This article will help you to troubleshoot the engine coolant temperature (ECT) sensor on your Ford 3.0L, 3.8L equipped vehicle.
As you can see, the engine coolant temperature (ECT) sensor will come in one of two configurations. It will either be the screw on type or the press in type (held in place by a clip)... This article's testing info covers both.
TIP1: If you're trying to test an overheating condition on your vehicle, this article won't help you, since a BAD engine coolant temperature sensor will NOT cause your vehicle to overheat.
So if your vehicle is overheating, I suggest taking a look at these two tests: How to Test the Thermostat (Ford 3.0L, 3.8L) and/or How To Test for a Blown Head Gasket (Ford 3.0L, 3.8L).
TIP2: The fastest, easiest and most pain free way to test the ECT sensor (and codes P0117, P0118) is with a scan tool with Live Data capability.
You don't need a professional $5K (US) scan tool to do it... a simple generic scan tool will do (don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
TIP3: Some of my testing suggestions involve tests with the engine running, this means you've got to be on your toes and alert. Think safety all of the time.
If the engine coolant temperature (ECT) sensor or its circuits are BAD, the first thing you'll see is the check engine light shining nice and bright and:
When there's a problem with the ECT sensor, the sensor will do one of the 3 very predictable things:
P0117: Engine Coolant Temperature (ECT) Circuit Low Input.
P0118: Engine Coolant Temperature (ECT) Circuit High Input
You're probably itching to get testing and skip this part... but I suggest you take a look at it since this section will explain (briefly and in non-tech terms) how the ECT sensor works.
In a nutshell... the coolant temperature sensor on your Ford vehicle is a thermistor. A thermistor is a resistor whose resistance changes with temperature.
This changing resistance blocks some or a lot of the voltage flowing thru' it. Depending on how much voltage is being blocked (this is known as a voltage drop), the PCM senses this and translates this into a temperature reading.
Here are more specifics:
The engine coolant (ECT) temperature sensor has two wires in the connector.
When the coolant is cold (or ambient temperature), the sensor's internal resistance is higher.
The PCM translates this higher voltage drop as a low coolant temperature.
As the engine warms up... the coolant starts to heat up, which in turn causes the coolant temp sensor's resistance to decrease.
The following table will help you to see this temperature to resistance to voltage drop relationship:
|50° F (10° C)||58.75 Ω||3.51 V DC|
|68° F (20° C)||37.30 Ω||3.07 V DC|
|86° F (30° C)||24.27 Ω||2.60 V DC|
|104° F (40° C)||16.15 Ω||2.13 V DC|
|122° F (50° C)||10.97 Ω||1.7 V DC|
|140° F (60° C)||7.70 Ω||1.33 V DC|
|158° F (70° C)||5.37 Ω||1.02 V DC|
|176° F (80° C)||3.84 Ω||0.78 V DC|
|194° F (90° C)||2.80 Ω||0.60 V DC|
|212° F (100° C)||2.07 Ω||0.46 V DC|
OK, let's get testing... in the next page.
“I never made a mistake in my life. I thought I did once, but I was wrong.”
Charles M. Schulz