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Transmission shifts late and hard after fluid change

I have a 1990 Chevy k1500. I have just recently replaced the transmission fluid and filter because I thought it would be a good idea while changing my oil. Only after the change did I find out that changing fluid after so many miles can be bad news. Now my transmission is shifting late and hitting the gear hard. I have checked the level while it was running on level ground and it was fine. Most posts I see point to the clutch pack not having the old grudge to make it shift smooth and say that the transmission just needs a rebuild. I also read one post saying the transmission just isn’t used to running with new fluid and the pressures for the gear changes are different, so the transmission just needs to be reset to re learn how to shift with clean fluid. Is this true? How can i do it? Is there any other options other than a rebuild I may have?

Did you use the exact fluid recommended by Chevrolet?

Check to verify that the modulator valve is connected to vacuum.

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The dipstick says use dextron II. I used the valvoline max life ATF. It says it is recommended for Dex/merc

I don’t think the transmission service you did is the cause of your symptoms. It was about to happen anyway I expect. A proper transmission service can only help. The only thing that could possibly affect the transmission adversely is if the fluid change somehow stirred up some gunk which is now blocking some port or another. I guess that is possible. But unlikely, like I say, if the service was done correctly, probably just a coincidence.

Automatics are configured with 3 or 4 gadgets called “gear sets”. Each gear set has two inputs, one for the clutches, and one for the drum. The one for the drum operates what is is called a “band”. Each input to a gear set is a hydraulic fluid pressures, and are either on or off. Logically I mean. The pressure for a logic “on” might be between 10 and 15 psi, and for logic “off” might be between 2 and 8 psi, that’s the idea anyway. So off/off might be neutral for that gearset, and on/on might be a 1.3:1 ratio, and on/off might be direct drive, a 1:1 ratio.

The pressure comes from a pump inside the transmission, the pump pressure tends to be proportional to the vehicle speed. So the transmission uses the pump pressure to decide – this is done with a part called the valve body – what each of the gear set’s inputs should be. It also needs to be able to adjust its decision for what gear to use for engine load, so there’s a method inside the transmission to do that, either with a vacuum modulator (as on my Ford truck) or a throttle position modulator (common to GM I think) , or on most modern vehicles there’s computer involvement, often using electrical solenoid valves.

So something wrong with all that. First thing I’d do is ask a shop to measure the pump pressure. And go from there. You may indeed need a rebuild, but it could also be just a bad solenoid.

One idea, if you feel lucky, do the transmission service a couple more times. You might want to purchase the fluid at a Chevy dealership so you can make sure it meets the required specs. Replace the filter each time. And sieve the contents of the bottom of the pan & old fluid you drain out for signs of metal debris. You can only get about 30% of the fluid out each drain and fill. So maybe just doing it a few more times will fix the problem, knock on wood. Best of luck.

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Not a good idea to change the tranny fluid on a sleeping transmission.

Unless the transmission fluid has been serviced on a regular basis over the years, a sudden shock of new tranny fluid can knock crap loose within the transmission.

This ends up in the transmission valve body. And the transmission burns up.


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“…transmission just isn’t used to running with new fluid and the pressures for the gear changes are different…”

Possibly. What did the old fluid look/smell like?

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It was a little dark, didn’t smell burnt, not much metal on the magnet

1990? how many miles? original trans? ever been rebuilt?

The transmission seems to be righting itself more and more everytime I drive it. Not hitting the gear as hard anymore, still winding 1st out but not as long. Used to shift at like 27 and now it’s going at like 19. Seems better every time I drive it. Is it possible the fluid just had to work it’s way back through the tranny? Or just had to break the new fluid in? Other possibilities?

Yes original, never rebuilt, 170xxx, bought at 140xxx. Not sure if fluid was ever changed before. I assume so as the old fluid didn’t seem too bad.

Thank you for the insight! Good knowledge, much appreciated

You’re probably experiencing the new fluid mixing more thoroughly with the residual old fluid. As George SJ advised, repeating the process a time or two and making sure you are using the exact recommended fluid will assure a higher percentage of the right stuff is in there. The fact that it’s better now is a good sign that you don’t have clogged passages.

So you would advise continuing to change the fluid? Now or at another 30k?

Inspect the detent/throttle valve cable, it may be sticking causing the late shifts. Other than that some dirt may have found its way into the governor causing it to stick.

Yes, good idea to change it a couple more times, with around a hundred miles between changes.

…and while you’re at it, make sure you are using the recommended fluid - not something that claims to be compatible with, or as good as, or better than - the exact recommended fluid.


Folks change motor oil 2-3 times/yr. but won’t change trans fluid. I do the drain/fill. Takes 5qts. Costs $25. But I have a drain plug for trans. car is new to me so I am doing 2 drain/fills

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I am not an automatic transmission guy and have never actually owned one. I was helping someone with one several years back and posted here for advice. The basic problem they were having were rough shifts at part throttle and shuddering like it wasn’t fully engaging before it finally caught and held. The car had about 100k on it at the time and the fluid was likely never changed.

Anyway, someone here suggested that she pour a can of Berryman B12 into the transmission a few days before having the fluid changed. I think it was someone who worked on transmissions that went by “Transman” here. Boy that name could take on a lot of meanings these days. Anyway, I would have thought this was too harsh of a solvent for a transmission but it didn’t seem to hurt anything and the transmission smoothed out before the change. She took it to the GM dealer and they replaced the fluid and the filter using Dexron 6 synthetic fluid. She kept the car about another two years and the transmission never had any other symptoms.

I suspect the detergents in the new fluid busted loose deposits that partly clogged something and these are now continuing to dissolve in the new fluid. No matter what you do I think the advice to change the fluid several more times with the correct fluid. Unless you take it somewhere there is no way for you to get all the fluid exchanged all at once.

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Transman### was from down in Georgia and was the best transmission guy I’ve ever seen on this board. He hasn’t been back for years. Had a transmission shop down there (Macon I think) and did lots of trans experiments just for fun. Had a couple very nice looking kids too and we miss him.

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