Before trying to fix your car’s heater, you should understand how your car’s heater system works. On this post, an explanation of how your car’s heater system functions, and then, further down, some pointers on heater system trouble-shooting. A car heater is designed to warm the passenger cabin of your car; this system makes use of excess heat from the engine’s internal combustion process. A car’s engine, in fact, when in operation produces so much excess heat that if excess heat is not removed, it would destroy the engine. It is your car’s cooling system that removes the excess heat; some of an engine’s excess heat is released through the exhaust system. Most of the rest of an engine’s excess heat is absorbed by a circulating liquid coolant, a mix of water and antifreeze. The heated liquid coolant is carried from the engine through the hoses to the radiator, which transfers the heat from the coolant to the outside air through a convection process. The heater in your car is similar to the radiator in the front of your car; in fact it looks like a small radiator. The difference is the heater is mounted inside the car and air is blown through the fins of the core. The heater hoses transfer engine coolant to the heater, this allows the heat from the engine coolant to be utilized and warm the passenger compartment air. The heat transferred lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again, a continuous system. Whereas the radiator is located at your car’s front grill, the unit that transfers heat to the passenger cabin (heater core) is located inside the dashboard. This unit, something of a mini-radiator, is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core, and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warmth through the heating vents into your car’s passenger cabin.


Because your car’s heating system works off of the cooling system, sometimes, heating system malfunctions are caused by problems in the cooling system. One of the first things to check when your car heater is not working is your engine coolant. If there is too little coolant in your car’s cooling system, the heater will not be able to produce adequate heat. In any event, to keep your car running optimally, you should regularly check the coolant level. For safety reasons, this absolutely must be done only when the engine is completely cooled down. This safety point cannot be over-emphasized. Opening the cooling system of a hot engine presents a terrible burn hazard, as the fluid in your car’s cooling system is dangerously hot and pressurized when the car has been in operation. It is best to let the car cool overnight before inspecting the coolant level. Most cars have coolant reservoirs with opaque white plastic tanks and measuring lines allowing you to see if the coolant is at the recommended level. Coolant expands when heated, and for that reason, most coolant reservoirs have a Full-Cold and a Full-Hot line.

If there is too little coolant in the reservoir, you should be alert to the possibility that there could be a leak somewhere in your car’s cooling system, but you may nonetheless remove the reservoir cap and add coolant to the Full-Cold line. Obviously, if a very short time after you refill the coolant reservoir, the coolant level is low, there is a leak in your car’s cooling system. Many points in your car’s cooling system are liable to develop leaks through which coolant can escape. Too little coolant in the cooling system will result in a malfunctioning heater.

A leak in the heater core occurs most often in escaped coolant on the front passenger-side floor of the passenger cabin. Most automobile antifreezes are either green or red, though they might also be another color, and that despite a sweet smell they are toxic and should never be ingested. Particular care must be taken to dispose of used coolant properly, as the sweet smell can attract children and pets, and disposing of used coolant at random contaminates the environment. The best way to repair a leaking heater core is to replace the core. Even more common than a leaking heater core, however, are leaking heater core hoses.

The heater hoses are held in place by spring loaded hose clamps and can be removed with the proper tool. Visible signs of leakage are the most obvious indicator of heater hose leaks, but you should also physically inspect the hoses for signs of deterioration. Squeezing one hose at a time, you should feel that the hose is soft and flexible. There should not be any irregular bulges in the hoses. You should not feel any dry rot, which manifests as brittleness and/or cracking or weaknesses in the hoses. If you detect any of these conditions the hoses must be replaced. Consult your owner’s manual, as it will likely have details specific to your car’s heating system. Before you replace any hoses measure the length and diameter of your old hoses so you will know the size your new ones must be. Many cars use three different sizes of heater hoses; the standard diameters are 1/2”, 5/8” and 3/4”. When removing the old hoses, take care not to use any yanking or turning motion that could damage the heater core. Repair technicians often score the old hoses open with a blade and then gradually coax them loose before removing them.

Controlling the heater system varies in most cars, but primarily there are two designs, the first design is the "manual control system", which constantly requires manual adjustment from the driver to maintain the desired temperature. And the automatic temperature control system which is a computerized management system, this system can keep the cabin at a selected temperature or split the set temperature from the driver or passenger side compartments. When a heater core fails it can leak coolant inside the car on the driver or passenger side floor board. Replacing a heater core is different on every vehicle, for the exact procedure consult a repair manual. Replacement can vary from undoing 5 screws and replace it, to the " they started with the heater core, then, built the rest of the car" kind of job which removing the entire dash is not unheard of and can take many hours. A heater valve is used to stop the coolant from flowing into the heater core when the heater system is off or the air conditioner is turned on. When the engine is cold the heater will not work a heater core must have heated engine coolant to work.

 

Greetings Worf
1990 F250 XLT Lariat
Manual control: High fan setting produces awful 'grinding' sounds, reduced at lower fan speed, but still audible.
blower motor resistor replaced ;(
* Is the combination of the grinding noises and the blown resistor:
- obstruction?
- blower motor bearings?
** the cabin gets heated

Thanks for your thoughts