The ECT sensor says - How hot it is...

The ECT sensor is one of the most important components on a modern car. It tells how hot or cold the engine coolant. But, there are specifics to this sensor. Making the wrong assumption will cause trouble for you, the DIY-Tech or auto-man-woman. Remember, just because the computer says so, it doesn't mean the ECT sensor is bad.

Theory of Operation

The engine coolant temperature sensor (ECT) is a device that changes resistance as temperature changes. Its operating characteristic is linear, which means that when plotted on a chart the plotted line is straight. Inside the sensor is found what is called a thermistor, which is an electronic temperature sensitive variable-resistor. The ECT (thermistor) is a negative temperature coefficient sensor. This means that as temperature goes up resistance and voltage goes down or vise-versa. The ECT sensor receives a 5.00 volts reference voltage from the ECM. The ECT works by changing its internal resistance according to coolant temperature and therefore also changing the voltage drop across itself.

Some manufacturers use a dual scale ECT signal determined by two different pull-up resistors inside the ECM. A high (3.60 K Ohms) and low (345 Ohms) impedance circuit is provided internally by the ECM for the 5.00 volts reference. At low coolant temperatures the ECM uses one of the pull-up resistors (3.60 K ohms). As temperature increases and crosses over a certain temperature value (around 120 deg. F) the ECM switches the 5.00 volt reference internally to the other pull-up resistor (345 Ohms), so that in effect you’ll see 2 volts at 86 deg. F and again 2 volts at 194 deg. F.

This is done to improve ECM control on a hotter engine. In essence, when plotted on a chart this voltage signal has a dual line curve. The actual ECT sensor however works the same way as any other coolant sensor. The difference is that in the dual resistor type system, the ECM internally switches voltage scales after the 120 º F temperature mark is reached resulting in the dual charted voltage curve.

Important! The above voltages are fairly generalized. Variations will be found among different manufacturers, but the general operation is the same.

The ECT sensor comes as either a 2 or 3-wire type. The vast majority of ECT sensors are of the 2-wire type. These are connected on one side to an ECM provided sensor ground and the other side to a reference line (usually 5.00 volts), also from the ECM through the dropping resistor inside it. As temperature goes up the resistance of the ECT (thermistor) goes down and so does the voltage drop across it. The resistor in series with the ECT inside the ECM is provided for current limiting and as a voltage divider. This way in the event of an ECT wiring short the ECM would not get damaged.

This is why on many occasions, it is possible to simply jump the two ECT terminals to determine cooling fan operation or injector control reaction. The 3-wire type ECT sensor works the same way as the 2-wire, it just has an extra thermistor inside to provide a signal to the temperature gauge or separate ignition module. In essence the 3 wire ECT sensor has the ECM coolant sensors and the gauge temperature-sending unit built inside the same sensor casing. A wide user of the dual ECT sensor is Mercedes Benz and some other Euro manufacturers, whereby the second ECT sends a signal to the ignition module as well. With this systems, the dual thermistor ECT sensor feeds both the ECM and the ICM or the ignition control module. Most early European systems had separate fuel and ignition modules.

There are some older systems of the early 1980’s that used a dual thermistor ECT sensor, with only two signal wires coming out of it. This rare type of ECT sensor had the two thermistors tied together. The two connector prongs were connected directly to each side of the thermistors, while the center thermistor-to-thermistor connection was tied to ground, though the sensor’s body casing. This ECT sensor was widely used by some early European systems, like Renault and Peugeot. The dual thermistor ECT sensor should not be confused with the ECM’s internal dual pull-up resistor systems, having dual scales. The dual thermistor ECT sensor is simply two ECT sensors combined into one casing.

The ECM uses the ECT input to determine various component operation and engine control modes. It uses the ECT signal input to make calculations for the following.

• Fuel delivery or injector pulse-width.

• Cold start enrichment mode.

• Cooling fan operation.

• Determine open and close loop initiation.

• Idle control (IAC) operation.

Ignition timing corrections.

The ECT is also used by the ECM to control the operation of the following components.

• Delay EGR, TCC and canister purge operation on a cold engine.

• Knock corrections depending on engine temperature.

• Some systems use the ECT input to control transmission shifts points and quality.

Conditions that Affect Operation

Any condition that changes the resistance of the ECT circuit other than a temperature change is bound to have an adverse effect on vehicle performance. High CO levels, engine flooding, faulty cooling fan operation, etc are a result of a defective ECT sensor or circuit. An inoperative cooling system (stuck thermostat, inoperative cooling fans, etc) will also change the resistance of the thermistor inside the ECT sensor causing an engine performance problem.

[There are two possible scenarios as far as the ECT signal is concerned. A fault leading to excessive resistance or a lack of resistance in the circuit.] Extreme corrosion at the ECT connector will lead to excessive resistance in the circuit. This condition will raise the ECT signal voltage leading the ECM into calculating that the engine is cooler than it is. A cold engine will make the ECM increase injector pulse-width, therefore, creating excessive CO at the tail pipe. In extreme high resistance conditions, engine flooding can occur, since the ECM reacts as if the engine temperature is at subzero temperatures. If on the other hand, a lack (shorted) of resistance is present at the ECT circuit, the opposite of the previous will hold true. Lower than normal ECT circuit resistance will lower the voltage signal across the sensor tricking the ECM to act if the temperature is higher than what it really is, which in turn makes the ECM reduce injector open time or pulse-width. Such a condition can cause a lean misfire or even a no start on a cold engine, due to the fuel starvation effect caused by the ECM’s reduction of injector pulse width.

NOTE: Be aware of the fact that some computer systems, depending on the manufacturer, will substitute an erroneous ECT signal with an acceptable value on the scan tool. This is done so as to prevent the engine from stalling, allowing the driver to reach a repair shop. Always be mindful of the fact that what you see on a scan tool might be a substituted value and not the real thing. It should be standard procedure in all cases to back your diagnostic scan readings with an actual multi-meter or a hands-on measurement to double check your work.

In the event of an ECT sensor fault, the ECM usually looks at the IAT (Intake Temperature Sensor) as an indicator for engine temperature. Also keep in mind that a low coolant condition will signal the wrong reading to the ECM. This can result in unnecessary ECT replacement. As a final note on conditions that affect the ECT sensor, it is worth mentioning a curious phenomenon. The ECT is also affected by the general electrolytic conditions of the coolant fluid. There are cases, due to lack of maintenance, where the actual coolant becomes slightly acidic. This turns the engine and coolant into a sort of battery, thereby, totally skewing the ECT sensor’s reading. The slight acid content of the coolant will produce a small voltage, which can interfere with the sensor signal. This is partly the reason for the use of plastic ECT sensors by some manufacturers. The same acidic coolant can also create a thin film around the ECT’s casing, preventing it from providing an accurate temperature reading.

Component Testing

Make sure that the cooling system is working properly. All cooling fans should operate as they should and the thermostat should not be faulty.

• The first thing to check for when the ECT is suspected is the actual coolant level. A low coolant level condition will make the ECT reading faulty, since there is no contact between the coolant and the sensor’s thermistor.

• Make an overall inspection of the coolant condition. Verify that there is no voltage present at the radiator. Using a volt-meter, dip one probe into the coolant and the other to the engine body. No voltage should be present.

• Perform a visual inspection of the sensor and its connector. Check for sulfated or corroded connector pins that could cause a high resistance in the circuit.

• Perform a ground voltage drop to determine any faulty or loss of ground. Turn the key ON and simply connect the negative lead of the multimeter to battery negative and the positive to the ECT connector ground wire. With KOEO no more than 100 m Volts drop should be present and with KOEC no more than a sustained 300 m Volts voltage drop should be present.

Verify the 5 volt reference line. Most systems are 5 volts. If in doubt check specifications.

Connect a scan tool and monitor the ECT PID. At the same time connect a multimeter to each of the sensor’s 2 wires (if it’s a 3-wire sensor make sure you are probing on the sensor ground and the ECM’s ECT signal wire). Get as close as possible to the ECM main connector so as to detect any wiring problem. Verify that the scanner ECT voltage PID reading is the same as the multimeter. This will rule out if the ECM is substituting the ECT value due to a problem or a possible ECM sensor ground fault that is skewing the ECT signal.

NOTE: In some faulty systems, the vehicle’s ECM will very briefly display the actual ECT sensor value on the scanner when the key is first turned on (a second or so). While turning the key on pay particular attention to the scan ECT reading.

While monitoring both the ECT sensor PID scan reading and the multimeter voltage signal, disconnect the ECT connector and look for a reference voltage (most systems are 5 volt reference). When the connector is disconnected this will in effect create an infinite resistance across the ECT sensor circuit. An infinite (high) resistance translates into a very cold engine reading. Also wiggle the wires to uncover any intermittent wiring problems (short).

Important! Do not expect an injector pulse width lengthening if the system is substituting the ECT value. This is precisely why the system substitutes a value so that the engine is affected as little as possible and allow the driver to reach a repair shop.

• Again while monitoring both the ECT scan reading and the multimeter voltage signal connect a jumper wire across the ECT connector and verify 0.00 volts (low resistance). This signals the ECM that engine is extremely hot. If both of these diagnostic steps pass the test, then it’s an indication that the ECT wiring is good. Again also wiggle the wires to uncover any intermittent wiring problems.

• Disconnect the multimeter. While observing the scanner for a temperature change also take an infrared reading using an infrared gun. Compare both readings to detect if the ECT sensor is biased either high or low. Certain coolant additives and leak stops will coat the sensor with a film that prevents an accurate temperature reading. Although this is not an electrical problem per-se, it will cause an erroneous reading. This fools the ECM and skew the air fuel mixture.

This final diagnostic step is intended to detect biased or shifted readings, which will not set off a faulty code. This logical ECT sensor diagnostic routine will hopefully lead you in the right direction.


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