Most Engine Coolant Sensor works by NTC principle: as temp increases, the resistance (measure in Ohm) decreased:
For example, I measured the brand-new Volvo ECT ad got these values
- -20C: 16,000 Ohms (16 kOhms)
- 0C: 7,300 Ohms
- 20C: 2,600 Ohms
- 80C: 300 Ohms
- 90C (Operating Temp): 200 Ohm
- 100C: 150 Ohms
So, when it is cold, the ECU injects extra fuel to start the engine. The problem is, when the ECT fails, it usually fails in “HOT” position (short-circuit inside the sensor), I have these 2 symptoms:
- Aux fan runs on its own for about 6 minutes or so.
- Hard start when COLD (ECU thinks it is hot and no need to inject extra fuel when it is COLD).
If you got stuck in the middle of nowhere with this problem and need to go home, you can use a resistor that simulates COLD engine (such as 20C with 2,600 Ohm or about 2 KOhms), to start the engine. Once warmed up, you can re-connect to the defective sensor to drive home.
- Volvo PN 9186486, I got it for $40 + shipping from Waltrip Volvo (Franklin TN). Made by SWF (Germany).
- Ebay has same SWF for $39
- T40 Key and 19-mm Wrench
- Some random resistors in case you want to play with the ECU