XLR front end "bumping" at hard slow turn

[James Landi]

About the seal the deal on a relatively low mileage 2005 XLR, and as my wife is test driving, she notices a definite "bumping" as she pulls the car into a parking spot.  Timed with tire rotation, the "bumping" occurs turning either direction as well as in reverse when the steering wheel has locked either way.  The "bumping" feels as if the car is going over an object in on the road surface, or as if a CV joint were failing on a FWD car.  Any thoughts?    James

[Roger Zimmermann]

Is that vehicle a permanent 4 x 4? I remember that the Chevy S10 we sold in Europe years ago was usually a RWD with  4 x 4 on demand. However, there ware some permanent 4 x 4 sold and they had a similar characteristic when driven in a tight curb.

[Steve Passmore]

Could also be a fault with the tyres?

[john estes]

This could be the cause. Easy fix. The bulletin states on hard acceleration but this condition also will happen on tight turns when making parking lot maneuvers.  07-04-20-002B: Rear Axle Noise or Vibration When Making Turns or Under Hard Acceleration (Replace Fluid and/or Posi-Traction Friction Plates) - (Apr 14, 2010)
Subject:   Rear Axle Noise or Vibration When Making Turns or Under Hard Acceleration (Replace Fluid and/or Posi-Traction Friction Plates)
 
Models:  2004-2008 Cadillac XLR, XLR-V
2005-2008 Chevrolet Corvette
2006-2008 Chevrolet Corvette Z06

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Condition
Some customers may comment about a noise or vibration condition coming from the rear axle. This condition can occur during turns or under hard acceleration.

Cause
This condition may be caused by breakdown of the rear axle fluid or, on certain models, degradation of the posi-traction friction plate material.

Correction
Perform the following steps to correct this condition.

For 2007-2008 Cadillac XLR and Chevrolet Corvette Z06

Drain and refill the rear axle fluid. Refer to Differential Oil Replacement in SI. Use the DEXRON® LS fluid listed below. This fluid already contains limited-slip friction modifier.


For 2006 Chevrolet Corvette Z06 and 2004-2006 Cadillac XLR, 2006-2008 XLR-V and 2005-2008 Chevrolet Corvette (base model/non-Z06)
 1. Drain and refill the rear axle fluid. Refer to Differential Oil Replacement in SI. Use the DEXRON® LS fluid listed below. This fluid already contains limited-slip friction modifier.
 2. If the customer returns with the same condition, replace the posi-traction plates with the parts listed below. For replacement instructions, refer to Ring Gear and Differential Housing Disassemble and Ring Gear and Differential Housing Assembly in SI.
Important: The listed replacement differential disc kits may contain a different number of friction plates than the vehicle was manufactured with. This is part of the revised part number application and is correct.

Draining Procedure


 1. Raise and support the vehicle.
 2. Clean any dirt from around the differential drain plug.
 3. If not equipped with a differential cooler, remove the drain plug (1) and washer (2) from the differential.
Object ID: 1491407Click here for detailed picture of the image.

 4. If equipped with a differential cooler:
4.1. Using a bent tip screwdriver, remove the quick connect fitting retaining ring (3) from the quick connect fitting (4) and discard the retaining ring.
4.2. Remove the differential cooler inlet pipe from the differential nut.
 5. Drain the fluid.
Filling Procedure


 1. If not equipped with a differential cooler, install the drain plug (1) and washer (2) to the differential.
Tighten
Tighten the differential drain plug to 35 Y (26 lb ft).

 2. If equipped with a differential cooler:
2.1. Install a new retaining ring.
2.2. Connect the differential cooler pump inlet pipe (2) to the differential. A distinct snap should be heard or felt when assembling the cooler pipe to the fitting.
2.3. To ensure the cooler line is properly installed, give the cooler pipe a gentle pull.
 3. Clean any dirt from around the differential fill plug.
 4. Remove the fill plug and washer from the differential.
Important: DO NOT add any limited-slip additive (friction modifier). With this new fluid (DEXRON® LS Gear Oil 75W-90) it is no longer required to add friction modifier. If friction modifier is added, it will cause the fluid to lose some of it's friction-reducing properties as well as reducing seal life.

 5. Fill the differential with the new DEXRON® LS gear oil, P/N 88862624 (Canadian P/N 88862625). Refer to the table listed below for lubrication specifications.
 6. Check the fluid level to ensure it is even with the bottom of the fill plug hole to no lower than 6 mm (0.25 in) below the opening.
 7. Install the fill plug and washer to the differential.
Tighten
Tighten the differential fill plug to 35 Y (26 lb ft).
Important: Steps 8-13 are for vehicles equipped with the differential cooling system. For vehicles not equipped with the cooling system, proceed to Step 14.

 8. Raise the vehicle so the wheels are about 30 cm (12 in) off the floor. Make sure the wheels can spin freely without obstruction from the hoist or any nearby items.
 9. Start the vehicle and put the transmission in second gear. Slowly accelerate to 16 km/h (10 mph) and hold this speed for one minute.
10. Turn the vehicle off .
11. Raise the vehicle.
12. Check the fluid level in the differential. Add fluid until the recommended level is reached.
13. Lower the vehicle.
14. Once the differential fluid has been changed, the vehicle must be driven in 8-10 tight figure eight maneuvers to heat the fluid and allow the fluid to be worked into the clutch plates.

[Roger Zimmermann]

I was searching too far away! That Positraction problem is as old as the controlled diff itself; I had that on a 1980 Olds...It was already a known issue.

[Gene Beaird]

OR, modern front end alignments. With radials, the cars will sometimes 'pop', or slip sideways when a tight turn. It is especially noticeable in reverse. The Ackerman adjustment that makes the cars so smooth on the interstate rears it's ugly head in tight maneuvers.