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My 04 LS 3.8l shifts very smoothly. But sometimes it thumps hard through gears. Downshifting too. Anyone know what this could be?
 

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My 04 LS 3.8l shifts very smoothly. But sometimes it thumps hard through gears. Downshifting too. Anyone know what this could be?


First step: check the transmission fluid level according to the directions in your vehicle owner’s manual.

Then, what is the age of the transmission fluid and filter? Is it due for a change?
 

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New fluid and filter 5 months ago. Filled to owners manual specs. Old fluid was very dirty. Don't know when it was changed before that. I bought it in march.
 

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Did you use the correct Dexron VI fluid and a quality (ACDelco) filter?
Yes because it is critical that you use a fluid that wasn't used as OEM fill till 2006 in your 2004 car............................

Did the sarcasm come thru well enough?

Dexron VI should work as it is reverse compatible and it is a better fluid, but Dexron III was the factory fill, and is fine if you used it. If seals are worn and it is an internal leak causing the goofy shift the thicker to start with Dexron III would mask it longer. It does shear down to VI viscosity with use.

My wife's 2004 3.4l car had a random hard shift from 170k till 235k when we got rid of it, hopefully you are so lucky.
 

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My 2005 started hard-shifting at 92k miles, and this would continue happening once the car was warmed up and only occasionally, like 5% of trips. I dropped the transmission pan and there was a lot of residue and the magnet was loaded. Apparently, the hard shifting is caused by a computer sensing a slow shift between gears, which means slippage. This produces an error code and the trans computer causes the fluid pressure regulator to go to full pressure, which means hard shifts. Turning the ignition off fully for a few seconds to reset the computer usually allowed it to work normally, but it's not a fix. I tried adding a $30 flush/solvent for 1500 miles, then I drained the trans and refilled it using a synthetic fluid. Didn't help at all. Internet forums point to various causes. I needed a trans rebuild. Mechanic said the accumulators were leaking, which contributed to the slippage. So, have it professionally checked ASAP and maybe you can avoid a rebuild.
 

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Any thoughts about it being vacuum and or the shift censors that are inside the rear of the transmission
 
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