In a previous article we discussed troubleshooting the Throttle Position Sensor and the important role it plays in driveability. Another analog sensor of primary importance is the Engine Coolant Temperature Sensor, or ECT for short. In the older engine diagnostic terminology before OBD II, most manufacturers called it the Coolant Temperature Sensor, or CTS.

The ECT tells the computer what the engine temperature is so that optimum driveability is realized while the engine is warming up and when the engine has reached operating temperature. In "B.C." days (Before Computers), cold engine performance was under the control of an automatic choke whose adjustment was critical. Even when properly adjusted, the engine had to warm up before the car would perform properly. Often the engine had to be run at high rpm at idle just to keep it running when very cold. Then the engine would stumble and hesitate until it got warm.

Once the engine reached operating temperature, all we worried about was overheating. Who would have thought then that computers would someday control engine performance through all stages of engine temperature, maintaining good fuel economy, low emissions and smooth performance?

Earliest computer systems used a very simple circuit involving a temperature switch. The circuit is shown in Figure 1.

The switch was a thermal-sensitive switch mounted in the coolant system to monitor coolant temperature. As long as the engine coolant was below engine operating temperature, the switch remained OPEN. The 14 volts on PCM Pin D (14V which came from the PCM) told the PCM that the engine had not reached operating temperature.

During cold engine run, the PCM would retard spark timing and run the engine's fuel mixture a little richer to help compensate for cold driveability conditions so characteristic of a typical gasoline engine.

The PCM continuously monitors Pin D to determine when the engine reaches operating temperature, as shown in Figure 2.

As soon as the engine does reach operating temperature, the switch responds by closing its contacts and grounding Pin D. The voltage on Pin D drops to ground voltage or almost 0.0 volt. The 14 volts supplied by the PCM is dropped inside the PCM across a load resistor so no harm is done to the PCM. As soon as the PCM sees 0.0 volt on Pin D, it immediately begins to modify fuel delivery by enleaning the fuel mixture and advancing spark timing to provide the proper conditions for handling a warm engine.

The circuit is very simple, but so was the computer's control over engine driveability by monitoring for switch contacts closing. There was no in-between. The PCM thinks the engine is cold, even when the engine is almost warm. If the switch is closed, the PCM thinks the engine is warm even if the engine is barely warm. Finite control of fuel mixture and spark timing isn't possible with a coolant switch.

The next generation of coolant temperature sensing employed a thermistor, which provides more control over engine performance throughout the entire range of engine operation. A thermistor is a variable resistance made of solid state material that changes resistance with temperature. The symbol for a thermistor is a resistor symbol with an arrow through it, as shown in Figure 3.

The thermistor used in vehicle applications has a negative temperature coefficient, which means resistance decreases as temperature increases.

The Thermistor Coolant Sensor circuit is shown in Figure 3, where the sensor is referred to as ECT. The ECT is usually grounded back through the PCM. If the ECT has only one wire going to it, that means the ECT is grounded where it mounts to the engine.

Inside the PCM and connected to Pin D is a 350 ohm load resistor, R1, connected to the 5 volt reference circuit. Resistor R1 and the ECT form a voltage divider of the 5 volt reference supply. Since R1 is a fixed 350 ohms, the voltage at Pin D depends on the value of the ECT. When the engine is cold, ECT resistance is very high. Some vehicles use an ECT that has a resistance of 100,000 ohms, while others may be 50,000 ohms.

When the engine is cold, the ECT has most of the resistance in the circuit and consumes most of the voltage. Resistor R1, only 350 ohms, drops only a little voltage in comparison to the ECT. This makes the voltage on Pin D almost 5 volts when the engine is cold. Let's suppose the ECT is 100,000 ohms when cold. As the engine begins to warm up, the resistance of the ECT begins to decrease. In the initial stages of engine warm-up, the ECT may be 75,000 ohms but R1 is still 350 ohms. The voltage on Pin D would still be close to 5 volts. For the voltage on Pin D to drop to 2.5 volts requires the engine to get almost up to operating temperature so that the ECT is also 350 ohms. Then R1, at 350 ohms, drops half the voltage, and the ECT, at 350 ohms, drops the other half making the voltage on Pin D 2.5 volts.

From here, the resistance of the ECT drops quickly until at 210 degrees Fahrenheit the resistance of the ECT may be only 70-80 ohms. The voltage on Pin D is then about 1.0 volt at operating temperature. As the ECT voltage begins to fall, the PCM is programmed to adjust fuel mixture and spark advance many times each second to respond to ECT signal voltage changes which are continuously varying downward. This provides finite control of engine performance during engine warm-up. That is why a car with a thermistor for an ECT can have excellent driveability characteristics when cold. During engine warm-up, there is no noticeable difference in engine performance even after the engine is at operating temperature. Should there be differences in driveability from cold to warm engine, it is a good idea to check the ECT signal voltage at Pin D.

Figure 4 shows how to test the ECT circuit.

It's simple - just watch the ECT voltage as the engine warms up and note the reading once the engine is at operating temperature. Expect a reading from 0.8-1.2 volts for most cars. Always check a known good vehicle if at all possible to establish what a good ECT voltage at operating temperature is if the vehicle manufacturer does not supply the information.

If the reading is higher than normal, the PCM will think the engine is not as warm as it really is and enrich the fuel mixture and retard timing which makes for warm driveability problems. Expect higher emissions and reduced fuel economy.

A higher-than-normal reading may also be caused by a poor ground at the PCM, which the ECT is depending on for proper operation. If the PCM ground is OK, check sensor resistance with a digital ohmmeter. Most manufacturers provide a table of ECT resistance for a given temperature.

If the reading is lower than normal, the PCM will think the engine is hotter than it really is and enlean the fuel mixture and advance timing, which makes for cold driveability problems.

Expect difficulty in cold starting. A lower than normal reading may also be caused by a defective ECT, or the ECT signal wire to Pin D might be shorted to ground.

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Vince Fischelli is president of Veejer Enterprises Inc. (www.veejer.com) in Garland, Texas.

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