Fluke 867 is my favorite voltmeter
- From: "BretLudwig" <bratzirules@xxxxxx>
- Date: Sat, 19 Jul 2008 19:10:42 -0500
Why did they discontinue this excellent tool?
Completely irrelevant to tube audio but very relevant to why I like it:
January 2002" Toyota Wide Range Air:Fuel Sensor, John Thornton, Underhood Service,
This month�s article is not going to focus on a specific driveability
problem, but rather a specific testing technique. Have you had the
opportunity to see a Toyota Wide Range Air:Fuel sensor yet? You may have
seen one and not even noticed it. Why? Because this new type of "oxygen"
sensor looks very similar to a conventional oxygen sensor. Photo 1 shows a
Wide Range Air:Fuel sensor in a 2001 Camry with a 2.2L engine.
At first glance, it looks like a standard 4-wire heated O2 sensor. Well,
it is definitely not a standard 4-wire O2 sensor. Toyota first started
using Wide Range Air:Fuel sensors with some 1997 models in California. By
1999, their use had spread to other Toyota and Lexus models.
Wide Range Air:Fuel sensors are sometimes called Linear Air:Fuel sensors.
Both Honda (in some limited Civics) and Cadillac (1999 Catera) have used
this unique type of air:fuel measuring device. But what is the benefit?
Let�s answer that question with a chart shown in Figure 1.
This chart from Toyota shows the relationship between air:fuel ratio and
Wide Range Air:Fuel sensor output (relative) voltage. However, some
explanation is in order.
We know that a conventional O2 sensor toggles or switches around the
stoichiometric point (14.7:1). The PCM never knows the exact air:fuel
ratio, only that it is richer or leaner than 14.7:1. When O2 voltage is
above the typical target voltage of 0.45 volts, short-term fuel trim tells
the PCM to slightly narrow injector pulsewidth. When O2 voltage is below
this target voltage, short-term fuel trim tells the PCM to slightly widen
injector pulsewidth. The result is the familiar O2 sensor voltage pattern
that we see.
The Wide Range Air:Fuel sensor does not operate on this principle. A Wide
Range Air:Fuel sensor produces an output that accurately identifies the
current air:fuel ratio over a wide lean-to-rich range. The PCM not only
knows what the air:fuel ratio really is, but it can command to a specific
ratio. How good is it? Well, only an engineer could tell us for sure.
Nonetheless, I decided to perform some emissions testing to satisfy my own
curiosity.
My first step was to bring a 2001 Camry with a 2.2L 5S-FE engine to an I/M
240 lane for emissions testing. This vehicle is ULEV certified per the
decal, so I was expecting some low HC and CO numbers. Figure 2 shows the
results of this 240-second dyno run.
The numbers were excellent! All three measured gases (HC, CO and NOx)
showed 0.00 grams/mile over the 240 seconds. The tested was performed four
times; all with similar results.
While the dyno runs were being performed, I put my gas analyzer probe in
the tailpipe. Generally, measuring HC in parts per million (ppm) and CO in
percentage (%), on the road (or on the dyno) can be good predictors of I/M
240 results. Figure 3 shows part of the results. The values shown in
Figure 3 represent a moment in time. Please take my word for this; those
were typical of the values seen over the 240 seconds. The HC never (and I
mean never) exceeded 0 ppm. The CO bounced around up to 0.03%. Peak NOx
was 144 ppm, and that lasted for just a few seconds.
I also recorded some road tests with my gas analyzer and saw similar
results. Understand that emissions control is a system; it is not just a
component. But I�m pretty sure the Wide Range Air:Fuel sensor plays an
important role in this system.
With that introduction info behind us, let�s get into the testing of
this sensor. Please review the chart in Figure 1. You�ll notice on the
vertical scale there is a voltage that corresponds with a specific
air:fuel ratio.
The chart implies that this Wide Range Air:Fuel sensor produces a voltage
in direct relation to the air:fuel ratio. This is not true. The voltage
shown on the chart is what one would see if one were using the factory
Toyota scan tool to measure the air:fuel sensor parameter. Toyota states
that the output of this sensor can only be measured with a scan tool. (It
appears that the Toyota factory scan tool and the Vetronix Mastertech with
the Toyota OE software are the only tools that support this parameter
currently.) The sensor�s output is not a changing analog voltage, but
rather a small (< 0.020 amps) bi-directional current. Internal circuitry
within the PCM converts the analog current output into a voltage. It is
this converted voltage that can be seen on the scan tool. To me, this is a
diagnostic challenge. There always is a way to check something. Maybe the
technique is direct, or maybe it is indirect.
Let�s start off with what Toyota shows on their scan tool, and then we
will look at two alternative testing methods.
Figure 4 shows a 30-second snapshot taken from the Toyota factory scan
tool that is manufactured by Vetronix. The Wide Range Air:Fuel sensor
output is shown as AFS B1 S1. The second parameter shown is fuel trim (AF
FT B1 S1). The engine is at idle, fully warmed up and in closed loop. As
you probably have already noticed, the voltage shown for the Wide Range
Air:Fuel sensor doesn�t change much.
As per the graph shown in Figure 1, 3.3 volts corresponds to the 14.7:1
air:fuel ratio. As the voltage displayed on the scan tool decreases, the
oxygen content of the exhaust decreases (rich exhaust). As the voltage
displayed on the scan tool increases, the oxygen content of the exhaust
increases (lean exhaust). Keep in mind that the oxygen content of the
exhaust corresponds to a specific voltage. Therefore, the PCM knows the
exact air:fuel ratio based on the sensor�s output signal.
Note that the scanner Wide Range Air:Fuel sensor is 3.29 volts with a
correction of 0.99 (1% negative). This is almost ideal. The PCM is
maintaining an air:fuel ratio that is very close to stoichiometric.
As the scanner voltage decreases (from 3.30 to 2.80 volts), exhaust gases
are said to be rich (exhaust oxygen deficient). As the scanner voltage
increases (from 3.30 to 3.80 volts), exhaust gases are said to be lean
(excess exhaust oxygen). This voltage will not oscillate like that of a
conventional oxygen sensor. The Wide Range Air:Fuel sensor voltage is
relatively stable. The scanner voltage will change due to extreme rich or
lean conditions in the exhaust.
Next, study the example shown in Figure 5. Is the exhaust rich or lean?
What is the fuel trim indicating? Well, at 2.63 volts the exhaust is on
the rich side. Remember, voltages less than 3.3 volts indicate a rich
exhaust while voltages greater than 3.3 volts indicate a lean exhaust. The
fuel trim at 0.81 (or 81%) equates to a 19% rich bias. The air:fuel ratio
is 19% rich of stoichiometric. I was flooding this engine with propane.
So, one way to check this sensor is with a scan tool. The only catch is
that the scan tool we use must support this parameter.
Before we go any farther, another important point needs to be made
regarding the operating temperature of this Wide Range Air:Fuel sensor.
For this sensor to function properly it must operate at about 1,200� F
vs. the typical 600-700� F that most O2 sensors operate at. Check out
the heater current shown in Figure 6 on page 30.
Channel 1 is connected to a current probe clamped around one heater wire,
and Channel 2 is connected to the PCM (control) side of the heater
circuit. Heater current is duty cycled by the PCM. Peak current is just
over 6 amps. I have not seen heater currents like this before. Certainly,
there are manufacturers who are pulsing (duty cycling) the heater control
line. Six amps is a fair amount of heater current.
I want to emphasize one more time that these sensors work at a much higher
operating temperature than we tend to be familiar with. If there is a
heater circuit problem, there will be a Wide Range Air:Fuel sensor
problem.
Let�s now examine circuit voltages. Figure 7 is a simplified drawing of
the Wide Range Air:Fuel sensor. The two heater wires have not been shown.
The sensor has two signal lines. One line has 3.3 volts on it, and the
other has 3.0 volts on it (relative to engine ground).
These two voltages do not change. If this is true, then the voltage (300
millivolts) across the sensor�s two signal lines does not change. I had
a hard time with this when I first started checking these sensors. No
matter what I did with the throttle, with propane or with controlled
vacuum leaks, I could not get those voltages to change. They are fixed.
What changes is the current flow through the sensor. Again, the voltages
are fixed, therefore, we have to test in a different fashion. We�ll
discuss two methods.
Please keep in mind for this sensor to operate it must be close to
1,200� F. This says the heater must be functional. The heater circuit
must always be checked for proper current flow. Additionally, any tests we
perform will require heater operation.
The first Wide Range Air:Fuel sensor test can be done with a lab scope or
a multimeter. Credit for this test must be given to Snap-on. I don�t
know who originated this test, but it is described in a help menu found in
the Snap-on Vantage. Disconnect the sensor�s 4-wire connector and jumper
the two heater wires so as to complete the heater circuit. The heater must
be functional for the sensor to work.
Do not jumper the sensor�s signal lines, but connect a scope or
multimeter to them. Connect the positive lead of the scope to the 3.3-volt
line, and connect the negative lead of the scope to the 3.0-volt line. This
is shown in Figure 8.
The scope has been connected to the two disconnected signal lines from the
Wide Range Air:Fuel sensor. The heater is still connected and operational.
Figure 9 shows the sensor�s response to propane. While idling, propane
is being added to the engine�s intake. The sensor produces a voltage
similar to that of a conventional oxygen sensor. About 1 volt indicates a
rich exhaust.
This is a good method for testing one portion of the Wide Range Air:Fuel
sensor. Without going into the theory of operation, the Wide Range
Air:Fuel sensor is made up of two cells. One of those cells is similar to
a typical O2 sensor. The other cell is used to pump oxygen into or out of
a reference chamber. The pumping action and, ultimately, the exhaust
oxygen content are determined by a very small current measurement. Per my
research, the current levels are less than 0.020 amps. Note: SAE papers
#930232 and #930233 contain detailed information about sensor operation.
The final method for testing involves the use of an ammeter. While the
above method is a good technique, measuring current puts us closer to what
is really happening with this Wide Range Air:Fuel sensor.
Figure 10 shows the setup. A digital multimeter has been connected in
series in the 3.3 volt signal line. Set the DMM to the milliamp scale. Use
jumpers so as to complete the heater circuit and the 3.0 volt circuit of
the air:fuel sensor. Note the meter orientation to the circuit. The
meter�s red lead is connected to the sensor and the meter�s black lead
is connected to the PCM. The meter is in the 3.3-volt circuit. Photos 2 and
3 show what my Fluke 87 displays with a rich exhaust and a lean exhaust.
In this example, a rich exhaust produced a positive 4.89 milliamps, and a
lean exhaust produced a negative 1.53 milliamps. If the meter leads had
been reversed, so would have been the polarities seen on the meter.
This test can be taken one step further. Instead of using a conventional
DMM, a graphical multimeter can be used. The Fluke 867 graphical
multimeter was used to acquire the upcoming patterns. As before, the meter
is connected in series with the 3.3-volt circuit. Use jumpers to complete
the heater circuit and the 3.0-volt circuit of the air:fuel sensor.
Figure 11 shows how the Fluke 867 was connected for the following
recordings. Please note, in this next group of tests the positive lead of
the meter was going to the 3.3-volt feed line of the PCM. Figure 12 shows
the relationship.
The next five figures are snaphots taken from the Fluke 867. Short of the
factory scan tool, I believe this is an effective and accurate way to test
the Toyota Wide Range Air:Fuel sensor.
In Figure 13, the vertical scale is �14.2 milliamps, and the horizontal
scale is from 0 to 12.8 seconds. This was taken at idle. I am snapping the
throttle open and close. Wide Range Air:Fuel sensor current drops to -6.9
milliamps (rich exhaust). The sensor responds to changes in the air:fuel
ratio. This is good.
In Figure 14, the vertical scale is �14.2 milliamps, and the horizontal
scale is from 0 to 12.8 seconds. Fuel injector #1 has been disconnected.
An open injector should bias the exhaust lean. Average current appears to
be about 2.8 milliamps. Per the Toyota scanner, the air:fuel ratio sensor
voltage was 3.5 volts. Remember that voltage above the base of 3.3 volts
indicates a lean exhaust.
In Figure 15, the vertical scale is �14.2 milliamps, and the horizontal
scale is from 0 to 64 seconds. With the engine at an elevated rpm, I am
adding and removing propane to the intake. The Wide Range Air:Fuel sensor
responds appropriately.
In Figure 16, the vertical scale is �14.2 milliamps, and the horizontal
scale is from 0 to 25.6 seconds. I am performing a brake torque in
reverse. Sensor current drops to -11 milliamps. As you now know, this
indicates a rich exhaust.
In Figure 17, the vertical scale is �14.2 milliamps, and the horizontal
scale is from 0 to 25.6 seconds. While cruising at about 30 mph, I did an
aggressive WOT acceleration and then a long deceleration. The current is
dropping (going rich) for about the first 15 seconds, before it starts to
increase (lean decel).
I hope these five figures demonstrate how effectively this Wide Range
Air:Fuel sensor can be checked with an ammeter. A graphing DMM is handy,
but not necessary, a typical DMM will work. If our scan tool cannot
support this parameter on a Toyota, we have an effective alternate test
for this unique sensor.
To summarize, we looked at three methods for testing the Toyota Wide Range
Air:Fuel sensor. The first, and probably the easiest, was through the use
of a scan tool. The only issue was that not all scan tools support this
parameter (yet).
The second method was to disconnect the two signal lines and connect them
to a lab scope. The heater circuit had to remain intact. The sensor was
then tested in a similar fashion to a conventional O2 sensor.
The third approach was to connect a DMM in series with the 3.3-volt signal
line, and jumper the remaining three wires back to their respective
circuits. With this technique, we could actually monitor the current the
PCM was measuring as the exhaust oxygen content changed. This test brought
us closest to actual circuit operation.
When one of these unique "oxygen" sensors appears in your bay, I hope this
article has given you the knowledge and confidence to properly evaluate
it."<<
http://www.babcox.com/editorial/us/us10226.htm
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