VW Polo 1.4D year 2003 9N1 car computer network got crazy repair manual
Today I faced with this car and his got crazy CAN network. In his madness he don very funny stuff like won’t crank, shows up some crazy stuff until the diagnostics like “cannot communicate with four wheel drive module” or “no steering assistant module present” and so on, even they have no four wheel drive module because the car is two wheel driven.
For first I will say some basic stuff about the CAN network protocol. CAN or Controller Area Network is really designed for the automotive industry, but nowadays they present in other communication area where are needed a network without a host computer. In other words, this network constructed so they can manage digital messages between microcontrollers or devices through two wires and do not need any host computer.
What you have to know is, in automotive industries in most case the network cables twisted together. No matter is it a CAN or a LIN network wiring.
Here you can see how a CAN network is builds up. The nodes are a module in the car. The modules are the masters of the car electronic devices. You have modules for ABS, engine management module, entertainment module etc. Therefore, you have to know on which module you wish to do some settings or diagnostics. There are resistors on each end of the CAN bus which values are 120Ω.
Today every car has a DLC or Data Link Connector whit 16 pins. This is not the case on trucks.
On a 16 pin DLC you have to use the pin 5, pin 6 and pin 14. Where pin 5 is GND, pin 6 is CAN-H and pin 14 is the CAN-L wiring.
Lets see how can we do a simple measuring on the CAN bus.
Tools what I use in most of time is
– OBD (On Board Diagnostic) diagnostic interface whit the required software
– Digital multi meter, or analog whatever
– 2channel DSO (Digital Signal Oscilloscope)
First I disconnect the car battery and measure the resistance between the CAN-H and CAN-L.
The resistance should be around 60Ω but it could be between 50-70Ω. If it is outside of this range then you have some problem on the CAN line. Maybe a shorten to ground, positive voltage or between the CAN wire or something else unusually.
My next step is to hook up my DSO probe CH1 to CAN-H and CH2 to CAN-L or CH1 to pin-6 and CH2 to pin-14 on the DLC. The probe croco clips should be grounded. Adjust the scope around 20-50uS and the voltage to 5v.
Take care how you handle your scope, you can damage your scope if you take no care where you put the probes. You can burn the tool!
Now you should see some square waves on the scope like this:
The blue waves are the CAN-H and the red are the opposite CAN-L signal of the CAN-H.
If you see something like a sine wave, or some square waves are left out or so then you have a problem in the CAN network which can be the faulty behavior of the other modules in the vehicle.
If you see something like this than you have a problem with the CAN network:
Some models of cars have a diagnostic unit integrated into the cars gateway or between the DLC and the car electronic.
What this mean?
If you try to measure the resistance and the message flow on the CAN network then you will got nothing. You will get maybe a sine wave on the scope. Do not spend lot of your time to figure out why the signals not met the signals of CAN protocols. Try to figure out how is build up the communication between the targeted car DLC and the Gateway unit in the car.
This mean, the pin-6 and pin-14 on the DLC are still the CAN-H and CAN-L line but the diagnostic control unit which, is integrated, will not send any message out to the DLC pins without a request. After a request was sent through the DLC pin-6 and pin-14 the diagnostic unit will transmit the message to the CAN network through the car and send out the result to the DLC. When the message was sent out, and no other request is requested the communication will be ended until a new request arrive.
When I connected my scope to the DLC I got this signal which is exactly the case I explained before where the CAN bus network is interrupted to send out traffic to the DLC.
If you see a signal like this and this is coming from the CAN network without the diagnostic module between the DLC and the CAN internal network then you have a faulty CAN network.
Otherwise, you have to send a request to the diagnostic interface.
Here is a schematic how the DLC and the control unit is wired up in my case of the VW Polo:
Here you can see the original VW Polo comfort module, which did the problem in my case.
It is located under the dashboard on the driver side.
What can you do if you figure out does your CAN is interrupted on the DLC? Simple stick your scope probe to another place. No matter where, it should be the CAN wire. Maybe you have a CAN bus wire in the door or the audio equipment or the ECU ( Electronic Control Unit ) for the engine. Figure it out what is for you the best place. Check in some database or schematic what is the exact color of the CAN bus wire.
After I connected the probe to the CAN bus on the Comfort module I saw there was some error in the communication. After a few hour of checking the wirings I found out does one of the wire was pulled out from the connector under the dash board where the ABS module was and exactly the CAN bus wire. I can’t take a picture about the faulty wiring because it was too deep in the dashboard and no way to go into whit my camera. Sorry…
After I put the wire back into his place the CAN bus network was working again. Now, I had to check for errors whit my diagnostic tool. Deleted all the errors and had to recode the Comfort module because there was a coding does the car have an automatic transmission even they have a manual transmission. In the module was present all-wheel drive option but the car was two wheels driven. I had to learn the central computer unit to lock and open the door remotely…
After all the programming stuff the CAN bus was still working and the car healed from his sickness.
I hope this tutorial will be helpful and save lot of repair time.
This article was prepared for you by Christian Robert Adzic from Novi Knezevac-Serbia.
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