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You are talking about your blower fan for the air conditioner and heater right? More information will be needed to solve the problem.
When you say it barely changes speeds, does that mean it doesnt speed up, or doesnt slow down? By that I mean, is it "stuck" on low, or high?
Most blower fans run at different speeds through the use of a multi-stage resistor built into the fan switch circuit. Low speed is a high value resistor, medium speed is a low value one, and high speed is the fan running without any resistance.
If your fan is running on high and doesnt seem to want to run on lower speeds, the resistor pack may be failing. If the fan is running on low speed only, it may be that the fan itself is beginning to fail, or that the lubricant in it has gotten gummy.
On any other vehicle it sounds like the blower fan switch. The fan switch provides power or ground to the resistor and there would be a high speed blower relay to bypass the resistor. But in your case the fan switch is part of the controller. I don't have a diagram for a 92 but the 94 model shows relays for low medium and high speeds. Each relay gets power from a fuseable link. The high relay gets power thru the low relay if the low relay is not engaged. If you have the model with 3 relays you may need to check them.
Most likely neither. The most common cause is that an electrical device called the blower motor resister block, has failed. This device provides all the slower speeds. It is generally mounted in the HVAC [Heating, Ventilation & Air Conditioning] box, very close to the blower motor. FYI, we mechanics call the fan motor inside the cabin the blower motor. The electric fan motor behind the radiator is called the cooling fan motor. If the resistor block is OK, then you have either a bad fan speed switch or a melted electrical connector at that switch. 95% of the time, the resister block is the problem. Charlie
If you bypass the resistor pack and connect the motor directly to the fused 12v supply then the fan should run at maximum speed. If so, it's the resistor pack that's blown: a very common fault on a huge number of makes and models.
The low speeds(all of them) are distributed threw the fan speed switch to the blower motor resister.The resister is the problem.It is located very near the blower motor.It has two screws mounting it to the blower box.It has a plug in with about 5 to 7 wires.Unplug this plug to the resister.Remove the screws.Pull the resister toward you,it mounts and rest inside of the blower case,this is how it is cooled.Now when you replace the resister,replace the blower motor.Even if the bower motor is running fine, it is pulling to many amps for the resister.It will happen again soon,if you don`t replace the blower motor.If I can help you any more,please let me know.