Question about 1994 Jeep Grand Cherokee

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Vent selector will not change the air from defrost position

I know that it works off of vacuum but i am not getting any vacuum to the control module

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  • Anonymous Feb 28, 2009

    Where on the engine does the a/c module get it's vacuum?

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You have a vacum leak.

Posted on Feb 28, 2009

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Posted on Jan 02, 2017

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Need pitcure of vaccum lines on 1999 Ford Ranger v6 3.o air and heat blows only threw the defroster.


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Item Part Number Description 1 18A318 Vacuum Control Motor-, Panel/Defrost Door 2 18A478 Panel/Defrost Door (Full Vacuum Position) 3 - Defrost Air Flow 4 - Side Window Demister Air Flow 5 18B545 Temperature Blend Door (Full Heat Position) 6 19860 A/C Evaporator Core 7 19A813 Air Inlet Duct Door (Full Vacuum Position) 8 - Outside Air Inlet 9 18A318 Vacuum Control Motor-, Air Inlet Duct Door 10 - Recirculated Air Inlet 11 18527 Blower Motor 12 19A566 A/C Vacuum Reservoir Tank and Bracket 13 - Vacuum from the Engine Intake Manifold 14 19B888 A/C-Heater Function Selector Switch 15 - Vacuum to the 4x4 Hub Solenoids 16 18495 Water Diverter Valve (Full Vacuum Position) 17 18476 Heater Core 18 18A559 Floor/Panel Door (Full Vacuum Position) 19 - Floor Air Flow 20 18A318 Vacuum Control Motor-, Floor/Panel Door 21 - Panel Vent Air Flow

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Why wont my 2001 ford windstar blow out of vents ? only on defrost?


Air Flow Control
Air flow control is accomplished in the following manner:
  • Primary control is through the function selector knob , mounted on the heater function selector switch, which is part of the heater control .
  • The function selector knob has the following positions: OFF, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST.
  • The heater function selector switch combines a vacuum selector valve with an internal electrical switch.
  • The vacuum selector valve directs source vacuum to various vacuum control motors (18A318) . Refer to the System Airflow Schematic and Vacuum Control Charts.
  • An internal single-pole electrical switch is also controlled by the selector. The switch controls the electrical supply to the heater blower motor switch (18578) .
  • The position of the function selector knob determines the manner in which the system will operate.
  • Each position of the function selector knob is detented for positive engagement.
  • You probably have a vacuum leak , Pull heater control panel an check for vacuum . A broke vacuum hose or The vacuum selector valve is bad ! Or check for a vacuum hose in the engine compartment being off somewhere !

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1997 Ford Taurus front defrost not working


. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ?
Air Flow Control
Air flow control is accomplished in the following manner:
  • Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
  • The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
  • The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
  • Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
  • The position of the function selector knob determines the manner in which the system will operate.
  • Each position of the function selector knob is detented for positive engagement.
  • Vacuum Control Motors The vacuum control motors (18A318):
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    • direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
    • The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).


vacuum control motor-i4bq0ig33y1522pr2d2rrkk3-2-0.gif

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1 Answer

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Works off of vacuum, check all vacuum hoses for leaks or dry rot.

Dec 04, 2013 | Honda Cars & Trucks

1 Answer

Air conditioning will not switch from defrost vents


Vacuum control motors are used to position these airflow control doors.
SECTION 412-01: Air Distribution and Filtering 2000 Expedition/Navigator Workshop Manual DESCRIPTION AND OPERATION Procedure revision date: 06/17/1999 Air Distribution There are two sources of air available to the air distribution system:
  • outside air
  • recirculated air
Recirculated air is only used during MAX A/C, AUTOMATIC (if equipped with EATC) and OFF modes.
Air distribution within the vehicle is determined by the function selector switch position, if equipped with manual A/C, or is controlled by the electronic automatic temperature control system in the AUTOMATIC mode but can be overridden by the driver if desired. Airflow control doors are used to direct airflow within the heater air plenum chamber. Vacuum control motors are used to position these airflow control doors.
The air distribution system is designed to provide airflow from the defrost nozzle when no vacuum is applied to any of the vacuum control motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak.
Air enters the passenger compartment from the:
  • instrument panel A/C registers.
  • heater outlet floor duct.
  • windshield defroster hose nozzle.
  • side window demisters.
  • rear seat airflow duct (if equipped).
Passenger compartment air is exhausted from the vehicle through open windows or cab air vents.


expedition air flow hvac system-xvcmtr1jbqa4kfrdszeh0lpe-3-0.gif Item Part Number Description 1 18A318 Vacuum control motor - panel/defrost door 2 18A478 Panel/defrost door (full vacuum position) 3 - Defrost airflow 4 - Side window demister airflow 5 18B545 Temperature blend door (full heat position) 6 19860 A/C evaporator core 7 19A813 Air inlet duct door (full vacuum position) 8 - Outside air inlet 9 18A318 Vacuum control motor - air inlet duct door 10 - Recirculated air inlet 11 19805 Blower motor 12 - Vacuum to emission control 13 - Vacuum from the engine intake manifold 14 19A563 A/C vacuum check valve 15 19A566 A/C vacuum reservoir tank and bracket 16 19B888 A/C-heater function selector switch 17 18476 Heater core 18 18A559 Floor/panel door (full vacuum position) 19 - Floor airflow 20 18A318 Vacuum control motor - floor/panel door 21 - Panel vent airflow
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1 Answer

I am unable to move selector switch from any position - something in the internal workings is broken b/c the knob moves but the air vent direction does not change? how does it work? what is the name of the...


The system runs on vacuum supplied by the engine intake manifold. The switch uses the vacuum to open/close the various doors in the system by actuating diaphrams. The default is usually defrost, and the system will blow on the windshield if there is no vacuum supplied or if there is a vacuum leak in the line or in the switch. See if you can trace the small vacuum hose that comes from the engine through the firewall and up to the controller/switch and check for a leak.

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