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Submitted by Tman70 / 12-22-04



All Ford/Lincoln/Mercury vehicles with Automatic Temperature Control so equipped (Generally from early 1980’s to early 1990’s).

Other Nomenclature

Cold Engine Lockout Switch, Blower Thermal Switch, Cold Engine Blower Cutout Switch, Cold Engine Thermal Switch, Blower Temperature Switch, Blower Thermal Switch.


Usual Symptoms (complaints)
  1. Blower (fan) doesn't run on FLOOR setting only.
  2. Blower won't blow when engine is cold.
  3. Blower only works sometimes.
  4. Fan won't run.
  5. Can't get heat.
  6. Takes a long time for heat to come up.
Location and Usual Operation
  1. The Thermal Blower Lockout Switch is located inline with the engine coolant supply (feed) to the heater core, usually about the middle of the passenger side rocker arm (valve) cover. It is black plastic and has one 2 wire electrical connection and one 2 hose vacuum connection. (See above)
  2. The Thermal Blower Lockout Switch (TBL) is normally found installed in vehicles equipped with Automatic Temperature Control (ATC) and a vacuum operated sub-system for directional control of in vehicle heating and cooling.
  3. The electrical operation of the blower motor is controlled by the TBL only when the ATC is set on the FLOOR setting. On the FLOOR setting, the TBL is powered through a set of contacts on the ATC Function Selector Switch (FSS) and one of two DB/LG wires (of circuit 249) which splice together before they reach the TBLS itself. One of these two wires is powered in all FSS positions except FLOOR, the other in all FSS positions except HI/LO and OFF.
  4. The TBL provides an open circuit to the blower motor until engine coolant temperature reaches approximately 120 degrees F. The TBL also applies full vacuum to the OUTSIDE-RECIRCULATE door vacuum motor to close off outside air until the coolant reaches the 120 degree F temperature.

    NOTE: The TBL does not restrict the normal flow of coolant in any application. It is not a coolant shut-off switch.
Usual Problem Causes
  1. Internal electrical connections become oxidized or corroded resulting in loss of electrical flow through the switch. Remedy: Clean contacts.
  2. Change in coolant temperature fails to activate valve switch or fails to shut it off. Remedy: Clean internal parts or replace switch.
  3. No vacuum flow through switch. Remedy: Clean internal parts or replace switch.
Disassembly of Switch

Removal from Vehicle
  1. Drain coolant to a level below the switch level.
  2. Disconnect the vacuum connector and the electrical connector.
  3. Remove the two retaining clamps on the hoses. NOTE: You will want to use worm drive clamps when reinstalling.
  4. With a rotational motion, remove the switch from the hoses. NOTE: The switch housing is constructed of breakable plastic, so, be careful.
Disassembly of Housing
  1. A 3/16" six point socket should be used to remove the locking screw from the side of the housing.
  2. The upper part of the housing can then be rotated 45 degrees counterclockwise to release it from the lower part.
  3. The upper housing can then be opened up by simply pulling on the two connector points.
  • Standard methods can be used on the electrical contacts. (spray cleaner, emery paper, etc.)
  • Neoprene safe cleaner should be used on the vacuum pad. (dish liquid, silicone cleaner, etc.)
  • Springs and thermal switch can be cleaned using more aggressive cleaners. (carburetor cleaner, wire brushing, etc.)
  • Housing can be cleaned using plastic safe cleaners.
Testing Thermal Switch
  • The Thermal Switch pin should be visible on the top portion of the actual sensor.
  • Water should be placed in a pot or pan to approximately 1" deep. 
  • Heat water and using a thermometer bring it to 130 degrees F.
  • Place the Thermal Switch into the water standing upright.
  • The center pin should then extend out approximately ¼" from its starting position to verify proper operation. Once cooled, it can be returned to its original position by hand compression.
Reassembly and Installation
  • Reassembly should be done following the attached pictures.
  • Reinstallation is the reversal of removal instructions.
  • Dielectric non-conductive) grease should be applied to the electrical contacts to inhibit corrosion and oxidation.
  • Silicone based lubricant can be used on the vacuum pad.

    NOTE: The switch is NOT directional or positional.
    NOTE2: This switch can be jumped across the 2 wires (using a short piece of 12 gauge wire and 2 male ¼" solderless spade connectors) as a means of diagnosis, however this is not a recommended repair procedure.
  1. Electrical connector housing
  2. Heater hose connection
  3. Vacuum connector housing
  4. Thermal switch
  5. Weep hole sponge
  6. Moveable electrical contact
  7. Electrical contact pressure spring
  8. Return spring
  9. Moveable holder
  10. Rubber vacuum switch
  11. Retaining screw

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