Transmission shifts late through all gears. Turbo 400 in a 1990 GMC 1 ton 4x4 w/454 engine. It shifts about 5mph over what it should.

1 Ck A/T fluid level & quality
2 Scan/ck for code(s)
3 With scan tool connected,road test vehicle/Ck parameters....
....time for a pro !!

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I suspect transmission oil drain back to oil pan during night.
if you want to confirm.
engine stop take oil level place a mark on dipstick then let it sit overnight. in the morning take oil level if it raise then oil have to charge the torque converter you hear a cavity front pump noise.
when hot take oil level engine hot in park have to be at top of oil level

You may have a body throttle position senson. You need a scanner to confirm that. You may have a vacuum leak somewhere around the throttle body. You may have a fuel pump getting weak so I would check fuel pressure. You may have an EGR valve that is partially stuck open.

is it a turbo 350-400, 350 has a shift cable 400 has a vacuum tube

Have the trans filter& gasket changed. Valve body maby sticking fresh fluid and filter might help.

1-2 shift is delayed and harsh, may not shift into OD: Check Throttle Position Sensor (TPS) for smooth and linear electrical response over the entire range of motion. If not, replace.

This is one of the things i can think of that would cause the issue you gave me. Start with that since it's simple and the other thing to is you can try resetting the ECM.

Turn the ignition key to the run position (engine not running) and pull the ECM fuse out for 30 seconds. Place fuse back in and start the engine, the engine may stall but let it. The ECM is relearning the engine and transmission and then go for a test drive after idling for 1min. The transmission my be running back good as new, if not. Have the TPS checked if not replaced. Good luck and keep me posted.

sounds as if you should service the transmission they wil replace oil and filters and gaskets etc ..

Speed sensor is bad,It is located on the rear axle.
Once replaced,Make sure to reset the computer.

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hello: i have a 1994 GMC SIerra 1/2 ton 2 wd, the first two gears shift find but when you get up to about 55 or 60 mph, the transmission wont shift.