Driveline clunk explanation


Old 01-04-03, 06:18 AM
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Driveline clunk explanation

Can anyone please post what this TSB is, 99-04-20-002A . It is for a 1993, Chevrolet 1500CK 5.7 stepside pick up truck.
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Old 01-04-03, 06:31 AM
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You can go to any GM dealer and they will print it out for you or make a copy.

If someone has access to Alldata here from home, they could print it for you.

Easier for you to go to the dealer and just inquire.
Old 01-04-03, 09:04 AM
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Its a pdf (acrobat reader) file. Scroll down to the the driveline section on the first page, to the left.

Old 01-04-03, 11:26 AM
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"Info - Driveline Clunk #99-04-20-002Info - Driveline Clunk #99-04-20-002

Driveline Clunk
2000 and Prior Chevrolet and GMC Light Duty Truck Models
This bulletin is being revised to update the models section and add information
to the Important statement. Please discard Corporate Bulletin Number 56-44-01A
(Section 4 - Drive Axle and Section 7 - Transmission).
The condition described in this bulletin should not be confused with Driveline
Stop Clunk, described in Corporate Bulletin Number 964101R (Chevrolet 92-265-7A,
GMC Truck 91-4A-77, Oldsmobile 47-71-20A, GM of Canada 93-4A-100) or Bump/Clunk
Upon Acceleration, described in Corporate Bulletin Number 99-04-21-004.
Some owners of light duty trucks equipped with automatic transmissions may
comment that the vehicle exhibits a clunk noise when shifting between Park and
Drive, Park and Reverse, or Drive and Reverse. Similarly, owners of vehicles
equipped with automatic or manual transmissions may comment that the vehicle
exhibits a clunk noise while driving when the accelerator is quickly depressed
and then released.
Whenever there are two or more gears interacting with one another, there must be
a certain amount of clearance between those gears in order for the gears to
operate properly. This clearance or freeplay (also known as lash) can translate
into a clunk noise whenever the gear is loaded and unloaded quickly, or whenever
the direction of rotation is reversed. The more gears you have in a system, the
more freeplay the total system will have.
The clunk noise that owners sometimes hear may be the result of a buildup of
freeplay (lash) between the components in the driveline.
For example, the potential for a driveline clunk would be greater in a 4-wheel
drive or all-wheel drive vehicle than a 2-wheel drive vehicle. This is because
in addition to the freeplay from the rear axle gears, the universal joints, and
the transmission (common to both vehicles), the 4-wheel drive transfer case
gears (and their associated clearances) add additional freeplay to the
In service, dealers are discouraged from attempting to repair driveline clunk
conditions for the following reasons:
Comments of driveline clunk are almost never the result of one individual
component with excessive lash, but rather the result of the added affect of
freeplay (or lash) present in all of the driveline components. Because all of
the components in the driveline have a certain amount of lash by design,
changing driveline components may not result in a satisfactory lash reduction.

While some owners may find the clunk noise objectionable, this will not
adversely affect durability or performance.
Copyright General Motors Corporation. All Rights Reserved."

Like I said before, "minor clunk no problem, big clunk, big problem".
Old 01-04-03, 11:37 AM
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I use to be able to view these bulletins through autozone's site, but now you have to go to alldata or to a dealer. Oh well, am grateful someone had it for you. All I could find was that brief overview not an explanation.

Old 01-04-03, 01:46 PM
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This is what you and I responded to each other a few weeks ago when I asked the question about the clunk problem I was having.
I just happened to run over this TSB from the advise of Joe to another question on the forum to look at alldata. I'm super thankfull for everyones help on this matter and especially yours otter for posting the information.
Old 01-04-03, 01:56 PM
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Could you post 99-04-21-004 Bump/Clunk upon Acceleration for the same truck.
If it is not to much trouble, If it is a hassle I can go to the dealership. Thanks a million.
Old 01-04-03, 02:55 PM
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Here it is....

Service InformationBump/Clunk Upon Acceleration (Flush and Replace Transfer Case
Fluid) #99-04-21-004
Bump/Clunk Upon Acceleration (Flush and Replace Transfer Case Fluid)
1999-2000 Cadillac Escalade
1998-2000 Chevrolet and GMC K1-2 Pickup and full-size Utility Models
With NP246 Automatic Transfer Case (RPO NP8)
This bulletin is being revised to add information to the Models and Correction
sections and changes to the Parts Information. Please discard Corporate Bulletin
Number 86-74-03 (Section 7 -- Transmission)
Some owners may comment about a "bump" feeling when the vehicle accelerates from
a stop. This condition usually is only a single bump feeling or single event. Do
not confuse this condition with a launch shudder condition, which feels as if
the vehicle is shaking or shuddering and is a multiple event (more than one
The condition may be caused by a slip-stick condition between the rear propeller
shaft slip yoke and the transfer case output shaft.
A new transfer case fluid has been developed to correct this condition. This
blue colored fluid is the next generation Automatic Transfer Case Fluid. This
fluid contains a better friction modifier that should eliminate the slip-stick
condition. Flush and replace the transfer case fluid on affected vehicles using
the following procedure and the part number listed below. Refer to the Transfer
Case subsection in the appropriate Service Manual for information on draining
and filling the transfer case.
Refer to Corporate Bulletin Number 99-04-21-001, dated April 1999, for important
information on removing the transfer case drain and fill plugs.
Drain the transfer case fluid.
Refill the transfer case with 2 qts (2 L) of new fluid, P/N 12378508 (In
Canada use P/N 10953626).
After refilling the transfer case, drive the vehicle a minimum of 8 km (5 mi).

With the transfer case at operating temperature, drain the transfer case fluid
Refill the transfer case again with the new fluid.
The vehicle may require approximately 160 km (100 mi) of driving before the
condition is completely corrected.
Parts Information
Part Number Description Qty
*12378508 (In Canada use P/N 10953626) Auto-Trak II Transfer Case Fluid 4
qts (4 L ) Containers

*The above P/N represents a case of twelve containers.
Parts are currently available from GMPSO.
Warranty Information
This procedure requires the draining and refilling of the transfer case fluid
twice. To submit for reimbursement, please follow these steps.
Use K4290 and put the labor time for the 1st drain and refill in the labor
hours field.
Put the 2nd drain and refill labor time in the other labor hours field.
Submit the part number, the total number of parts used and the total parts
dollars in the respective parts fields.
This claim will require dealer self-authorization using authorization code
For vehicles repaired under warranty, use:
Labor Operation Description Labor Time
K4290 Plug, Transfer Case Drain - Replace Use published labor operation

No Problemo, however I don't think that the above applies to your truck.
Old 01-04-03, 04:01 PM
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Thanks otter..You do good..
Old 01-04-03, 04:41 PM
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no problem,

just in case the clunk is coming from your leaf springs, here's a snippet of something I found on the web:

"Exhaust hanger O-rings on leaf springs: Yes, its the cheapest fix and you can do it yourself. Just stretch them over the ends of the lower springs to just past the plastic peg sticking out on the lower side of the leaf. These exhaust hanger O-rings are about 1/4-inch thick and about 1&3/4-inch inside diameter, they are a common exhaust system aftermarket part. They look like big heavy duty O-rings and you need strong hands to install the ones I used. What it does is cushion the lower leaf against the upper one and stops the rattle. Its either do this or live with it. There is no other fix that I know of! "
Old 01-04-03, 07:20 PM
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I will try it. Thanks again. cm

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