Connect Detect and fault diagnosis

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Steve Smith
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Connect Detect and fault diagnosis

Post by Steve Smith » Tue Dec 31, 2019 10:04 am

Connect Detect (CD)

After a recent presentation at Autoinform Harrogate and a conversation with Pete Melville (HEVRA) I wanted to qualify the suggestion of using PicoScope Connect Detect (CD) to determine the cause of specific sensor faults.

Before I begin, more information on CD can be found here

Basically, the PicoScope 4425 & 4225 models can display the status of your x1 test lead connection (e.g. to the component) based on the impedance of the circuit to which your test lead is connected. Therefore, you know when you have made a good connection to the component/circuit prior to measuring.
CD functions with circuits of low impedance (resistances up to 100 kΩ)

The images below demonstrate how our PicoScope is connected to a typical fuel rail pressure sensor circuit and the response from CD when all connections are good.

IMAGE 1.png
Image 1
IMAGE 2.png
Image 2
Taking this one step further, imagine a fuel rail pressure sensor with a sporadic output, I think we can all agree there are numerous possibilities as to why. A typical fuel rail pressure sensor has a 5 V supply, sensor ground and a signal wire carrying a voltage dependent upon fuel pressure. Assuming the physical fuel pressure is good, but the sensor signal is corrupt, we could have:

Sensor supply wire intermittent open circuit
Sensor supply wire intermittent short to ground or + 12 V

Sensor signal wire intermittent open circuit
Sensor signal wire intermittent short to ground or + 12 V

Sensor ground wire intermittent open circuit
Sensor ground wire intermittent short to + 12 V

High resistance is another “typical” fault and must be considered when using CD as a fault-finding tool. Whilst high resistance will increase circuit impedance, if the total resistance remains below 100 kΩ (in the fault condition) it will inhibit the ability of CD to conclusively identify the style of fault condition (e.g. open circuit or short to ground)

The conversation between Pete and I suggested: “If CD uses impedance of the circuit to determine connection status, could we use CD to reveal the style of fault?”


This depends on a number of factors such as:

• Your measurement point with the circuit under test
• The influence upon circuit impedance based on the style of fault
• The behavior of the circuit in response to one of the faults listed above. Not all sensors and circuits behave in the same manner in response to these faults
• CD update speed, which is governed by the time base setting of the scope.
CD status is updated at the end of each buffer, therefore status updates are received sooner on faster time bases

Let’s take a fuel rail pressure sensor with the signal voltage dropping to zero volts with the engine running. This could be due to numerous faults but let’s assume a momentary open circuit or a short to ground in the signal wire. Our initial assumption is that both fault conditions potentially generate identical symptoms, however with the fuel rail pressure sensor circuit, the reality is completely different! With that said, could CD be used to reveal the status of the circuit (open or short to ground) during the fault condition?
IMAGE 3.png
Image 3
Theory
A short to ground has a low impedance and so CD indicates the status of our connection as “Green”, whereas an open circuit has infinite resistance/impedance resulting in CD indicating “Red”. As an end user you now have an understanding as to why the voltage has fallen to 0 V (style of fault) on the fuel rail pressure sensor signal wire.

Reality
Images 4 & 5 below demonstrate the fuel rail pressure sensor signal wire shorted to ground (0 V) yet the status of CD remains faded and green, indicating to the user the circuit is not open and there must be an impedance with a resistance of less than 100 kΩ at the test lead signal wire. In other words, the test lead probe is still connected to the circuit and the circuit is not open
IMAGE 4.png
Image 4
IMAGE 5 SIG SC GND.png
Image 5
One variable to consider is the measurement point within the fuel rail pressure sensor circuit as this has a huge influence on the response of CD

Below we have an open circuit in the fuel rail pressure signal wire after the measurement point of Channel B test lead
IMAGE 6.png
Image 6
Once again, no change in the status of CD as the circuit impedance has not changed sufficiently to warrant an indication to the user. Notice we still have the correct voltage from the rail pressure sensor signal wire at idle speed given the open circuit is after the measurement point.
IMAGE 7.png
Image 7
In the above scenario the ECM would record a fault code for fuel rail pressure signal error and would display such an error via live data, this demonstrates the need for probing sensor signals at the ECM and actuators at the components (such as EGR Valve Motor) Here we incorporate the harness into our measurement, but as ever, not always possible in the real world!

Next, we have an open circuit in the fuel rail pressure sensor signal wire before the measurement point
IMAGE 8.png
Image 8
Once again, no change in the status of CD as the circuit impedance has not changed sufficiently to warrant an indication to the user. The tell-tale sign here is that our signal wire has now increased to 5 V which is typical of an “open” with this circuit/connection arrangement
IMAGE 9.png
Image 9

We have therefore covered 2 possible fault conditions (Open and short to ground) with the fuel rail pressure signal wire but with 3 different outcomes depending on your measurement point within the circuit. In all cases, CD had no cause to indicate to the user that the connection of the test lead to the circuit under test had changed (which is correct)

What is more important, we have confirmed that in this scenario we cannot rely on CD to inform the user of the style of fault as we first thought in our initial conversation

Connect Detect therefore remains the perfect tool to verify your test lead connection to the circuit prior to measurement and for any change to that connection during the capture (e.g. test lead unexpected disconnection)

More fault combinations
Looking at the fuel rail pressure sensor circuit (image 1 above) I have included a short video below covering “open faults” (before & after measurement points) in all 3 wires and shorts to ground within the “5 V Supply” and “Signal” wires of this circuit.
TEST 9 ALL FAULTS INC. EARTH.mp4
Video
(2.84 MiB) Downloaded 154 times
These deliberate faults reveal the effects upon the voltage in all 3 wires during the fault conditions and highlight the characteristic response of the circuit/ECM.

For example, with the engine running at idle speed, the 5 V sensor supply was opened before the measure point on channel A, then restored. It was then opened after the measurement point then restored again, followed by the 5 V sensor supply being shorted to ground.

The following capture displays 8 deliberate faults placed on the fuel rail pressure circuit and the response from each of the 3 wires (Note the sensor ground wire was not shorted to ground as it is already a ground reference via the ECM)
IMAGE 10.png
Image 10
Notice how a long-time frame of 10 s/div has been chosen (CD is deactivated) along with a rising edge trigger point set at 10 V for the 5 V sensor supply on channel A!

This is deliberate as I do not expect the 5 V supply to ever reach this 10 V value, hence the capture will scroll from right to left of the screen until the scope is stopped by the user. This technique is ideal for capturing those intermittent faults “hands free” as once you experience the glitch (for example on road test) you will have the time it takes the capture/glitch to scroll from the right of the screen to the left (100 seconds) before the data is lost! (here we only ever capture a single buffer)
Remember once the glitch has left the screen (with these scope settings) it is lost forever so ensure you stop and save the capture for review when safe to do so.

Below we have the faults with their corresponding waveforms


5 V Sensor supply open before measurement point
IMAGE 11.png
Image 11

5 V Sensor supply open after measurement point
IMAGE 12.png
Image 12

5 V Sensor supply shorted to ground
IMAGE 13.png
Image 13

Sensor ground open after measurement point
IMAGE 14.png
Image 14

Sensor ground open before measurement point
IMAGE 15.png
Image 15

Tips for using Connect Detect

• Connect Detect functions with x1 Test leads only
• Use a fast time frame for rapid update of connection status, once you are happy with a stable connection select your desired time frame
• The LED’s adjacent to the BNC inputs of your scope also indicate the status of CD. This is ideal when you cannot see the scope screen during capture
• Remember CD status is updated at the end of each buffer
• Do not rely on CD to inform you of the style of fault (open of short) as covered above
• CD requires a circuit impedance of less than 100 kΩ to function correctly, therefore some sensors with a naturally high impedance (Knock and Lambda sensors) may not function with CD
• Do not rely upon CD for continuity testing
• Use CD purely as an indication of the status of your test lead to the circuit under test

I hope this helps, take care…….Steve

barrycourtney1989
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Posts: 4
Joined: Wed Mar 18, 2020 1:32 pm

Re: Connect Detect and fault diagnosis

Post by barrycourtney1989 » Mon Apr 06, 2020 2:54 pm

Hi Steve
I am using the latest pico 6 software and it does not have connect detect. Does it have to be turned on?
kind regards varry

Martyn
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Location: Cambridgeshire

Re: Connect Detect and fault diagnosis

Post by Martyn » Mon Apr 06, 2020 7:02 pm

Which PicoScope model do you have?
Martyn
Technical Support Manager

barrycourtney1989
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Posts: 4
Joined: Wed Mar 18, 2020 1:32 pm

Re: Connect Detect and fault diagnosis

Post by barrycourtney1989 » Mon Apr 06, 2020 8:05 pm

its 4425

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 815
Joined: Sun Aug 25, 2013 7:22 am

Re: Connect Detect and fault diagnosis

Post by Steve Smith » Tue Apr 07, 2020 7:30 am

Hi Barry, could you check you have PicoScope 6 Automotive installed and not PicoScope 6 Test and Measurement.

The Automotive version is found here

The following link will help with activation here

I hope this helps, take care......Steve

barrycourtney1989
Newbie
Posts: 4
Joined: Wed Mar 18, 2020 1:32 pm

Re: Connect Detect and fault diagnosis

Post by barrycourtney1989 » Tue Apr 07, 2020 9:06 am

Hi Steve
I downloaded the software from the link you sent and still doesn't have connect detect. thanks

barrycourtney1989
Newbie
Posts: 4
Joined: Wed Mar 18, 2020 1:32 pm

Re: Connect Detect and fault diagnosis

Post by barrycourtney1989 » Tue Apr 07, 2020 9:22 am

sorry steve its a 4423 does this have connect detect

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 815
Joined: Sun Aug 25, 2013 7:22 am

Re: Connect Detect and fault diagnosis

Post by Steve Smith » Tue Apr 07, 2020 12:02 pm

No worries, unfortunately not.

Connect Detect is only available with the 4x25 and now the 4x25A

The 4423 will always be an awesome scope without CD

Take care.....Steve

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