What should you do first if a car upshifts late and harsh with no slippage?

The correct action to take when a car upshifts late and harsh with no slippage is to check the throttle linkage or modulator. This is critical because the throttle linkage and modulator directly influence transmission shifting behavior. If the throttle linkage is misadjusted or if the modulator is malfunctioning, it can result in delayed or harsh shifting as the transmission does not receive the proper signals to shift at the correct time.

Changing the fluid and filter, as indicated in the chosen answer, while a good practice for maintaining transmission health, does not address the immediate issue of harsh and late shifting. It can also lead to further confusion if the problem persists after maintenance.

Adjusting the pressure regulator could potentially help with shifting issues, as it controls fluid pressure within the transmission, but this would not be the first step to address an observable symptom like harsh shifting. Lastly, checking clutch pack clearance is relevant when diagnosing slippage or engagement issues but is less likely to be the cause of harsh, late shifts without slippage occurring.

In conclusion, the most logical first step is to check the throttle linkage or modulator as they have the most direct impact on the described symptoms.