Dears:

In normal mode(not Sport), transmission reluctant to upshift to 4th from 3rd, more pronounced while going uphill, when the rpm would be 3500+ and the 5HP24 still decides to stay in the lower gear, even if you give it more gas. Tried using manual mode of the J-Gate, same issue. Car is '98 VDP. No fault codes, and otherwise no other issues.

Diagnosis?

 

First thing would be a sticky sport switch as happens in these car. But since also happening in manual mode I doubt it. It sounds like the TCM.

 

Is there any sign of fluid loss? A hard reset might do it. Search for transmission calibration process, there's a set of stages the driver puts the trans thru in learn mode after the reset. Once this is done leave the car to idle as it loads the new driving map.

 

Dear Sean B:
No sign of fluid loss. Why would a hard reset mend the issue? Is it because the TCM has stored too much confusing shifting patterns? And when you said let it idle for a while, did you mean right after the hard rest, or after you have taught the TCM during first drive? (is it like, 40 shifts?)

 

Do you mean that the shifting works fine in the sport mode?
How many miles, and when was the last trans oil change?
Are you sure the filter was also changed?

 

Sounds simple - Check your tranny fluid level and report back. I bet it is low.

You don't have to see a leak to be low on fluid!
Vector

 

Because it puts the car back to it's original factory settings, resets everything and puts it in learning mode. You let it idle for a while so the various components shake hands with each other and the throttle body can set itself up.

But as others have said, get your fluid changed if it's never been done. You should also look into having the modified pressure valve fitted to prevent any future woes. Search the forum for Transgo Valve.