Kenworth T600 problem with temperature gauge

Hey everyone. I just brought a 2005 KW T600 with a C15 the twin turbo. On the way driving it home about 300 miles my temperature gauge didn't make it up to 100 degrees. It did move but barely even touched a 100. The heat was blowing a little warm. But really not warm enough. The over flow container looked properly filled. I think the temperature sensor was working because the heat did reflect what the temperature gauge said the temperature of the engine was. But I really don't know. Do any of you guys have experience with a 05 kw t600? Or have any understanding that you can share period? Thanks in advance

 

Start the engine from cold start and feel the upper rad hose to get gradually warm. Upper hose should stay cool until 180/190 thermostat opening temp. If the upper hose stays the same temp as lower you have coolant bypassing the thermostat or thermostat stuck open or opening early.

 

I'm going to try that first thing in the morning. And I appreciate you taking the time to tell me that. Thanks