96 dodge ram 12v 3500 auto. Just replaced the radiator. I have heat and the line on the engine that runs to the heater core are hot but the top rad hose is not. and the gauge isnt showing anything. I checked the connection on the temp sending unit and its fine. Help Please!

 

Still have air in the system. Remove the t-stat housing and t-stat to purge the air.

 

Won't I loose coolant and have air in the system when I put it back together? Thanks

 

Once you have the block filled to the thermostat level you can then top off the radiator and the thermostat will open as the engine warms up. After the thermostat opens you can finish topping off the system

 

Then the temp gauge should work once I get coolant in the head?

 

If the gauge and sender are good the answer is yes.

 

OK I took off the housing and replaced the t stat then filled the rad up with coolant until it came out of the block then I bolted the housing up. topped off with coolant let the truck run for 30 mins still having the same problem. Top rad hose and rad are cold. the lines running to the heater core are hottttt. Should I try it without the t stat?

 

Did you drive the truck or just let it idle? In the cold weather it takes a diesel quite a while to warm up. You could ground the wire to the sender to make sure the gauge is working.The sender could still be bad. If the needle doesn't move with the wire grounded the gauge circuit is bad somewhere. Without the thermostat the engine would run too cold. In the summer it wouldn't be such a problem.

 

I drove the truck around the block. maybe 1 mile tota after I let it idle for 30 mins. I don't want to drive it too far and mess something up. Why would the heater core lines be hot and I have good heat but the rad and top hose are cold?

 

With a new radiator and cold weather It might be cooling so well that the thermostat doesn't stay open very long. Try putting some cardboard in front of the radiator and see if you can get it to warm up. Is the gauge showing any thing at all?