Coolant Temperature Sender

From WikiLEC

The Coolant Temperature Sender (not to be confused with the Coolant Temperature Sensor used by the ECM) is also a negative temperature coefficient (NTC) thermistor that allows the temperature gauge to display engine temperature. The temperature gauge is provided with a precision 10V supply and current flows through the gauge and via the sender to ground.

Empirical data for the effect of various resistors on the temperature gauge is as follows (sender resistance | sender voltage | gauge position | temperature):

35.6 | 4.00 | 8/8 | 115°C

48.7 | 4.80 |

50.0 | 4.90 | 6/8 | 105°C

60.0 | 5.30 |

70.0 | 5.75 | 4/8 | 95°C

85.3 | 6.20 |

100. | 6.60 | 2/8 | 85°C

120. | 7.00 | 1/8 | 80°C

153. | 7.50 | 0/8 | 75°C

200. | 8.00 |

This fits well with the gauge having a resistance of 52 ohms and the thermistor having Ro=1700 ohms and Beta=5000.

This results in a reasonably linear relationship between temperature and sender voltage: V ~ 14 - 0.087 × T(°C). For example, the sender voltage at 95°C is 5.74V and 90°C is 6.17V.