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Replace heater motor resistor.
replace the blower motor switch
replace the blower motor resistor
You need to replace blower motor resistor.
It is on the Right side of the Heater Core Covering.
For the 1989 Dodge Ram, the heater blower motor resistor can be found on the passenger's side, on the bottom of the heater box. It is beside the blower motor.
Your truck has both. the resistor is between the switch & blower.thje resistor controls the blower speed
Remove plastic air duct from under passenger side dash, and you will see a white ceramic rectangular resistor with a couple wires sticking out on the firewall by the blower motor cover. Remove screws on the resistor and remove wires to replace.
The blower motor speed resistor is burned out. You can replace only the resistor but the resistor probably burned out because the blower motor is going bad. If you want to fix it for good, replace both the blower motor and blower motor speed resistor.
You have to remove the wipers and the cover that's underneath them. There is a hole on the passenger side that allows access to the resistor from under the hood.
To replace a blower motor on the resistor first you take off the nozzle and then you go into the motor through the nozzle and unlock it then repeat and oil and done.
The resistor block or blower motor resistor is used to control the speed of the blower motor. That is how you get the lower speeds on your air conditioning or heater. The higher the resistance the lower the speed of the motor.