Article 958 - Temperature Switch Diagnosis

TSB #52 - December 2, 1966
(1965 Ford, 1966 Ford and Fairlane, 1967 Ford, Fairlane)

Analysis of temperature switches returned from the field indicates that a high percentage of them should not have been replaced. To reduce the rate of temperature switch replacement, a systematic diagnostic procedure has been prepared. It is important that you follow this procedure as outlined below:

Do not apply 12 volts directly to temperature switch terminals at any time during the test procedure. This voltage will damage the temperature switch unit.
1. Blue Light - COLD. Perform this test only if engine is cool (below 125 °F). Turn ignition switch to ON position. The blue light marked "COLD" should glow. If the blue light does not glow:
    (a) the engine temperature is above 125°F.
    (b) the electrical circuit is not completed or
    (c) the switch unit is not properly grounded to engine.
Connect a jumper wire from the temperature switch hex portion to the engine. If the blue light now glows, tighten the switch as necessary to establish a good ground connection from the switch to the engine. If the blue light still does not glow, proceed with further testing.

Remove the (2)-terminal connector from the temperature switch and connect a jumper wire from the connector white-green stripe wire to the engine, Illus. SI 151-A. If the blue light now glows, verify that the switch is defective. Connect an ohmmeter from the switch COLD terminal to the hex portion, B of Illus. S1151-A. If the resistance is not zero ohms, remove and replace the temperature switch. Be sure to use electrically conductive thread sealer C3AZ-19554-B. Reconnect the (2)-terminal connector and check that the blue light glows. If the blue light does not glow, the wiring or light bulb is defective (open circuited).

When replacing the light bulb, be sure that the ignition switch is in the OFF position.

2. Red Light - HOT. Perform this test only if engine temperature is less than 245°F. If the red light stays on with the ignition switch turned to the On position a defective temperature sending switch is indicated. Remove the (2) terminal connector from the temperature switch and connect an ohmmeter from the temperature switch HOT terminal to the engine (C of Illus. S1151-A), there should be an open circuit (infinite ohms). If the meter reads zero ohms replace the switch. Now if the light is off when the ignition switch is in the ON position turn the switch to the start position and the prove-out circuit should light the bulb. If the bulb does not glow the bulb or the prove-out circuit is defective. Remove the bulb and check the bulb for continuity. If the ohmmeter indicates open circuit (infinite ohms) replace the bulb and again turn the ignition switch to start position to light the bulb with the prove-out circuit. If the bulb still does not light, check the wire continuity from the temperature sending switch to the light bulb to the ignition switch and repair any breaks in the circuit. This should complete the prove-out circuit and the bulb will light when the ignition switch is again turned to the start position.

Temperature Indicating Lights System Operation:
The temperature indicating system provides the driver with warning indication of engine coolant operating temperature by means of indicator lights. This system consists of a temperature switch unit mounted in the intake manifold (8 cylinder) or engine head (6 cylinder), lead wires and two temperature indicator lights mounted on the instrument panel. The lights in the instrument panel are identified by letters "HOT" on red color and "COLD" on blue color. (Illus. S1152-A).

When the engine is cold and the ignition switch is in ON (IGN) position, the "COLD" blue light glows and remains on until the engine begins to approach operating temperature. When the temperature of the engine coolant reaches approximately 125 °F, the blue light will go out, indicating that the engine has reached 125 ° or more.

Should the temperature of the engine coolant reach approximately 245 °F, the "HOT" red warning light will glow, indicating that the engine is overheated.
NOTE: The "HOT" red light will not indicate low coolant level.

These indicating lights are controlled by the temperature switch. The temperature switch has a temperature-sensitive bimetallic arm which completes the circuit through switch body to engine ground.

With the ignition switch in START position, the "HOT" red light should glow even though the engine is cold, thus proving that the light bulb is operable. A set of contacts in the ignition switch (normally open) completes the proving circuit to ground in START position.

Illus. S1151-A
Illus. S1152-A