I have a 1999 mustang. When i went to turn the heat on for winter. the buttons turns a little hard when the fan is on full blast. Also there is no heat coming out unless i mess with the heat dial for a while. i'm sure it has to do with something in the heater box.. but what exact item?
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Make sure the radiator pressure cap is the correct one and that the hose is not soft--check the lower hose and heater hoses for softness as well. They should be firm, but not hard or cracking. The temperature sensor is reporting normal operating temperature, so it and the thermostat are working correctly (hot water would blow out of the radiator if it were stuck). Check the cooling fan for free turning and no noise. Check it's grounding connection. Check the fuse box for blown fuses to the fan and to the A/C compressor. Make sure all the relays and fuses are firmly pressed into their sockets. Check the A/C compressor clutch winding for resistance--should not read infinity. Does the blower motor for the heater/A/C run when either is turned on? The A/C switch on the dash may be defective.
Over top of the blower motor you will see a 4 wire connector hooking up to what is called the blower motor resistor,the wire colors will be OG/BK,LG/WH,YE/RD,BK. It will be held on by 2 clips or screws,this is what is bad and needs to be replaced.
Sounds like you have partialy blocked heater core,not enough flow to the heater or heater tap either not working or only partialy open.You may have to remove heater hose at the fire wall and flush with garden hose.Also check if heater hoses are clear and running freely by using the same garden hose If you put hose up removed heater hose you should get a strong run of water coming out of the heater radiator.