I come seaking knowledge from the transmission gods.

This is my wife's truck that she has driven from day one. She tells me the shifting has changed. She says the shifts are happening later at higher RPM than before. She is very technically savy and I know something has changed when she says it. So, I took it to the stealer and had them check it out. They tested the line pressures and looked for codes. None were found, so I'm just out the $85. My problem is I know my wife is not making this up and that she might be sensing something starting to go bad. Now, I did do a transmission fluid flush and changed to Amsoil synthetic at 45K. She has 58K on it now, so I would think that was it, but have heard of Amsoil causing slipping in other applications.

Any help or ideas would be greatly appreciated.

 

You may have a transmission on its way out. Although my '99 F-150 has a 4R70W and yours is probably not, some of the trannies do tend to blow the shifter accumulators. My truck started shifting late when I left the house first thing in the morning. It did this for quite a while, and nobody could (would) tell me what was up. Then all of a sudden I got the flashing O/D light; I took it in to the Dealership, and the tranny guy knew exactly what was wrong. Thankfully, I am a GMC veteran, and I had purchased the 100K powertrain warranty.

Hope this is not your problem.

BTW, if you did the fluid change and didn't change but what was in the pan, you didn't do a lot. The transmission holds 15 quarts, IIRC. Use the Search feature and see if you can drain your torque converter like us F-150 folks can.

Hope this helps.

 

I have the 6 year, 100K warranty as well, but am almost at the end of the 6 years.
I did do the full flush when I changed. I used almost 5 gallons total to make sure I got all of the old stuff out.
Did you have to get a new transmission or did they do a rebuild on yours?
Which transmission do you think I have if not the 4R70W?