Rick's Free Auto Repair Advice

Test engine coolant temperature sensor ECT

How to test an engine coolant temperature sensor

An engine coolant temperature sensor is usually a variable resistor that changes resistance and thus voltage as the coolant temperature changes. To test the sensor, coolant temp sensoryou can remove the electrical connector and check the resistance value of the sensor and compare it to specs. If it’s way off, replace it. But if it’s close, you really need to perform a voltage test, since it’s far more accurate than an ohm test.

To test the voltage, you’ll need to either back probe the electrical connector or pierce the wire while the connector is still connected to the sensor and the engine is running. If you’re unsure how to do either of these, click here for more information.

Here’s a temp/resistance/voltage chart for a typical GM sensor early ’90s

Coolant Temp Resistance Voltage

-40℉     100,000 Ω      5.00V
+33℉         9,600 Ω      4.50
+57℉         4,095 Ω      4.00
+78℉         2,975 Ω      3.50
+100℉      1,800 Ω       3.00
+120℉      1,350 Ω       2.50
+143℉          835 Ω       2.00
+163℉          432 Ω       1.50
+186℉          305Ω       1.00
+210℉          185Ω       0.75
+234℉            60Ω       0.50

Symptoms of a failing engine coolant temperature sensor

An engine coolant temperature sensor can be out of calibration enough to cause a no-start condition or poor fuel economy, yet not set a trouble code. If your vehicle won’t start when cold, the easiest way to check the engine coolant temperature sensor is to depress the gas pedal while cranking the engine. If it starts, that’s a pretty good sign the sensor is off. When you press the gas pedal you’re overriding the computer’s air fuel cold start mixture and you’re adding fuel.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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