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Jerry, Master Mechanic
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2004 Buick Century: awhile the transmission..shifting..low speed..slip

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I have a 2004 Buick Century. Every once in awhile the transmission makes a clunking sound when shifting up from low speed. Transmission does not slip or anything. Had it checked out by a local mechanic and was told there was nothing wrong. What is causing this and is it something serious?

Hello and welcome to Just, Super mechanic here. About your vehicle.

There has been a service bulletin for that type of concern. Here is what it states.


Bulletin No.: 01-07-30-042F

Date: February 05, 2010

Subject: Information on 2-3 Upshift or 3-2 Downshift Clunk Noise



2010 and Prior GM Passenger Cars and Light Duty Trucks
2010 and Prior HUMMER H2, H3
2005-2009 Saab 9-7X
with 4L60-E, 4L65-E or 4L70-E Automatic Transmission (RPOs M30, M32, M70)

This bulletin is being revised to add the 2010 model year and 4L70E transmission. Please discard Corporate Bulletin Number 01-07-30-042E (Section 07 - Transmission/Transaxle).

For 2005 model year fullsize utilities and pickups, refer to Corporate Bulletin 05-07-30-012.

Some vehicles may exhibit a clunk noise that can be heard on a 2-3 upshift or a 3-2 downshift.



During a 2-3 upshift, the 2-4 band is released and the 3-4 clutch is applied. The timing of this shift can cause a momentary torque reversal of the output shaft that results in a clunk noise. This same torque reversal can also occur on a 3-2 downshift when the 3-4 clutch is released and the 2-4 band applied.



This condition may be worse on a 4-wheel drive vehicle due to the additional tolerances in the transfer case.

This is a normal condition. No repairs should be attempted.

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