TK SL 300 Slow cooling after 50F

[Milion Nas]

Hello i am having a problem with a Thermo King SL300. I have changed the throttling valve and after a month or so, i am having a problem.
Until 50oF the temperature is falling very fast but below 50 the rate is very slow.
There is no error code. I am not sure if the problem has to do with the changed valve.
Any suggestions?
Thanks

[cadwaladr]

What are the pressures and ambient temperature

[Milion Nas]

Suction is 2bar and 250psi is the pressure

Ambient is 25oC or 77oF
Sorry for any troubles with pressure units.

[cadwaladr]

What's the box /cargo area temperature when you quote these pressures,the high pressure sounds ok,can you get another gauge on the compressor side of the throttling valve and was this valve overhauled by a competent person,the shims inside the valve adjust the pressure setting of the valve,check both sides of valve while it's in heat mode or defrost mode.

[gravy258]

the throttling valve pressure on the compressor side should be 28psi in heat/defrost.
Whats the temperature difference across the evaporator coil ? This can be checked by pressing the select button three times to DIS. You should be looking at least -4c. (sorry not in F)
Does this unit have modulation ? have a look in the back on right hand side.
Is the corking tape still covering the TX valve sensor bulb on the suction pipe in the evaporator ?