Rick's Free Auto Repair Advice

Test engine coolant temperature sensor

How to test a coolant temperature sensor

A bad engine coolant temperature sensor can really screw up starting and performance. If you suspect the sensor isn’t working properly, you can test a coolant temperature sensor or simply replace it. It’s one of the few parts in your car that’s cheap enough that it’s often easier to just replace it rather than spend time testing it. If the coolant temperature sensor is off by a lot, it can prevent the engine from starting, make it run poorly and guzzle gas

Where is the coolant temperature sensor?

The location varies but in most engines it’s located near the thermostat.

where is the engine coolant temperature sensor

Engine coolant temperature sensor is usually located near the thermostat housing

In many case you may find two sensors; one for the computer and a second one for the temperature gauge. The sensor for the computer will have at least two wires, while the sensor for the gauge often has just one wire.

How does the coolant temperature sensor work?

The engine coolant temperature sensor is a thermistor, so electrical resistance changes in response to changes in temperature. That, in turn, changes the return

ECT, engine coolant temperature sensor

Coolant temperature sensor

voltage to the PCM. Engine coolant temperature sensors can be either a negative temperature coefficient (NTC) design where they provide high resistance when they are cold, and resistance drops as they heat up, or a positive temperature coefficient (PTC) where they provide low resistance when they are cold.

Test coolant sensor ohms or voltage?

Many inexpensive shop manuals tell you to test an engine coolant temperature sensor using an ohmmeter. You can do that, but it’s not a very accurate test. Most pro’s test actual voltage coming out of the engine coolant temperature sensor. See the diagram below for where to test voltage. It’s always on the return signal wire.

 test an Engine coolant temperature sensor

How to test an Engine coolant temperature sensor

Coolant Temperature Sensor
Temperature  °F       Resistance            Voltage
-40°F                          100,700Ω             4.90 V
+30°F                         9,600Ω                  3.90 V
+60°F                         4095Ω                   3.00 V
+80°F                         2,975Ω                  2.44 V
+100°F                       1,800Ω                  1.83 V
+120°F                       1350Ω                   4.00V
+140°F                       835Ω                     3.60 V
+160°F                       432Ω                     3.20 V
+180°F                       305Ω                     2.80 V
+210°F                       185Ω                     2.20 V
+220°F                       60Ω                      2.00 V

©, 1017 Rick Muscoplat

Posted on by Rick Muscoplat

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