The temperature is measured electrically, the temperature gauge being of the so-called bimetal type. The unit consists of a pickup mounted on the egine, and a gauge fitted in the combined instrument which is fed over a so-called voltage stabilizer.
The pickup is of the semi-conductor type, that is, lt contains a semi-conductor the electrical resistance of which alters with the temperature. Current through the pickup is proportional. to the temperature indicated by the gauge.
The amount of current psessing through the instrument and pickup determines how much the bimetal spring in the instrument is heated and thus how much temperature the needle indicates. A warmer engine permits more current to pass through the pickup and results in a higher gauge reading.
The sender and gauge are not repairable and the entire unit must be replaced if damaged. The gauge can be tested with an ohmeter. The resistance should be about 58 ohms. This can be suitably measured between the nuts an the reverse side of the instrument plate. The sender should also be checked with an ohmmeter, Its resistance should be about 31.4-47.6 ohms at 100 °C (212 °F)
The gauge can also be checked by connecting to a 12 volt battery across a voltage stabilizer and with a correct sender in series. During warming up, the instrument should show the corresponding temperature. A check can suitably be made with a thermometer (sender and thermometer lowered in heated water).
The checking value for the gauge are:
Beginning of green area 45 °C 113 °F
At first dividing line between green areas 70 °C 158 °F
At second dividing line between green areas 80 °C 176 °F
At dividing line between green and red areas 100 °C 212 °F
