In general, a temperature sensor for an automobile detects a temperature of engine coolant, transmission oil or engine suction air, and transmits the detected temperature to an ECU (Electronic Control Unit). The temperature sensor uses a ceramic element for detecting a middle and low temperature range (the maximum detectable temperature: 150° C. to 170° C.).
FIG. 1 is a cross-sectional view showing a conventional temperature sensor for an automobile, and FIG. 2 is an enlarged view of circle “A” in FIG. 1. A temperature sensor for detecting a temperature of engine coolant is used as an example.
As shown in the drawings, a conventional temperature sensor for an automobile comprises a sensor body 1 and a connector housing 2. The sensor body 1 is formed in a probe type. The sensor body 1 includes a cylindrical inner empty space 4 and a bottom wall 3 for closing the bottom of the space 4. A flange 5 extends radially from the upper end of the body 1, and a connector receiving portion 6 extends upward from the periphery of the flange 5. A screw thread 7 is formed around the outer-middle surface of the body 1. The screw thread 7 may be tightened into a screw hole formed at a wall of an engine cooling jacket, for detecting the temperature of engine coolant.
The lower portion of the connector housing 2 is received in the connector receiving portion 6 of the sensor body 1. The connector housing 2 is fixed to the sensor body 1 by wrapping around the upper periphery of the connector receiving portion 6. The connector housing 2 is made from a plastic material by an injection molding process. A rod 8 extends integrally downward from the connector housing 2. Before the injection molding process, lower ends of two lead wires 10a and 10b are soldered to a ceramic element 9, and upper ends of two lead wires 10a and 10b are connected to two terminals 11a and 11b disposed at the connector housing 2. The ceramic element 9 is inserted within an epoxy or glass molding 12. During the injection molding process, the ceramic element 9 is disposed near the lower end of the rod 8 extending from the connector housing 2, and the lead wires 10a and 10b pass through the rod 8 to be connected to the terminals 11a and 11b. 
The rod 8 of the connector housing 2 is inserted into the empty space 4 of the sensor body 1. When the connector housing 2 is received in the connector receiving portion 6 of the sensor body 1, an O-ring 13 is interposed between the lower outer surface of the connector housing 2 and the inner surface of the connector receiving portion 6. Thus, the ceramic element 9 is located near the bottom wall 3 inside the empty space 4 of the sensor body 1.
When the above conventional temperature sensor is used for detecting a variation in temperature of engine-coolant, the ceramic element 9 located near the bottom wall 3 of the sensor body 1 detects the temperature of engine coolant. The heat of engine coolant is transferred to the ceramic element 9 through the bottom wall 3, the rod 8 and the molding 12. It is preferable that the ceramic element 9 has a good response to the variation in temperature. The ceramic element 9 typically has a size of 1.0×1.0×0.5 mm.
In addition, the temperature sensor disclosed in Korean Patent No. 254860 and Korean Utility Model Registration No. 248772, which have been assigned to this Applicant, may be referred as a prior art.
Those skilled in the art have attempted to improve the response of the temperature sensor. For example, a plurality of concave and convex portions may be formed at the outer surface of the sensor body to increase a heat exchange area. Also, an inorganic matter may be mixed with the epoxy or glass molding, within which the ceramic element is inserted, to improve a heat transfer efficiency of the molding. Further, a plurality of concave and convex portions may be formed at the outer surface of the molding to increase a heat exchange area. Also, the ceramic element may be formed compactly to easily react to the increase in temperature. In addition, to reduce a heat loss from the lead wires connecting the ceramic element to the terminals, a relatively thin lead wire may be used or a material having low heat-loss characteristics may be selected for manufacturing the lead wire.
However, if forming the ceramic element compactly, it has a good response to the increase in temperature, but it has a poor response to the decrease in temperature because the heat exchange area is too small to efficiently emit the heat generated at the ceramic element when current is applied through the ceramic element.