Why does sterling truck heater only blow cold air?

Answer 1

First, its a Sterling... you're gonna regret that purchase. Daimler A.G. claimed they'd continue supporting them for 10 years after they ended production in 2009, but they've been lacking in that, especially since Ford still supplies wiring harnesses and some other components, and they made no such obligation to continue supporting them. So, best of luck to you with that purchase... hate to say it, but they were better when Ford made them. Sorry to be a doomsayer, but I'm a mechanic with a municipality which has a bunch of Sterlings, and I was a driver and equipment operator for ten years before becoming a wrench, and I literally hate Sterlings with every fiber of my being. Anyhow, to the point..First, check your coolant level. Sounds pretty elementary, I know, but just check it. Top it off as needed, see if that rectifies the problem. If not, check to make sure the lines to the heater core don't have a cutoff valve which is shut off. Drivers often do this during the warmer months because it can get the AC a couple degrees cooler... or else it can make the AC work when the heater control valve goes tits up (remember what I said about Daimler crapping out on supporting those models for ten more years? Yeah...). Exactly how they're routed... depends on your model year and which engine you have. Detroit Diesel, Mercedes-Benz (from 2003 up), Cummins, and Caterpillar engines were all available in Sterling products, as well as one Mitsubishi engine (because the 360 was just a badge engineered Mitsubishi Fuso, and you never said what model Sterling you had). If the valves to the heater core are open or else aren't installed, check the temperatures of the lines to the heater core once the vehicle has been running, the thermostat has opened up, and the coolant is hot. If one of those lines is cold, you have a plugged heater core. If they're both hot, you're probably looking at a blend door issue.