Tip & How-To about Mercedes-Benz E-Class
2.2) ECT - Engine Coolant Temperature (sensor)
What is it? This is small electrical device
for measuring the coolant temperature in the engine
Where is it located? It is usually located
on the engine near to the thermostat housing.
The ECT is sited on the 'hot' side of the thermostat so that it senses
the coolant/engine temperature before the thermostat opens and allows coolant
to flow through the radiator.
How does it work? Modern temperature
sensors consist of a thermistor in a sealed unit. As the temperature rises the electrical resistance
varies proportionately; some thermistors increase their resistance with
temperature (PTC - positive temperature correlation) whilst others decrease
their resistance (NTC - negative temperature correlation). When the engine is cold at start up the
coolant sensor sends an appropriate signal to the ECU. The ECU responds by increasing the length of
the injection cycles to enrich the combustion mix. This is an electronic equivalent of pulling
the 'choke' out on a carburetor. As the
engine warms up the signals from the coolant sensor cause the ECU to shorten
the injection cycles making the fuel mix progressively leaner. The process of coolant sensor and ECU
interaction explains why engines have a slightly faster idle when starting cold
than when running hot.
Symptoms of faulty coolant sensor
Associated OBD2 error codes DTCs: P0115 - P119; P0125, P0126, P0128
How to check? Most often the coolant sensor is quite
separate to the temperature sender, so a correct read-out on the dash
board does not necessarily indicate correct sensor function. Using a
voltmeter the resistance across the electrical terminals on the sensor can be
measured. By removing the device from
the car and putting the end of the sensor in a pan of hot water it should be
possible to see an immediate change in resistance, it does not matter so much
that the resistance goes up or down but that there is a discernable change with
change in temperature. Generally high
resistance equates to cold temperatures and vice versa. If there is no resistance
change commensurate with temperature change then the sensor is at fault. If there is simply no resistance measurable
(open circuit) then the sensor is at fault. If the sensor is working correctly
check the connector, the wiring and the wiring insulation for faults and
possible shorting.
How to fix? Replace if found faulty
NEXT 3.1) CKP - Crankshaft position sensor
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