"I noticed that when I have the heat on the floor or vents, it's nice and hot. But as soon as I switch it to the windshield for the defroster, the air feels lukewarm and it takes forever to clear the glass. Is my heater core going bad, or is something blocked?"
Mechanic's Expert Answer"If the heat is hot at your feet but weak at the windshield, your heater core is likely fine. The most common culprit in a semi-truck is the Blend Door Actuator or the Mode Door. Check these 3 areas: The Mode Door Cable/Motor: When you switch from floor to defrost, a small motor (or cable) moves a plastic door inside the dash. If it's stripped or stuck, it won't seal off the other vents, so the hot air 'leaks' out elsewhere instead of going to the glass. Cabin Air Filter: A clogged filter restricts total airflow. If it's dirty, the fan can't push enough volume to travel the long path up to the windshield vents. The 'Defrost-A/C' Connection: Most trucks automatically turn on the A/C compressor when in Defrost mode to dehumidify the air. If your A/C system is overcharged or the expansion valve is sticking, it might be 'over-cooling' the air before the heater core can warm it back up. Pro Tip: Reach under the dash while switching the dial; if you hear a clicking or grinding sound, that’s a stripped plastic gear in your actuator motor."