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
• Cooling fan
open and close loop initiation.
• Idle control
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.
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.
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.
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
• 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
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
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.
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.