Basic How To Read Wiring Diagrams

[varooom]

I am going to show you an example of a real life issue, and how you can then use this to locate and read the wiring diagram to help you trace faulty wiring.

 

 

The car in question is a 2015 Skoda Octavia III

 

 

Step 1: The car is scanned and we save this diagnostic session (because car history is important to keep)

Address 10: Park/Steer Assist (J791)       Labels:| 5Q0-919-298.clb
   Part No SW: 5Q0 919 298 K    HW: 5Q0 919 298
   Component: PARKHILFE PLA H12 0054  
   Revision: --------    Serial number: 39491514005930
   Coding: 0131061041
   Shop #: WSC 73430 790 00063
   ASAM Dataset: EV_EPHVA2CAU3700000 003021
   ROD: EV_EPHVA2CAU3700000_003.rod
   VCID: 47992422FE9974B74E-8012

3 Faults Found:
1079572 - Rear Right Parking Aid Sensors
          B1079 14 [009] - Open or Short to Ground
          Confirmed - Tested Since Memory Clear
             Freeze Frame:
                    Fault Status: 00000001
                    Fault Priority: 3
                    Fault Frequency: 2
                    Reset counter: 98
                    Mileage: 222684 km
                    Date: 2021.04.19
                    Time: 14:36:26

1079316 - Center Rear Right Parking Aid Sensor
          B1078 14 [009] - Open or Short to Ground
          Confirmed - Tested Since Memory Clear
             Freeze Frame:
                    Fault Status: 00000001
                    Fault Priority: 3
                    Fault Frequency: 1
                    Reset counter: 2
                    Mileage: 222684 km
                    Date: 2021.04.19
                    Time: 14:36:26

1080852 - Side Rear Right Parking Aid Sensors
          B107E 14 [009] - Open or Short to Ground
          Confirmed - Tested Since Memory Clear
             Freeze Frame:
                    Fault Status: 00000001
                    Fault Priority: 3
                    Fault Frequency: 1
                    Reset counter: 2
                    Mileage: 222684 km
                    Date: 2021.04.19
                    Time: 14:36:27

 

 

Step 2: We need to locate the correct wiring diagram, this is for an Octavia III 2015 car

You will note from the next 2 pictures, that there exists two wiring diagram files from erWin website

 

Here is the second file...

 

As you can see from these two documents, the correct file we need to search though is our 1st document, as it "from November 2012" onwards

 

 

Step 3: We now need to search the document for a keyword, or we need to use the bookmarks to find the correct section

If you remember our diagnostic log, we can take the name of the control module scanned and use that as our keyword

Address 10: Park/Steer Assist (J791)       Labels:| 5Q0-919-298.clb

For our example, we have J791 we can input into a search field on the document

 

This takes us to this main section

 

 

 

Step 4: We will now need to scan through the next few pages until we find the correct wiring diagram, which will depend on what is fitted to the vehicle

The car in question has 6x rear parking sensors, and some of the pages only show wiring for 4x sensors!

 

Diagram No. 42 / 6

This shows 2 of the 6 sensors, G203 and G204

Diagram No. 42 / 7

This shows the next pair of 2 sensors, G205 and G206

Diagram No. 42 / 8

This shows final 2 of the 6 sensors, G716 and G717

 

Step 5: Now the hardest part is tying all 3 wiring diagrams together.

If we start with the basics, along the bottom row, each image has got a line, along with a row of numbers at the bottom, this is what ties one page into another so you can follow the trail of breadcrumbs so to speak.

 

Diagram No. 42 / 6

Diagram No. 42 / 7

Diagram No. 42 / 8

 

So if you can imagine printing out all 3x pages, you would have to lay these 3x pages from left to right in a linear fashion.

 

 

Step 6a: Now we need to examine and work out in our example, how each of the sensors is connected to the Park/Steer Assist (J791) module

 

Taking a look at this small section: -

1. Our module J791 is at the top, it connects towards our sensor via T26b connector (the numerical value = number of connections in the plug)
2. T26b then connects to T12e 12-pin connector, on left in luggage compartment.
3. Finally we have T3au that is the small 3-pin plug that connects onto the parking sensor itself

 

Ok, so that was just one section of the wiring from the control unit J791 to just one sensor

Now if we look at the 3x wires that head down to the sensor, we see on connector T3au/1 (left) and T3au/3 (right) two large black dots, along with a black horizontal line heading off to the right side of the image.

1. The line coming from connector T3au/1 (left) has a large circled X68 Positive connection (parking aid), in rear bumper wiring harness
2. This means that the wire coming from T12e/10 towards T3au/1 will have a "splice connection" where the single feed wire is split into two or more cables
3. The line coming from connector T3au/3 (right) has a large circled 352 Earth connection (parking aid), in rear bumper wiring harness
4. This means that the wire coming from T12e/10 towards T3au/3 will have a "splice connection" where the single feed wire is split into two or more cables

 

I will add at the bottom of the thread, what a splice connection looks like in real life

 

 

Step 6b: We now need to follow onto the next two pages of the documents to make note of what wires go where.

Here is something that should be obvious, but best to point this out.

 

• The connector T12e is named so as it only has 12 maximum connections
• Each of the sensors has 3 wires feeding it
• Given that there are 6 sensors in total for this example vehicle, you can multiply 3 wires per sensor, and 6 sensors for a grand total of "18" wires
• Last time I checked, 18 wires cannot go into a 12 pin connector
• So this is why the little black dots on the wiring diagram and the circled X68 & 352 are important, they represent a splice in the wiring.

 

 

Edited 24 July, 2022 by varooom

[varooom]

Credits to @Breezy_Pete & @JoeF for both posting this image

 

This shows us an image of a single wire that is spliced into two wires

 

 

Step 7: Here we have a tracing of all the wires in the loom from T12e to each of the 6x Sensors

Red text is for the Positive (only 1 feed wire is Positive, T12e / 10)
Black for Negative (only 1 feed wire is Negative, T12e / 7)

Blue for Sensor data (T12e / 1-4, 8 & 9)

 

 

Step 8: So how do we actually know where to look for a fault in the wiring loom?

This is where you must combine the knowledge of the wiring diagram and also the fault codes that we scanned.

 

If there was a fault in the main wiring loom from the module J791 via T26b (short to ground for example) then we can see that this would take out ALL 6 sensors, not just 3 out of the 6

Now we know that from T12e down to each sensor in turn is just 1 Positive feed, and 1 Negative feed wire, and where each of these 2x main feed wires then are spliced down towards each of the sensors.  We can now disconnect connector T12e from inside of the boot on the left (behind tail light, inside the body) and check each sensor that is faulting, by disconnecting the wire and checking for continuity from T12e pin 10 to the Positive and Negative on each sensor.

 

We can also visually inspect of course to see if any wires are cut, before we even have to probe any wires.

 

 

In the above example, 3 of the 6 sensors cannot communicate.  And checking the wires, breaks can be seen in some with corrosion, and also with some wires where no corrosion is seen, upon stripping the loom wires can be found to have been shorted out.

Therefore, repair or replace this loom section (in this example, new loom fitted due to water ingress on some of the plugs)

Edited 24 July, 2022 by varooom

[Breezy_Pete]

Good stuff @varooom. 🙂

Not sure how else to add this other than an attachment, but feel free to incorporate the contents explicitly if you know a good way.

It covers all sorts, but crucially how to work with the 'numbers in rectangular boxes' which frequently fox newcomers to the CFDs (current flow diagrams).

 

CFD explanations.pdf

 

One more thing; for anyone wondering about the seemingly misplaced laughter (L0L) in some of varooom's examples, that's a code meaning  Left Hand Drive. L0R is the equivalent for RHD. 😁

Edited 24 July, 2022 by Breezy_Pete

[varooom]

Thanks will look when near PC

[nige8021]

https://www.youtube.com/watch?v=jKnTjuSmwaE

[nige8021]

Some info on wiring repairs SSP+871003+++Wiring+harness+inspection+and+repair.pdf

[nige8021]

Another from the VAG SSP listing SSP 873003 How to read wiring diagrams.pdf the full list of SSP's https://cardiagn.com/vag-self-study-program-ssp/ 

[varooom]

Sorry trying to edit is being a pain.

Made the guide to collate as much information in one thread as possible, so thanks very much for all your contributions, it will be very helpful when someone else is needing help.

 

p.s. I have the SSP list, and then some more on top 😁

Edited 26 July, 2022 by varooom
More information

[ApertureS]

This is very very helpful for a lot of people! Well done!

 

Also something to add for people L0L and L0R will sometimes be displayed on the wiring diagrams - this relates to left hand drive or right hand drive.