The coolant temperature gauge in the BMW E30 instrument cluster is connected to the brown-plug coolant temperature sender(12621710511).
The coolant temperature sensor
The temperature sensor is an NTC thermistor, its resistance is higher when the temperature is lower.
It was inserted into a container with liquid silicone that was subjected to freezing and heating. Temperature values were recorded. They are lower that the temperature range of the engine, I may try to measure the higher temperatures later and recalculate the results.
| Temperature | Resistance |
|---|---|
| 3°C (38°F) | 1300 Ω |
| 25°C (77°F) | 620 Ω |
| 60°C (140°F) | 186 Ω |
The Steinhart–Hart equation coefficients:
A = 0.0021390576483277567
B = 0.00011856786248721395
C = 0.000001702703922601429
Coolant temperature gauge values
The values for the reference marks, they start from 0. The supplied voltage is 5 Volts.
| Mark | Resistance | Temperature | Input voltage |
|---|---|---|---|
| 0 (blue) | 160 Ω | 64°C (147°F) | 3.1 V |
| 1 | 85 Ω | 82°C (180°F) | 2.3 V |
| 2 | 54 Ω | 94°C (201°F) | 1.8 V |
| 3 (middle) | 35 Ω | 106°C (222°F) | 1.33 V |
| 4 | 25 Ω | 114°C (237°F) | 1.05 V |
| 5 (red) | 20 Ω | 120°C (248°F) | 0.86 V |
| 6 | 12.5 Ω | 132°C (270°F) | 0.6 V |
