Monday, May 23, 2011

How to replace Heated Oxygen Sensor on 2000 jeep grand Cherokee?

Heated Oxygen Sensor

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Operation



The Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Depending on the emission package, the vehicle may use a total of either 2 or 4 sensors.

An O2 sensor is a galvanic battery that provides the PCM with a voltage signal (0-1 volt) inversely proportional to the amount of oxygen in the exhaust. In other words, if the oxygen content is low, the voltage output is high; if the oxygen content is high the output voltage is low. The PCM uses this information to adjust injector pulse-width to achieve the 14.7to1 air/fuel ratio necessary for proper engine operation and to control emissions.

The O2 sensor must have a source of oxygen from outside of the exhaust stream for comparison. Current O2 sensors receive their fresh oxygen (outside air) supply through the O2 sensor case housing.

Four wires (circuits) are used on each O2 sensor: a 12volt feed circuit for the sensor heating element; a ground circuit for the heater element; a low-noise sensor return circuit to the PCM, and an input circuit from the sensor back to the PCM to detect sensor operation.

4.0L 6cylinder With California Emissions Package

On this emissions package, 4 sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2). With this emission package, the rear/upper upstream sensor (2/1) is located in the exhaust downpipe just before the rear mini-catalytic converter. The front/upper upstream sensor (1/1) is located in the exhaust downpipe just before the front mini-catalytic converter. The rear/lower downstream sensor (2/2) is located in the exhaust downpipe just after the rear mini-catalytic converter, and before the main catalytic converter. The front/lower downstream sensor (1/2) is located in the exhaust downpipe just after the front mini-catalytic converter, and before the main catalytic converter.

4.7L V-8 With California Emissions Package

On this emissions package, 4 sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2). With this emission package, the right upstream sensor (2/1) is located in the right exhaust downpipe just before the mini-catalytic converter. The left upstream sensor (1/1) is located in the left exhaust downpipe just before the mini-catalytic converter. The right downstream sensor (2/2) is located in the right exhaust downpipe just after the mini-catalytic converter, and before the main catalytic converter. The left downstream sensor (1/2) is located in the left exhaust downpipe just after the mini-catalytic converter, and before the main catalytic converter.

Downstream Sensor (Non-California Emissions)

The downstream oxygen sensor (1/2) is also used to determine the correct air-fuel ratio. As the oxygen content changes at the downstream sensor, the PCM calculates how much air-fuel ratio change is required. The PCM then looks at the upstream oxygen sensor voltage and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content). The downstream oxygen sensor also provides an input to determine catalytic converter efficiency.

Downstream Sensors (California Engines)

Two downstream sensors are used (1/2 and 2/2). The downstream sensors are used to determine the correct air-fuel ratio. As the oxygen content changes at the downstream sensor, the PCM calculates how much air-fuel ratio change is required. The PCM then looks at the upstream oxygen sensor voltage, and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content).

The downstream oxygen sensors also provide an input to determine mini-catalyst efficiency. Main catalytic converter efficiency is not calculated with this package.

Engines equipped with either a downstream sensor(s), or a post-catalytic sensor, will monitor catalytic converter efficiency. If efficiency is below emission standards, the Malfunction Indicator Lamp (MIL) will be illuminated and a Diagnostic Trouble Code (DTC) will be set.

Federal Emissions Package

Two sensors are used: upstream (referred to as 1/1) and downstream (referred to as 1/2). With this emission package, the upstream sensor (1/1) is located just before the main catalytic converter. The downstream sensor (1/2) is located just after the main catalytic converter.

Oxygen Sensor Heaters/Heater Relays

Depending on the emissions package, the heating elements within the sensors will be supplied voltage from either the ASD relay, or 2 separate oxygen sensor relays.

The O2 sensor uses a Positive Thermal Co-efficient (PTC) heater element. As temperature increases, resistance increases. At ambient temperatures around 70F, the resistance of the heating element is approximately 4.5 ohms on 4.0L engines. It is approximately 13.5 ohms on the 4.7L engine. As the sensors temperature increases, resistance in the heater element increases. This allows the heater to maintain the optimum operating temperature of approximately 930-1100F (500-600 C). Although the sensors operate the same, there are physical differences, due to the environment that they operate in, that keep them from being interchangeable.

Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.

In Closed Loop operation, the PCM monitors certain O2 sensor input(s) along with other inputs, and adjusts the injector pulse width accordingly. During Open Loop operation, the PCM ignores the O2 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

Upstream Sensor (Non-California Emissions)

The upstream sensor (1/1) provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensor. The PCM will change the air/fuel ratio until the upstream sensor inputs a voltage that the PCM has determined will make the downstream sensor output (oxygen content) correct. The upstream oxygen sensor also provides an input to determine catalytic converter efficiency.

Upstream Sensors (California Engines)

Two upstream sensors are used (1/1 and 2/1). The 1/1 sensor is the first sensor to receive exhaust gases from the #1 cylinder. They provide an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensors. The PCM will change the air/fuel ratio until the upstream sensors input a voltage that the PCM has determined will make the downstream sensors output (oxygen content) correct. The upstream oxygen sensors also provide an input to determine mini-catalyst efficiency. Main catalytic converter efficiency is not calculated with this package.

Removal & Installation




WARNING
Never apply any type of grease to the oxygen sensor electrical connector, or attempt any soldering of the sensor wiring harness.


CAUTION
The exhaust manifold, exhaust pipes and catalytic converter(s) become very hot during engine operation. Allow engine to cool before removing oxygen sensor.

  1. Raise and support vehicle.
  2. Disconnect O2S pigtail harness from main wiring harness.
  3. If equipped, disconnect sensor wire harness mounting clips from engine or body.
    WARNING
    When disconnecting sensor electrical connector, do not pull directly on wire going into sensor.

  4. Remove O2S sensor with an oxygen sensor.

To Install:


NOTE
Threads of new oxygen sensors are factory coated with anti-seize compound to aid in installation.


WARNING
DO NOT add any additional anti-seize compound to threads of a new oxygen sensor.

  1. Install O2S. Tighten to 30 Nm (22 ft. lbs.) torque.
  2. Connect O2S wire connector to main wiring harness.
  3. If equipped, connect sensor wire harness mounting clips to engine or body. When equipped, the O2S pigtail harness must be clipped and/or bolted back to their original positions on engine or body to prevent mechanical damage to wiring.
  4. Lower vehicle.

    Click image to see an enlarged view

    Fig.



    Click image to see an enlarged view

    Fig.

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