vehicle ambient temperature sensor 29 may be advantageously employed if present to provide ambient temperature readings to the measurement circuit 28 such elements are believed well within those skilled in the art referring to fig4 and 5 a sensor 31 suitable for encapsulation to form sensor module 20 is illustrated sensor 31 comprises a backing such as a circuit board 30 from one of the major surfaces of which has been etched a copper or metal coated pad 32 metal coated pad 32 includes rails 34 and 38 and a thin film sheet 36 located between the rails and connected to the rails along two opposed edges thin film sheet 36 should be sufficiently thin relative to the rails 34 38 to exhibit substantially greater resistivity than the rails as circuit board 30 is worn down from the top in the direction indicated by the arrow u201c a u201d and thin film sheet 36 is worn down along a free edge of the sheet between rails 34 and 38 the area of thin film sheet 36 decreases a consequent increase in the resistance of sheet 36 between rails 34 and 38 results electrical connection to rails 34 and 38 may be made by connection to pads 42 and 44 which are shown on a side of circuit board 30 bordering the etched major face alternatively the leads may be taken off.