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Probably something going on with the blower motor control Processor . Hooking a scan tool an checking for DTC'S - diagnostic trouble codes would be the first step in a diagnostic procedure for a problem like this .
Auxiliary Blower Motor Control Processor
The auxiliary blower motor control processor is an interface between the auxiliary HVAC control module and the rear blower motor. The auxiliary blower motor control processor monitors supply voltage and ground circuits to the rear blower motor. The auxiliary HVAC control module provides a pulse width modulation (PWM) signal to the control processor in order to command the blower motor speed. The auxiliary blower motor control processor uses the blower motor ground as a low side control to adjust the blower motor speed.
The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motors speed by placing the blower motor switch in a desired speed position or by selecting automatic operation. In manual operation, once a blower speed is selected, the blower speed remains constant, until a new speed is selected. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.
As the requested blower speed increases, the following conditions occur:
• The HVAC control module increases the amount of time that the blower motor speed control circuit is modulated to ground.
• The voltage and duty cycle, measured between the blower motor speed control circuit and ground, decrease.
As the requested blower speed decreases, the following conditions occur:
• The HVAC control module decreases the amount of time that the blower motor speed control circuit is modulated to ground.
• The voltage and duty cycle, measured between the blower motor speed control circuit and ground, increase.
Sounds to me the blower motor is on it's way out and should be replaced. Even though they sound like there working doesn't mean everything is ok with it there are different parts on it that could go wrong and they way to fix it is with another blower fan and not a bad idea to do the resistor at the same time located in the blower box with about 4 wires coming out of it with one or two screws holding it in place all is located under dash behind the glove box area.
you may have had something fall down into the defrost vent and it has worked its way down and is blocking the air flow thats trying to come out the vents,near the blower motor.also does it sound like the blower motor is spinning on high speed or at a lower speed,you could have a slow turning blower motor.
Check the blower motor speed control resistor pack is ok.
(The resistor pack is usually located in the blower fan ducting in the air flow).
Check condition of electrical connection terminals on the blower motor circuit - these terminals pass considerable current and over time may fatigue overheat and loosen.
Check operation/replace any blower motor control relay,
Check the blower motor will run if +12v directly applied to the blower motor.
Check operation of the blower motor speed control switch is ok.
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