1990 Nissan 300zx w/ shifting issue

Could be bad shifting sencor on tranny check to see if you have a shifting solenoid

have them check the tranny solinoid

when a tranny has this sort of problem the modular unit or actuator piston is registering the tailshaft/ engine speed ratio at the point when a shift is required. Have a auto shop check the pressures in the tranny as it may be a mechanical adjustment problem or if electrical it may be the wires in the oil or the unit itself

Put it in 1 from the colum and about 15mph put it into 2 and then drive or over drive and see if that works it might be tranny problems i think its called torque conveter
Or try gas let go gas let go gas and see if yhou can feel it my altima had that issue came out to be SHIFT CYLINOED

300zx auto will have 4 gears. 1-3 are normal running gears 4th is said to be an "overdrive" gear.

shift points should be under 4krpm, with light driving. my 300zx non-turbo usually shifts around 2-3K.

throttle position sensor could need adjustment or replacement. trans fluid should be checked to see if it's in good shape and full.

Does it shift normally in drive and just won't go into overdrive, (4th gear), or is it hung up in 3rd from a dead stop?

best way, remove tranny, then front cross member and then drop the engine out the bottom. having the tranny on it makes it very difficult to menuver

is there any pedal pressure when you step on the clutch? I had a similiar problem with my 91 300zx tt....it was stuck in third gear, no pedal pressure on the clutch whatsoever....i had cracked the pivot ball and the clutch slave 'popped out'. i had to pull the trans to fx the problem because i could not reattach the clutch fork onto the release bearing with the trans in the vehicle....you need to bleed the system...gravity bleed the slave until there are no air bubbles. make sure you top off the clutch master cylinder in the process.....then bleed the clutch operating cylinder.....it is in the passenger front hood near the power steering resivoir....top off the clutch master cylinder again. i hope this helps.

You need to do a transmission diagnostics. Its easy and will eliminate questions about the tranny and what's up with it including the complex sensors that inform the ECU how and when to shift. You'll find the transmission diagnostic process in the Transmission part of the online factory service manual.
Reading the indicator lights will give you a code and a diagnosis of the tranny issue. If your tranny checks out ok then you'll need to check the ECU. The manual also has a separate set of steps to diagnose the ECU. Since the ECU controls the shifting, if the tranny itself reports back as fine, the you'll likely have an ECU issue which should show up in the diagnostics and/or indicate what piece of equipment or sensor is causing the problem.
I would do these diagnostic procedures before replacing anything. Z32 parts are not cheap and you may be making unnecessary replacements. Since you already replaced the tranny and your problem persists I would advise going straight to the ECU iagnostics and identifying what is making it shift incorrectly.
Hope this helps.

I assume this is an automatic transmission. Is it a twin turbo or a non-turbo model? It sounds like you've lost fourth gear and are stuck in 3rd - those RPM would be about right. If you can provide more information, it will probably help in the diagnosis.