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.
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.
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 !!!.
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.
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
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.
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 !.