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DIAGNOSING PROBLEMS RELATED TO THE COOLANT TEMPERATURE SENSOR
by Mark Davidson

The Engine Coolant Temperature Sensor (“ECT” for short) is an analog sensor which, as it’s name says, measures the temperature of the coolant and feeds this data to the on-board computer. The computer then uses this data to maintain optimum driveability especially while the engine is warming up and until it reaches operating temperature. In the days before electronic engine control modules, the choke on the carburetor served this purpose, ie. to cause a rich fuel mixture at least until the engine was warmed up. Frequently the engine had to idle at high rpm to keep it running when cold. Often it would stall before it got warm.

Coolant SensorEarly model OBD vehicles used a very simple circuit involving only a temperature switch (Fig 1). This switch was a heat sensitive (thermal) switch mounted in the coolant fluid to monitor its temperature. So long as the temperature of the engine coolant was below the engine operating temperature, this switch would stay in an open position. The 14 volts on PCM Pin D (14 volts which came from the PCM) communicated to the PCM that the engine hadn’t achieved spec operating temperature.

During a cold engine test, the PCM would retard the spark timing and run the engine’s fuel mixture slightly richer to help compensate for cold conditions which are so characteristic of any typical gasoline engine. The PCM constantly monitors Pin D so as to determine when the engine reaches operating temperature (Fig. 2).ECT When the engine reaches it’s operating temperature, the switch responds by closing the contacts & grounding Pin D. The voltage on Pin D drops to near ground voltage (almost 0.0 volts). The 14 volts supplied by the PCM gets dropped inside the PCM across the load resistor. As soon as the PCM reaches 0.0 volts, it immediately starts to change it’s fuel delivery by leaning the air/fuel mixture and advancing sparkplug timing so as to be able to handle a warming engine.

The PCM hence “thinks” the engine is stone cold even when it’s almost warm. But if the switch stays closed, the PCM “thinks” the engine is warm (even when the engine is just mildly warm). Specific control of air/fuel mixture and spark plug timing isn’t possible with a coolant temperature switch alone.

Later model engine coolant temperature sensors used something called a thermistor… a device which provides higher control over engine performance throughout the range of engine operations. So then, a thermistor is a variable resistor made of solid state materials that will change resistance according to temperature. The type of thermistor which is used in automotive applications has a negative temperature coefficient; this means resistance decreases as temperature increases.

Connected to Pin D inside the PCM is a 350 ohm load resistor (R1) connected to a 5 volt reference circuit. The resistor R1 and the ECT make a voltage divider of the 5 volt supply reference. Since R1 is fixed at 350 Ohms, the voltage at Pin D depends on the value of the ECT. So then, when the engine is cold, the ECT resistance is naturally very high. Some vehicles will use a ECT which has resistance of 100,000 ohms and yet others may be 50,000 ohms.

When the engine is cold, the ECT will have the majority of the resistance in the circuit and thus consumes most of the voltage. The resistor “R1″, only 350 ohms, drops only a slight voltage amount in comparison to the ECT. This makes the voltage on Pin D nearly 5 volts when the engine is cold. We can suppose that the ECT is 100,000 ohms (when cold). As the engine warm ups, the resistance of the ECT will begin to decrease. In the initial stage of engine warm-up, the ECT could be 75,000 ohms but “R1″ is still 350 ohms. Thus the voltage of Pin D is still close to 5 volts. For the voltage of Pin D to drop to 2.5 volts will require the engine to get almost up to spec operating temperature so the ECT is also 350 ohms. Then “R1″, at 350 ohms, will drop half the voltage, and the ECT, at 350 Ohms, will drop the other half which makes the voltage on Pin D at 2.5 volts.

HOW TO TEST THE ECT CIRCUIT
This is a rather simple procedure — watch the ECT voltage as the engine begins to warm up and then check the reading once the engine is at spec operating temperature. You should expect a reading from 0.8 – 1.2 volts for most vehicles. Always check a known good vehicle if possible to establish what a good ECT voltage (at spec operating temperature) is if the car manufacturer doesn’t supply such information. If the reading is higher than spec, the PCM will “think” the engine isn’t as warm as it actually is and thus enrich the fuel mixture and retard the engine timing which will make for warm driveability problems. In this case, you can expect higher emissions and severely reduced fuel economy.

A higher than spec reading will also be caused by poor ground at PCM, which the ECT is dependent on for proper operation. If the PCM ground is acceptable, you should check sensor resistance with a digital ohmmeter. Many manufacturers will provide a table of ECT resistance for a certain temperature. But if the reading is lower than spec, the PCM will “think” the engine is actually hotter than it really is and enlean the air/fuel mixture and also advance timing, which makes for cold driveability problems. In this case, expect difficulty in cold starting (of course). A lower than spec reading may also be caused by a defective ECT, or, the ECT signal wire to Pin D could be shorted to ground.


(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)

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2 Responses to “Coolant Temperature Sensor”

  1. Norris Fait says:

    Hi

    I have a 2004 toyota carmery that runs like it should. Until it reaches operating temperature, then the car slows down to a top speed of 18 miles per hour. Is this the etc or is it perhaps another problem?

  2. matt parker says:

    having the same problem with a kia sendona diesel. been advised its the temp sensor. if any one has come across this please contact me

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