Heater works on high but not low or medium - 1983 Ford Ranger

It will not correct fan speeds, you have other problems, perhaps heater core is partially blocked not allowing air to pass over fins. Look at blower motor resistors or high sped relay. good Luck.

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Nov 2, 2013 - 2009 dodge avenger blower motor works on low, medium low,medium high but not high. Resister has ... SOURCE: Dodge 2001 Stratus - A/C fanonly blows on High ... working. Jan 04, 2010 ' 2003 Dodge Durango · 1 Answer ...

Could be a bad switch or it may be high blower relay.

Heater core is partially plugged or the control valve for the heater is not working.

either the switch or there is a resistor pack in the heater duct that gets burned out and only 1 side works if it is not the high medium low switch check resistor pack mounted in the heater duct.

I assume that you are talking about the heater fan?

Your heater FAN has at least 3 settings, and for our purposes we will assume low,med and high

Low and medium run through a resistor High is wired direct
Either your resistor is bad or your fan is
I know it seems wrong that the fan could be shot but it could be pulling so many amps that it won't work in any other mode than high
I just had this happen on one of my trucks
Start with the fan

All cars are designed with a blower fan motor that pushes air through the ventilation system. This air is processed by a series of vents to direct the air to various positions. Example: heater, air conditioner and defrost modes. Most blower motors are power by a basic electrical circuit that utilizes a multi resistor or a solid-state resistor to control different fan speed modes (low, medium low, medium and high). If the resistor malfunctions it may lose one or more of the lower fan speed settings.
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There are two relays for the heater control fan,a low blower relay and a high blower relay.It sounds like the hi blower relay is bad.

Replace the blower resistor.

You need a blower motor resistor,and the heater blower motor should be checked for high amp draw. check Napa for the part or go to Dodge..prpbably about 60.00 dollars.