Sensors

(O2S)Oxygen Sensor

The exhaust gas oxygen sensor, is the key sensor in the engine control feedback loop. The computer uses the O2 sensors input to balance the fuel mixture, leaning it when the sensor reads rich and enrichening it when the sensor reads lean. The O2 sensor produces a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. An oxygen sensor is essentially a galvanic battery that produces its own voltage. When hot (atleast 600 degrees F.), the zirconium dioxide element in the sensor's tip produces a voltage that varies according to the amount of oxygen in the exhaust compared to the ambient oxygen level in the outside air.The higher the exhaust oxygen content, the lower the oxygen differential across the sensor tip, compared to the outside ambient oxygen.
The voltage output ranges from 0.1 volts (LEAN) to 0.9 volts(RICH). A perfectly balanced or "stoichiometric" fuel mixture of 14.7:1 will give a reading of around 0.5 volts. Some O2 sensors have three wires and an internal heating element to help the sensor reach operating temperature more quickly. the heater also keeps the sensor from cooling off when the engine is idling. A good oxygen sensor should produce a fluctuating signal voltage that changes in the oxygen level in the exhaust.

• O2 Sensor Checks To check a zirconium O2 sensor, you will need a 10k-ohm impedance digital voltmeter. Back probe the O2 sensor signal wire (refer to manufacturers wiring diagram) without piercing the wire's insulation. Connect the red test lead to the signal wire, and the black test lead to ground. With the engine idling , if the O2 sensor and computer system are working properly, the sensors voltage should be cycling from low voltage to high voltage, between 0.1-1.0 volts. You should never see above 1.0 volts.
  To check the sensor's response to changing oxygen levels in the exhaust, first create an artificially lean condition by pulling off a large vacuum line. When extra air is introduced into the engine, the sensor's voltage output should drop below 0.5 volts. On carbureted engines, the sensor's rich response can be checked by creating an artificially rich mixture.Push the choke almost shut while the engine is idling. This should cause the O2 sensor's voltage to increase above 0.5 volts. If the sensor's output voltage fails to change in response to changes you've created in the level of oxygen in the exhaust, then the sensor needs to be replaced.
  If the voltage is continually high, the air/fuel ratio may be rich or the sensor may be contaminated. The 02 sensor can get contaminated with Room Temperature Vulcanizing(RTV) sealant, antifreeze , or leaded gasoline . When the O2 sensor voltage is continually low, the air/fuel ratio may be lean, the sensor may be defective, or the wire between the sensor and the computer may have a high resistance problem caused by corroded terminals or loose connection. If the O2 sensor voltage signal remains in the mid-range position, the computer may be in open loop or the sensor may be defective.
  When the O2 sensor is removed from the engine, a digital voltmeter may be connected from the signal wire with the red test lead, and to the sensor's case with the black test lead. Then heat the tip of the sensor with the propane torch. The tip should be hot enough to turn cherry red, and the flame must enter the opening into the sensor tip. While the sensor element is in the flame, the voltage should be nearly 1.0 volt. The voltage should drop to zero immediately when the flame is removed from the sensor. If the sensor does not produce the specified voltage, it should be replaced. Also a low reading or one that is slow to change means the sensor needs to be replaced.
• O2 Sensor Heater Check If the O2 sensor heater is not working, the sensor warm-up time is extended, and the computer stays in open loop longer. In this mode, the computer supplies a richer air/fuel ratio, and the fuel economy is reduced. Disconnect the O2 sensor connector and connect the voltmeter red test lead to the heater supply wire toward the computer, and the black test lead to ground. With the ignition switch on, 12 volts should be supplied on this wire. If the voltage is less than 12v, repair the fuse in this voltage supply wire, or repair the wire itself. With the O2 sensor wires disconnected, connect an ohmmeter across the heater terminals in the sensor connector on the sensor side. If the meter reads infinite or does not have the specified resistance, replace the O2 sensor.
  
• Driveability Symptoms The O2 sensor's normal life span is 30,000 to 50,000 miles. But the sensor may fail prematurely if it becomes clogged with carbon, or is contaminated by leaded gasoline or solvents from the wrong RTV silicone sealer.
  As the sensor ages, it becomes sluggish and slow. Eventually it produces an unchanging signal or no signal at all. When this happens, the engine will experiance driveability problems. It may lack power, idle rough, deliver poor fuel mileage, or flunk an emissions test. Sometimes an apparent O2 sensor problem is not really the sensors fault. An air leak in the intake or exhaust manifold or even a fouled spark plug, for example, will cause the O2 sensor to give a false lean reading. The sensor reacts only to the presence or absence of oxygen in the exhaust. It has no way of knowing where the extra oxygen came from. So keep that in mind when troubleshooting O2 sensor problems.

(ECT)Engine Coolant Sensor

The coolant sensor is usually called the "Master" sensor or main sensor, because the computer uses its input to regulate many other functions, which include:

• Activating and deactivating the Early Fuel Evaporation(EFE) system such as the electric heating grid under the carburetor or the thermactor air system.
• Open/Closed loop feedback control of the air/fuel mixture. The system won't go into closed loop until the engine is warm.
• Start-up fuel enrichment on fuel injected engines, which the computer varies according to whether the engine is starting up warm or cold.
• Spark advance and retard. Spark advance is normally limited until the engine reaches normal operating temperatures.
• EGR flow, which is blocked while the engine is cold to improve driveability.
• Canister purge, which doesen't occur until the engine is warm.
• Throttle kicker or idle speed.
• Transmission torque converter clutch lockup(TCC).

The coolant sensor is usually located on the head or intake manifold, where it screws into the water jacket. The sensors come in two basic types: Variable resister sensors called "Thermistors", becauase their electrical resistance changes with temperature; and theres simple on/off switch type sensors, which work like a conventional temperature sending unit or electric cooling fan thermostat by closing and opening a circuit at a preset temperature.
The variable resister sensors are "smarter" than the on/off switch type because they provide the computer with a more accurate indication of actual engine temperature. The computer feeds the sensor a fixed reference voltage of about 5 volts when the key is on and engine off. The resistance in the sensor is high when cold, and drops about 300ohms for every degree of fahrenheit as the sensor warms up. This alters the return voltage signal that goes back to the computer, which the computer then reads to determine engine temperatureThe switch type sensor may be desighned to remain closed within a certain temperature range (say between 55 and 235 degrees F.).

• Coolant Sensor Checks
  You can check and compare the sensor's electrical resistance with a ohmmeter and its voltage reading with a digital voltage meter(most read from 2.0 to 4.4 volts). Testing the coolant sensor consists of measuring its electrical resistance with a DVOM (Digital Volt Meter) at low and high temperatures.
  To check a variable resistance coolant sensor the engine should be cold, preferably the first warm-up of the day. With the key off disconnect the coolant temp sensor and check the resistance of the sensor while cold, refer to the vehicle's manufacturer specifications(all sensors vary by manufacturer). Reconnect the sensor and turn the key to the on position with the engine off, backprobe the connector to compare the voltage reading to the resistance, compare readings to manufacturer's specifications. Run the engine and allow it to reach operating temperature, backprobe the sensor and compare the voltage reading to manufacturer's specification. Turn the key to the off position and disconnect the sensor to check its resistance at operating temperature, compare the resistance to the voltage reading at operating temperature to manufacturer's specifications. If the sensor does not have the specified resistance, replace the sensor.
  To check a switch-type, check the resistance with the engine cold, normaly should show no continuity.With the engine warm at the specified temperature should show continuity. If the sensor does not have the specified temperature range, replace the sensor.
  

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