2000 freightliner dump with cummins no heat

Hey guys I drive a freightliner dump truck its a company truck, it has no heat at all, the shop mechanic is a fresh grad from diesel school he is scratching his head over this. I have researched high and low, I do have some good gas engine mechanics so I have been thinking along those lines. My truck does not have valves to turn on or off have looked, fan runs constant, will not build up heat gauge never moves when its below 40 outside, if it does its very little, to me thats a sign a thermostat is stuck open. Can be lucky in this case thats all that is wrong or is there more to it, was very cold friday high in the 20's and I had no heat and froze, I need heat this winter, I can do with out a/c thats a different issue that we can work on I have knowledge of that. I am new to diving these big trucks, never worked on them.

 

Yea sounds like mine got in it when i first started end of aug first of sept a/c worked so so then it went down hill compressor runs but no cold air comes out, I can do with out a/c but not heat I am a wuss when it comes to the cold. Plus running 10 plus hours a day till dark no time really to work on it.

 

Sorry for delay of a update, had been busy and they finally was able to get them changed before another cold snap hit, heat now works perfect keeps me nice and toasty, mechanic did say it was alot easier then he thought 4 bolts, then a trap door type thing opens and they are looking right at you.

 

We got it fixed awhile back it was the thermostats changed them out and now have great heat in fact it will roast you out, yes the fan stays on there is no switch and our tech doesn't know anything about it or where to look to fix it, and for me as long as I get heat when its cold I am happy.