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Re: Replacing O2 sensor in 99 Camry (AF sensors explained)

Nobody Important snipped-for-privacy@nospam.gmail.com> wrote:

You know, I've read this before, but it's not clear to me why this is so. Referring to:

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you see on page 20 the transfer function for a plain O2 sensor and on page 25 that of the fancy toyota sensor. These functions look the same to me, which implies that you could substitute a plain O2 sensor. What's wrong with this argument? (I'm not disputing you; I ask merely for information.)

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I see what your saying, but pages 20 through 25 explain why a substitution will not work. It's due to the detection circuit in the ECU, also the sonstuction of the sensor. An 'old style' sensor is a battery. It produces voltage based on the amount of oxygen in the exhaust stream. The more oxygen (lean)the less voltage, the less oxygen (rich) the more voltage. By cycling the injecor pulse width based on feedback from the primary sensor output voltage, you are left with what is an ideal stoichiometric average. Basically, the computer only monitoring a voltage source. The AF sensor has a fixed voltage output from the computer, and produces current instead to describe exhaust oxygen content. To be clear, here is text verbatim from Toyotas own course book; "Engine Control Systems II - Course 874, Section 7, O2 & A/F Sensor Diagnosis"...... "The A/F sensor is similar to the O2 sensor. It appears similar to the O2 sensor, but it is constructed differently and has different operating characteristics. "The advantage of using the A/F sensor is that the ECM can more accurately meter the fuel redicing emissions. "To accomplish this, the A/F sensor:

-operates at app. 650C degrees, much hotter that the O2 sensors which operate at 400C degrees

-changes its current (amperage) output in relation to the amount of oxygen in the exhaust stream.

--OPERATION--

"A detection circuit in the ECM detects the change and strength of current flow and outs out a voltage signal relatively proportional to exhaust oxygen content. "This voltage signal can only be measured using the Diagnostic Tester or OBDII scantool. The AF sensor current output cannot be accurately measured directly. If a Diagnostic tester or OBDII scantool is used, refer to the RM for conversion, as the output signal is different. "The AF sensor is designed so that at stoichiometry, there is no current flow and the voltage put out by the detection circuit is 3.3 volts. A rich mixture, which leaves very little oxygen in the exhaust stream, produces a negative current flow. The detection circuit will produce a voltage below 3.3 volts. A lean mixture, which has more oxygen in the exhaust stream, produces a positive current flow. The detection circuit will now produce a voltage signal above 3.3 volts. "The AF sensor voltage output is the opposite of what happens in the narrow range O2 sensor. Voltage output through the detection circuit increases as the mixture gets leaner. "The AF sensor voltage signal is proportional to the change in the air/fuel mixture. This allows the ECM to more accurately judge the exact AF ratio under a wide variety of conditions and quickly adjust the amount of fuel to the stoichiometric point. This type of rapid correction is not possible with the narrow range O2 sensor. With an AF sensor, the ECM does not follow a rich/lean cycle"

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How about that. For those of you with the intestinal fortitude to have made it this far, you should see now that the main difference in the system is the detection circuit in the ECM. Since there is no cycling to bo done in the air/fuel ratio's ECM, a regular galvanic battery-style O2 sensor would not work. On an interesting side note, there is a crazy wiring diagram in this course book of the detection circuit inside the ECM for an AF sensor. I asked the instructor of the course if he could go over it with me (because it sure as hell makes so sense, I think), and he said that he, as well as all the other instructors for Central Atlantic Toyota, were not really clear as to how it works. He told a story of a training conference put on by engineers in Japan for the Course Instrucors, and when one of fuel management engineers was asked to explain in detail exactly how the sensor and detection circuit work, every on was told "There are three people in Toyota who know how the system works. None of them are here today" I can scan it if someone has webspace to put it up on. And sorry for any spelling and grammatical errors, but I'm not going through this long ass text to edit.

Reply to
qslim
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A/F sensor, oxygen sensor are just ways people call a category of sensors for detecting oxygen content in the exhaust for the purpose of adjusting the fuel mixture. Bosch for one, call the five different types of sensors:

  1. unheated thimble,
  2. heated thimble,
  3. planar,
  4. wide-band and
  5. titania

Check it out:

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Q. Except for the connector assembly, aren't all oxygen sensors basically the same? A. NO. There are five fundamentally different types of oxygen sensors: unheated thimble, heated thimble, planar, wide-band and titania. Within each sensor type, sensors vary in the design of the ceramic element, heater element and protection tube design, all of which affect sensor operation.

Unheated Thimble: This was the first oxygen sensor design and Bosch has been producing them since 1976. These sensors either have one wire or two (for purposes of grounding), since they do not require power for a heater element. They use a zirconia ceramic element to generate a voltage in a rich (insufficient oxygen) gas environment. They can take several minutes to reach operating temperatures of 625 degrees Fahrenheit after a cold start, however this varies with application as the location of the sensor determines the rate at which it heats. Click here for a diagram of a thimble type oxygen sensor.

Heated Thimble: These sensors were introduced by Bosch in 1982 and function in the same manner as the unheated thimble, however inside the thimble is a special heating element that brings the sensor up to operating temperature in about 30-60 seconds. The additional wires to power the heater- typically a total of 3-4 wires (dependent on grounding scenario) can identify these sensors. Click here for a diagram of a thimble type oxygen sensor.

Planar: This sensor was introduced by Bosch in 1997 and also uses the zirconia ceramic sensing technology, but placed in a more compact thin flat shape (hence the name) instead of a thimble or conical shape. This allows the sensor to be smaller and lighter. They also require less electrical power as the heater is integrated into the smaller sensing element. By model year 2004, planar oxygen sensors are expected to account for over 30% of all new oxygen sensor applications and by 2007 for over 50% of new oxygen sensors. These sensors also have only four wires. Click here for a diagram of a planar oxygen sensor.

Wideband: This is the newest design of oxygen sensor and it offers the increased accuracy needed to meet the latest emissions requirements. Unlike all the other types of oxygen sensors, the wide-band sensor can actually measure the air/fuel ratio from 11:1 (excess fuel condition) all the way to straight air (no fuel)! (The ideal air/fuel ratio is

14.7/1.) This improved measurement allows the engine control system to measure the actual air/fuel ratio and eliminates the switching between lean and rich associated with a traditional type of oxygen sensor. These sensors use a planar zirconia ceramic element, so that they heat up much faster than other types of sensors- reducing cold start emissions. These sensors can be identified with having five or more wires. In addition, these sensors are used with the newly developed gasoline direct injection engines. Direct injection engines can use stratified charges, which produce a very lean mixture in the combustion chamber, and these sensors must be used because of their ability to measure from very lean to very rich accurately. Click here for a diagram of a wideband oxygen sensor.

Titania: These sensors use a different type of oxygen sensing technology and instead of generating a voltage signal that changes with the air/fuel ratio, the sensor's resistance changes. These sensors are used on less than 0.5% of all oxygen sensor equipped vehicles.

qslim wrote:

Reply to
johngdole

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I'm assuming but, it looks like the Bosch Titania O2 sensor is functioning the same as Toyota's A/F sensor. By changing the resistance in a circuit you could change the current. Probably Bosch says Potato and Toyota says PotAto. :-) davidj92

Reply to
davidj92

Toyota calls the wide-band sensor A/F. These are usually twice the cost of the narrow band ones.

The newer low emissions models I think may have the titania sensor. These cost over twice the wide-band sensors, like $300-400 a piece.

I'll go with the Bosch wide-band oxygen sensor. I might even try the one with the universal connector just to see.

Reply to
johngdole

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