Transmission - hard shifting into 2nd gear

[Fecklar]

Hello,
I have a 2000 GMC Safari (4L60E) with 275,000 Kms on it. Its a 2 wheel drive cargo van. Transmission was performing normal. The transmission fluid is clean and red and has no filings or debris on the dipstick. I change the tranny oil every 40,000 km. Then all the sudden yesterday it just started to shift very hard from 1st to 2nd gear and also from 2nd to 3rd but not nearly as firm. 3rd to 4th seemed normal. The SES light did not go on (and yes it does work). Please before anyone says with those kilometres, I need a new transmission, I have a fleet of these vehicles and most often I get 450,000 to 750,000 Km out of these because I maintain them so well and they are mostly all highway kilometres. Any suggestions would be greatly appreciated. I dont know any tranny shops around here that I trust.
Thanks in advance.

[Blue Bowtie]

I'd agree that a well maintained 4L60 behind a V6 should last 400,000 miles easily with a lot of highway driving.

The harsh shifting can be a sign of higher than normal oil pressure. This could be due to a sticking PWM valve, weak accumulator, weak or dirty temperature/pressure sensor array, or other problems. These problems will not necessarily set a trouble code, since the PCM sees the electrical functions of the trans working normally.

It may be revealing to plan another service, remove the pan and filter, and remove/clean/test the pressure sensor. You might also want to remove (unclip) the solenoids and check the solenoid spools and spools behind them for wear, sticking, and contamination.

Short of that, you can connect a line pressure gauge and bi-directional scanner to compare the pressure transducer to the actual pressure gauge reading, then monitor oil pressure while forcing solenoids with the scanner.