NTK ZFAS U2 - WIDEBAND OXYGEN SENSOR CASE STUDY

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KimAndersen
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NTK ZFAS U2 - WIDEBAND OXYGEN SENSOR CASE STUDY

Post by KimAndersen »

Let me introduce the vehicle under test - It´s a Skoda Octavia 1.6 TDI 77 kw engine type (CLHA) MY2014 which has driven 153.000 km without any problem - so its a " known good" car for testing.

With this NTK ZFAS U2 oxygen wideband sensor I will try to show you reader how my troubles and findings of "How to measure the pumping current going to the pump cell of this wideband sensor". Before you start to investigate any sensor at all you need some background knowledge about - how this sensor operates.

I quickly found out that the information on this sensor is very limited and my first thought was that it would be difficult to show without proper information. To my big suprise did I find some very usefull infomations about this rare NTK wideband sensor from outside the automotive industry - it seems to me that there alot secretive about this sensor.

With this documentation regarding NTK ZFAS U2 WIDEBAND on my availability it becomes a little clearer how this sensor operates in first place and then again it does not explain down in detail how it really works which makes me a bit confused.

First and foremost this document does not show any oscilloscope waveform captures so wouldn´t have any reference waveform to compare against - so what should a good NTK WIDEBAND sensor look like on a Picoscope !!!. Thats a really good question, that I can not answer at this moment.

The information that I used from the document in my research of the NTK ZFAS U2 wideband sensor is the specification of the internal resistance through the pump cell also known as the " IP cell". The internal resistance of the IP cell is 260 Ohm when this NTK oxygen wideband is new and this resistance is later on used in my calculation of the pump current through the IP cell or pump cell.

The calculation of the pump current is based on voltage drop across the pump cell which is between pin 2 (IP+) and virtual ground pin 6(IP-). Whith the use of a math channel - where I subtract ch A from ch B and divide this with the internal resistance at 204 Ohm, then I get the actual milliamp going through the pump cell (IP) - the math channel look like this A-B/204 ohm.

NTK_ZFAS_U2_WIDEBAND_SENSOR_CONNECTOR_6PIN
NTK_ZFAS_U2_WIDEBAND_SENSOR_CONNECTOR_6PIN

As the internal resistance of the pump cell is very important in this calculation when it comes to getting the correct pump current (mA) through the pump cell (IP) - I had to make a adjustment to the internal resistance used in the math channel. So instead of 260 ohm which is the specification on a new wideband sensor, have I used the resistance of 204 Ohm.

In my study of this sensor, I have used several measurement techniques such as DMM (power and voltage measurement), low amp current probe (Chauvin Arnoux K2) combined with both DMM and Picoscope and finally the use of VCDS software for logging pump flow pids. . off this lambda sensor.

The first and most obvious tool to use when measuring low amperage from a broadband oxygen sensor is the low current probe from Chauvin Arnoux K2, I thought !!!. The result of this test, which I measured the pump flow that went to the Cell (IP) pump, was poor. I think it has something to do with the low bandwidth of this low current probe - it was simply slow to respond to the changes in this NTK ZFAS U2 wideband sensor.

The next test did I use my trusty Fluke 88 DMM to measure the voltage drop across the pump cell ( IP ) which is between Pin 2 and Pin 6 at the lambda connector.

NTK_ZFAS_U2_VOLTAGEDROP_DMM_TEST
NTK_ZFAS_U2_VOLTAGEDROP_DMM_TEST

So what did I expect of the measurement !. With everything connected did I start the engine and watched the DMM - nothing happend to the DMM reading - the voltage drop showed zero volt (0 volt dc). Then after a couple of minutes did I raise the engine RPM to around 2000 for 5 minutes so the wideband sensor could heat up properly and still no change in the DMM reading. Then I drove the car just to see if it would make any change to my voltage drop reading and it did. After around 3 minutes of my road test, did the DMM suddenly showed a voltage reading that I was expecting.

It clearly to me that this a normal part of the ECU strategy on how to switch the NTK wideband sensor from a open loop to closed loop, but I haven´t expected it to take that long.

During my road test did I observe the voltage reading at the DMM and it showed 0.8 volt DC at fuel cut or deaccelation of the engine and under hard acceleration the reading was 0.4 volt DC and finally at idle speed it showed a steady reading at 0.6 volt DC.

One more thing about the DMM voltage drop test is to remember that this test is a relative voltage reading to each other ( between pin 2 and pin 6) and if this test was made with reference to battery ground terminal the DMM voltage reading would be 7.65 volt DC during the closed loop status !!!.

NTK ZFAS U2 VOLTAGEDROP_CURVE VS LAMBDA
NTK ZFAS U2 VOLTAGEDROP_CURVE VS LAMBDA


Let's start by looking at the VCDS log where I've taken the raw data and exported this to my CurveExpert software whereby its possible to make a curve over the pumping current going to pump cell and combine this with the lambda factor. As this Skoda Octavia equipped with a 1.6 TDI diesel engine - will the lambda values be in the lean region that will be higher than lambda 1.

According to the VCDS log file is the pump current scaling between -4 negative to 4 positive milliamp (mA). During my dynamically roadtest did I observe the lambda values and the lowest lambda reading was lambda 0.9516 and the highest lambda was lambda 14.64 during fuel cut or deaccelation of the engine.

NTK ZFAS U2 PUMP CURRENT VS LAMBDA CURVE
NTK ZFAS U2 PUMP CURRENT VS LAMBDA CURVE

Now it's time to connect the picoscope to this oxygen wideband sensor and see if its possible to make measurement of the current going through the pump cell (IP). I´ve used three channels in this test. where channel (A) is connected to (IP+) Pin2 White and channel (B) er connected to (IP -) Pin6 Black at the oxygen wideband connector plug. Only two channels are used in this pump current measurement A and B.

With the magic of a math channel where channel A-B/204 ohm we get pump current (mA) going trough the pump cell. Compare this picoscope capture with normal oxygen wideband sensor - it looks like pump current are being pumped in steps which I havent seen on any wideband sensor before - I know its not a regular wideband sensor this NTK ZFAS U2 sensor, maybe someone with a broader knowledge than me could explain whats going on in this sensor. The picoscope two black testleads and are connected to negative battery terminal - just to verify this.

This test is done at a public road, so it reflects real data coming from the lambda sensor in terms of a pumping current going through the pump cell (IP) with some help from a math channel. Frame 1 through frame 20 represend a acceleration and a long deacceleration of the engine where the pump current reaches its maximum of 4 mA which is equal to a excess of oxygen the exhaust system. The maximum rpm during this road test at around 4200 rpm

SKODA OCTAVIA 2014 1.6TDI PUMP CURRENT ACCELERATION / DEACCELERATION TEST
SKODA OCTAVIA 2014 1.6TDI PUMP CURRENT ACCELERATION / DEACCELERATION TEST

The next test is showing the pump current during a idle speed at 771 RPM and with value at 2.95 mA which equals a lambda value at 3.48.
All the pump current (IP) readings taken with the picoscope (4423) are cross checked the VCDS road test log file and they match each other in terms of accuracy as long as the math calculation are based on internal resistance of 204 OHM.

SKODA OCTAVIA 2014 1.6TDI WIDEBAND SENSOR IDLE SPEED 771 RPM
SKODA OCTAVIA 2014 1.6TDI WIDEBAND SENSOR IDLE SPEED 771 RPM

As I mentioned before are the internal resistance of a new NTK ZFAS U2 wideband sensor at 260 ohm and a adjustment was needed as this car had driven 153.000 km and some sort ageing of this wideband sensor is to be expected.

One more thing about the measurement that you maybe will observe is the use of "ADD MEASUREMNET" in the bottom of the capture. You can´t use the ruler to measure the pump current !!!. Let me explain it again - you can use the ruler !!! , but where should you make the measurement from.

When you look at the latest picoscope capture at idle speed the pump current ranges from negative 1.525 mA to positive 8.345 mA and with a delta reading 6.821 mA. This peak to peak pump current (mA) reading dosen´t match the VCDS log file that I made, which made me a bit confused on how to measure this wideband sensor.

After a long research and alot of testing did I found the answer to me accuracy of this pump current (IP) reading which was the use of ( Average measurement ) on both channel A and channel B and also on the "Pump Current measurement ", before the correct result showed up.

To finish this casestudy up - would I like say that I dont know everything about this rare NTK ZFAS U2 wideband sensor and there are alot more aspect to this casestudy on how to interpretate data coming from this sensor that you could imagine.

Is there anyone that have done similar tests !.

SKODA_OCTAVIA_2014_1.6_TDI_WIDEBAND_SENSOR_ROAD_TEST.psdata
SKODA_OCTAVIA_2014_1.6_TDI_WIDEBAND_SENSOR_ROAD_TEST
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SKODA_OCTAVIA_2014_1.6_TDI_WIDEBAND_SENSOR_IDLE_SPEED_771_RPM.psdata
SKODA_OCTAVIA_2014_1.6_TDI_WIDEBAND_SENSOR_IDLE_SPEED_771_RPM
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Regards
Kim

Steve Smith
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Re: NTK ZFAS U2 - WIDEBAND OXYGEN SENSOR CASE STUDY

Post by Steve Smith »

Hi Kim, thank you for taking the time out to post this awesome information

I have not carried out any testing with this brand of sensor but have completed a number of Wide-band Lambda Sensor Guided Tests :

The Bosch LSU 4.2 Lambda sensor Overview Test can be seen here https://www.picoauto.com/library/automo ... en-sensor/ with additional information here https://www.picoauto.com/library/traini ... gen-sensor

The following Guided Tests are also complete and will be released pending Technical Publication review

The Bosch LSU 4.2 Response test (single wire check)
The Bosch LSU 4.9 Overview Test
The Bosch LSU 4.9 Response test (single wire check)

I have now started work on the Denso 4 Wire Broadband sensor seen on Toyota/Lexus and Subaru models (to name but a view) These will also follow the same theme where you can test all inputs/outputs (Overview Test) or a Response Test using a single wire measurement to confirm O2 sensor response to load

Frank has a very good video here covering Wide-band Lambda Response
https://youtu.be/UdbZfXMMTQs

Thank you once again, it is much appreciated by all

Take care......Steve

KimAndersen
TwoWaves
TwoWaves
Posts: 245
Joined: Fri Nov 30, 2012 2:53 pm
Location: Denmark

Re: NTK ZFAS U2 - WIDEBAND OXYGEN SENSOR CASE STUDY

Post by KimAndersen »

Here is just a follow-up since my last post regarding the warm-up phase of this special broadband oxygen sensor. Here I talked about how long it takes for the broadband sensor to warm up or become active.

Just be patient until it's fully warmed up - this will mean between 3-4 minutes on a road test. As shown here in the picture, you can see when the sensor becomes active and where the voltage measurement goes from a constant 7,362 Volt DC to alternating voltage.

NTK ZFAS WARMUP ROADTEST
NTK ZFAS WARMUP ROADTEST

This measurement is taken between Pin2 ( pump cell ) and the negative battery terminal. By measuring the voltage relative to the negative terminal of the battery, the voltage will be higher than if it was measured across the pump cell (between Pin2 and Pin6) as I showed in my first post.

A very small update regarding the calculation of the pump current through the pump cell. The difference is - the way you measure the pump cell voltage. Here I have deliberately measured the voltage of the pump cell in relation to the negative pole of the battery. There is nothing wrong with the first calculation in the post above - they can both be used.

But how can you calculate the pump current through the pump cell with the higher voltage ! ! !.

There is actually a factor of 5 between the voltage drop across the pump cell and the voltage drop between Pin2 of the pump cell and battery ground. It is absolutely essential that you are aware of this difference in the measurement method, otherwise the calculation of the pump current through the cell will not be correct.

PUMP CURRENT FACTOR 5 METHOD
PUMP CURRENT FACTOR 5 METHOD

This secret factor 5 was not invented by me. I found an old document from OTC where the relationship between OEM PID and Global OBD II is described. But this factor of 5 is not enough to calculate the pump current - you need to know the internal resistance of the cell.

Just a correction for internal resistance as I mentioned in my first post - where I mentioned that the resistance in the pump cell on a new sensor is 260 OHM. It has an upper value of max 260 ohms on a new sensor. There are no lower values on this sensor. In my calculation above, I have used the value 204 OHM, which fits perfectly in the calculation.

Are there any good or bad experiences with this type of sensor!

Regards
Kim
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