superior/ Inaudible
• 9 = Excellent / Audible trace
• 8 = Very Good / Slightly noticeable
• 7 = Good / Noticeable to critical observer
• 6 = Fair / Noticeable to average observer
• 5 = Acceptable / Noticeable to most observers
• 4 = Unacceptable / Noticeable to all observers
• 3 = Unacceptable to the average observer
• 2 = Unacceptable to most observers
• 1 = Unacceptable to all observers

Drive Mode:

• Float = Lightly lift foot on and off the accelerator while maintaining target speed.
• Heavy Drive = Rapid acceleration through target speed.
• Coast = Deceleration through a target speed.
• Cruise = Maintaining target speed without accelerating or decelerating.
• Turns = Left and right turns to load outer axle shaft bearings.

Type Of Axle Sound:

• Growl = a low pitch sound (most noticeable from 15 to 60 mph) usually associated with a bearing.
• Whine = a high pitch sound (most noticeable at highway speeds) usually associated with ring and pinion engagement.
• Howl = an oscillating sound (most noticeable between 45 to greater than 65 mph) usually associated with runout of the major rotating components within the axle like the differential case, pinion gear, or ring gear.
• Drone = a moderate to low pitch sound not usually associated with the axle.

4).    Road test the vehicle to obtain the necessary data to complete the Subjective Rating and Type Sound form above. Record on the form your subjective rating and the type sound that may be of concern. Record on the form if the noise is affected by vehicle turns (axle loading). If the sound is affected by engine speed, record the type sound and the engine speed.

Road Test Procedure:

a). Turn off the vehicle radio.
b). Set the HVAC control to FLOOR mode and blower speed to mid level.
c). Obtain target speed through light acceleration.
d). Float the throttle so the vehicle is neither accelerating nor decelerating.
e). Accelerate rapidly 1 0 mph higher than target speed without causing a downshift.
f). Coast back down to target speed.
g). Cruise by holding the throttle to maintain target speed.
h). Turn the steering wheel left and right.
i). Repeat this road test procedure for each set of target speeds.

5).    Continue with the DIAGNOSIS if the condition is present. If the condition is not present or the subjective ratings are greater than a "7" (Good / Noticeable to critical observer), then return the vehicle to the customer with an explanation of your evaluation.

6).    If the condition is changed by engine speed, then the sound is related to the engine, transmission, transfer case, exhaust, etc.

 

Page 1 - Verify the condition Page 2 - Verify and Analyze Symptoms Page 3 - Isolate the condition Page 4 - Repair the condition, verify repair