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Diagnose Sensor Prob.

The coolant sensor is often called the "master" sensor because the powertrain control module (PCM) uses the sensor's input along with that from the oxygen sensor to go into the "closed loop" mode of controlling the fuel mixture. It also uses input from the coolant sensor to regulate the operation of many other important functions, including:
* Start up fuel enrichment on fuel injected engines. Injector dwell is increased to create a richer fuel mixture when the coolant sensor indicates a cold engine.
* Spark advance and retard. Spark advance is often limited for emission purposes until the engine reaches normal operating temperature.
* EGR flow is blocked while the engine is cold to improve cold driveability.
* Canister purge does not occur until the engine is warm to improve cold driveability.
* Energizing the electric heater grid under the carburetor on older engines to improve early fuel evaporation when the engine is cold.
* Operation of the throttle kicker or idle speed when the engine is cold.
* Transmission torque converter clutch lockup when the engine is cold.
* Operation of the electric cooling fan (if a separate fan thermostat isn't used) when a certain temperature is reached.

Located on the cylinder head or intake manifold where it screws into the water jacket, the coolant sensor may be one of two basic types. Most are variable resistor sensors called "thermistors" because their electrical resistance changes with temperature. Most are the "negative temperature coefficient" type which means the sensor's resistance decreases as the temperature goes up.

The other type of coolant sensor is an on/off switch that works like a conventional temperature sending unit or thermostat by closing or opening at a preset temperature.

The variable resistor type of coolant sensors are "smarter" than the on/off switches because they provide the PCM with an more accurate indication of actual engine temperature. The PCM feeds the sensor a fixed reference voltage (VRef) of usually 5 volts when the ignition is on. The resistance in the sensor is high when cold and drops as the sensor warms up to alter the return voltage signal back to the PCM, which the PCM uses to determine engine temperature.

The switch type sensor may be designed to remain closed within a certain temperature range (say between 55 and 235 degrees F., for example) or to open only when the engine is warm (above 125 degrees F.). Switch type coolant sensors are found on older GM "T" car minimum function systems, Ford MCU and Chrysler Lean Burn systems.

Because of the coolant sensor's central role in triggering so many engine functions, a faulty coolant sensor (or sensor circuit) can cause a variety of cold performance problems as well as emission failures.

The most common symptom that indicates a bad coolant sensor is an engine control system that fails to go into closed loop once the engine is warm.

Other symptoms that might be caused by a bad coolant sensor include:
* Poor cold idle (due to no EFE, heated air or rich fuel mixture).
* Stalling (rich mixture, retarded timing, slow idle speed).
* Cold hesitation or stumble (no EFE or EGR occurring too soon).
* Poor fuel mileage (rich mixture, open loop, spark retarded).
* Hard starting when hot (coolant sensor reads cold all the time)

Keep in mind that coolant sensor problems are more often due to wiring faults and loose or corroded connectors than failure of the sensor itself. Correct operation of the sensor can also be upset by installing the wrong temperature range thermostat (too cold for the application), or by incomplete filling of the cooling system or coolant loss. To give an accurate reading, the sensor element must be in direct contact with liquid coolant.

One way to test the coolant sensor is with a "sweep test:"
1. Start with a cold engine. With the ignition off, disconnect the wiring connector from the coolant sensor.
2. Attach an ohmmeter across the sensor's terminals.
3. Measure the sensor's resistance and record the reading.
4. Reconnect the sensor's wiring connector.
5. Start and run the engine for two minutes and then shut the engine off.
6. Disconnect the sensor's wiring harness again, and take an ohmmeter reading across the sensor's terminals.
7. Compare the two readings. There should be a difference of at least 200 ohms. If not, the sensor is defective or suffers from a buildup of cooling system sludge which makes it less sensitive to changes in engine temperature. Remove and inspect the sensor, clean if necessary and retest.

You can also measure the sensor's resistance at various temperatures, and compare the readings against the resistance values specified by the vehicle manufacturer. See the sample specifications for GM and Ford coolant sensors. Another way to check the sensor's operation is measure the sensor's reference voltage and return voltage signal as the engine warms up with a voltmeter. If you have access to a digital storage oscilloscope (DSO), you can also observe the sensor's waveform (see chart).

The reference voltage to the sensor from the PCM (VRef) should be about 5-volts on most applications. The return voltage signal should be around 3 to 4 volts when the engine is cold, and gradually drop to 2 volts or less as the engine reaches normal operating temperature.
No change in the return signal would indicate a faulty sensor.
No return voltage reading would indicate an open sensor.
No VRef reading would indicate a wiring fault.
Note: Some 1985 and newer Chrysler coolant sensors switch a 1000 ohm resistor into the circuit when the coolant temperature reaches about 125 degrees F, causing a sudden rise or step up in the return voltage signal before it continues to drop.

If a vehicle provides live sensor data through its diagnostic connector, you can also read the coolant sensor's output directly with a scan tool (usually in degrees Centigrade or Fahrenheit).

Trouble codes that indicate a problem in coolant sensor circuit:
* General Motors: Code 14 (shorted) and 15 (open)
* Ford: Codes 21, 51 & 61
* Chrysler: Code 17 & 22
* OBD II applications: P0117, P0118, P0125, P1114, P1115 & P1620

The coolant sensor can also be tested directly. By connecting a DVOM to the sensor's terminals, you can check the sensor's resistance when the engine is cold, and then again when it is at normal temperature. If the resistance reading doesn't change or is outside the range specified by the manufacturer, the sensor needs to be replaced.

You can also substitute a simulated sensor signal to find out if a problem is in the sensor itself or elsewhere in the engine control system. Unplug the coolant sensor from its wiring connector and attach the leads from a sensor simulator tool to the connector. Then dial in a fixed resistance and see what happens. If changing from high resistance (to simulate a cold engine) to low resistance (a warm engine) fails to put the engine control system into closed loop, the problem isn't in the sensor but is somewhere in the wiring or computer system.

To replace a bad coolant sensor, you usually have to drain some coolant from the cooling system. When you install the new sensor, coat the threads lightly with sealer to prevent coolant leaks. Do not overtighten the sensor. Refill the cooling system and make sure there are no air pockets trapped inside the engine. Turn the heater on as you're adding coolant to the system, and open any bleeder screws that are provided to vent air as the coolant level rises. Then recheck the level and add coolant as needed after the engine has been driven and allowed to cool down.

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