18 speed temp hot

I would start with that if it'll fit without interfering.

Can shoot the housing with an IR temperature gun too to confirm the temperature.

300 is HOT!!!! If its running at that temperature it'll be burnt up in short order. Anything above 250 you're cooking the oil.

 

18 spd usually requires a cooler. Did the 10 speed have one?

 

Does it always read 300? Does it start out at 300 or slowly climb up? Does it fluctuate during the day?

 

Where does the gauge needle park when key is off?
On the Peterbilt gauges they normally park around the 150 degree mark when key is off. Turn key on and it will move needle to read actual temp if above 100. That is one way of knowing if the gauge or sender is still working correctly.
I’m just not sure if the KW’s do the same thing.

I’m like everyone else and agree the sender in the trans does not match the gauge in the dash. Easy fix.