Heater blower motor will not go to hi speed,I have replaced blower motor and control switch,helped a little but not much what else can I try? 1985 f-250 manual trans. 4wd with a/c

No vacuum at control unit, usually hose off at engine or broken hose due to engine heat

Trace the wire back from the blower motor. I don't know about your vehicle, but U.S. cars have a "resistor pack", a small bundle of resistors for the different speeds. These little coils get pretty hot, so they are placed onto the heater case where air inside the case will blow by the resistor pack and help cool it. The resistor pack is the last part of the wiring to the blower motor. You have the blower switch control on the dash, then it goes to the resistor pack and then to the blower motor.

If the fan will not run on ANY speed in your Grand Marquis, the problem is most likely NOT the control switch.

Check for power and ground at the blower motor connector. If power and ground is present, replace the blower motor. Otherwise, the problem is most likely a failed blower speed control module (VERY common). The blower motor speed controller is located in the A/C evaporator housing just below the heater hoses.

Note: It is also common for a bad blower motor to CAUSE the controller to go bad. You MAY have to replace BOTH.

If you have a manual system, you have a resistor. If electronic, you have a blower motor speed controller. Please rate if this helps.

here is info:

Control System Outputs-Manual A/C
Blower Motor Switch Resistor
The heater blower motor switch resistor has the following features:

• The assembly is located on the evaporator housing in the engine compartment.
• Three resistance elements are mounted on the resistor board to provide four blower motor speeds.
• Depending on the heater blower motor switch position, series resistance is added or bypassed in the blower motor circuit to decrease or increase blower motor speed.
• An overheating protective device (thermal limiter) will open the resistor coil circuit when the temperature reaches 121°C (250°F) interrupting the blower motor operation in all speeds except HI.
• The thermal limiter cannot be reset and is not serviceable.

Control System Outputs-EATC
Blower Motor Speed Control
The blower speed control is located on the evaporator housing in the engine compartment.

• The function of the blower motor speed control is to convert low power signals from the electronic automatic temperature control module to a high current, variable ground feed for the blower motor (18527).
• The blower motor speed is infinitely variable and is controlled by the electronic automatic temperature control module software.
• A delay function provides a gradual increase or decrease in blower motor speed under all conditions.

Hi,
Don't go there just yet ! Most heater motor speed problems relate to the heater resistor ! It's located in the ducting close to the motor assy and usually has 4/5 wires on to it and a 2 screw fixing. This componant must be cooled (hence air duct location) and is solely responsible for motor speeds ! You can check motor fan by test lamp/meter ! Good Luck

you have to replace the blend (heat/cold) door motor, you can try to remove motor and turn door and try to jam something in door handle slot ,that the part that the motor turns,hope this helps.

this is for manual heater system. If you have electronic (digital display) you don't have one. Instead you have a blower motor speed controller.

here is info

Control System Outputs—Manual A/C
Blower Motor Switch Resistor
The heater blower motor switch resistor has the following features:

• The assembly is located in the engine compartment on the evaporator housing toward the center of the vehicle.
• Three resistor elements are mounted on the resistor board to provide four blower motor speeds.
• Depending on the heater blower motor switch position, series resistance is added or bypassed in the A/C blower motor circuit to decrease or increase A/C blower motor speed.
• An overheating protective device (thermal limiter) will open the resistor coil when the temperature reaches 121°C (250°F) interrupting the blower motor operation in all speeds except HI.
• The thermal limiter cannot be reset and is not serviceable.

replace fan control switch

here is what my book says:
Blower Motor Speed Control To Remove:

1. Remove the right lower instrument panel insulator.
2. Disconnect the electrical connector from the blower motor speed control.
3. Remove the blower motor speed control mounting screws.
4. Remove the blower motor speed control.