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.
Important Suggestions And Tips
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, since 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.
Symptoms Of A Bad Engine Coolant Temp (ECT) Sensor
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:
- Diagnostic trouble codes P0117 or P0118 stored in the PCM's memory.
- Bad gas mileage.
- Extended cranking. In other words, a hard start.
- Your vehicle may crank but not start.
When there's a problem with the ECT sensor, the sensor will do one of the 3 very predictable things:
- It will report an extremely high temperature. Usually around 250° F, even tho' the engine is cold. This condition leads the PCM to register a P0118 DTC.
- It will report an extremely low temperature. This temp is usually around -40° F. This condition leads the PCM to set a P0117 DTC.
- Reports a ‘normal’ temperature, but this temperature does not correspond to the actual coolant temperature.
P0117 And P0118 Essentials
P0117: Engine Coolant Temperature (ECT) Circuit Low Input.
- This DTC tells you that the PCM is seeing a temperature reading above 250° F (121° C).
- In technical terms, the PCM is seeing a very low voltage drop across the ECT sensor's circuit, which it translates into a very high temperature.
- This DTC indicates one of several things:
- Coolant temp sensor's wires are shorted to Ground.
- Coolant temp sensor's wires are shorted to each other.
- Coolant temp sensor is bad.
- PCM is bad.
P0118: Engine Coolant Temperature (ECT) Circuit High Input
- This DTC tells you that the PCM is seeing a coolant temperature of -46° F (-50° C).
- In technical terms, the PCM is seeing a very high voltage drop across the ECT sensor's circuit, which it translates into an extremely low temperature.
- This DTC indicates one of several things:
- Coolant temp sensor's wires have an 'open-circuit' problem (usually due to a cut wire).
- Coolant sensor's connector has become unplugged from the sensor itself.
- Coolant temp sensor is bad.
- PCM is bad.
Coolant Temperature Sensor Basics
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:
- 1
The engine coolant (ECT) temperature sensor has two wires in the connector.
One is the power wire and this power is in the form of 5 Volts. These 5 Volts are created inside the PCM.
The other wire is the Ground wire. Ground is provided internally by the PCM too. - 2
When the coolant is cold (or ambient temperature), the sensor's internal resistance is higher.
This higher resistance causes less of the 5 Volts to pass thru' the sensor, thus causing a higher voltage drop.
This voltage drop can be measured with a multimeter in Volts DC if you probe the two wires (with the connector connected to the ECT sensor).
The voltage that does not pass thru' the sensor (on its way to Ground) will now pass thru' your multimeter and you'll see this as a voltage number on your multimeter's screen.
The PCM translates this higher voltage drop as a low coolant temperature. - 3
As the engine warms up, the coolant starts to heat up, which in turn causes the coolant temp sensor's resistance to decrease.
This decreasing resistance now lets more of the 5 Volts to pass thru' the sensor onto Ground.
So, going back to the multimeter example, this would mean that less voltage is passing thru' the meter and so it'll register a lower voltage value.
The PCM then translates this lower voltage drop as a higher temperature. - 4
The following table will help you to see this temperature to resistance to voltage drop relationship:
Temperature Resistance
(K Ω)Voltage Drop 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.
