So, when my transmission gets up to 140 degrees or warmer it will not downshift to 1st gear when coming to a stop. I can manually downshift to 1st gear then place the shifter back in drive and it'll work like normal. When the transmission temp is below 140 it downshifts as it should. The only mods to the transmission are a transgo shift kit, borg warner governor pressure solenoid, and a BD Diesel transducer upgrade. I checked the tv cable, but it was already adjusted properly. What could the issue be?

 

Could be that the PCM sees a much different trans temp than your gauge shows.
Scanning and reading the value the PCM sees with a good scanner could help you determine this.

 

I have the same issue that I’ve been fighting for months. When cold. Shifts good. Once warm it goes into limp mode. I’ve done everything from cleaning all grounds, changing solenoids and transducers from DNJ to mopar to Borg Warner, I even opened up the flow path to the gov solenoid so it can get fluid all the way around the sensor. Mine is a fully built all billet Suncoast 47RE with a triple disk converter. My issue is electric. New batteries, alternator, and chased wires. Slaved in fresh PCM’s and still no dice. Also multiple valve bodies. Nothing fixes it. So now I’m going to a stand alone controller and fingers crossed this ends my headache. I’m currently 5 months behind on progress with my build because of this nuance.

 

Anyways, following the standard diagnostics procedure:

Measure governor pressure while accelerating and decelerating cold and warm.
Compare gauge readings with the values the PCM sees.

You can also try to substitute the +5V supply for the governor pressure sensor with an adjustable supply that you adjust to 4.7-4.8V for starters.