Can the window lift be actuated from the driver side?

Yes:

In this case both door control units, the CAN data bus lines and the win-dow lift motor are all OK. The fault will probably be found in the winwin-dow lift switch on the passenger side.

No:

Can other functions (e.g. mirror adjustment) be operated?

If it is possible to carry out other functions it must be assumed that the door control units and the CAN data bus are OK. Possible causes of the fault are the window lift switch on the driver side or the window lift motor on the passenger side. This can be determined by carrying out a functio-nal test from the passenger side. If the window lift works, the window lift motor can be excluded as a potential cause. The switch on the driver side must be considered as the potential source of the fault.

If no other functions can be carried out on the passenger side from the driver side, the fault could be in the CAN data bus or in the control units.

Systems:

Troubleshooting in the data bus

Conform image: Both signals CAN-H and CAN-L can be seen.

Non-conform image: Only one signal is visible.

To link the oscilloscope to the CAN data bus, connection should be made at a suitable spot. This is usually at the plug-type connection between the control unit and the CAN data bus line. In our example vehicle there is a potential distributor on the passenger side, in the cable channel beneath the sill strip (photo).

Systems: The CAN data bus

Comparison between conform and non-conform images on the oscilloscope

This is where the individual data bus lines from the control units meet. The oscilloscope can easily be connected to this potential distributor.

If no signals can be seen on the connected oscilloscope, the data bus has a problem. In order to find out exactly where the fault is, the individual plug-type connections can now be disconnected. The oscilloscope must be monitored during this procedure. If signals can be seen on the oscillo-scope after a plug-type connection has been disconnected, the data bus is working again. The fault is located in the system belonging to the plug-type connection. All the connectors previously removed should be reinser-ted. The subsequent problem is to assign the plug-type connection that belongs to the faulty system to a control unit. Vehicle manufacturers pro-vide no information about this assignment.

To make the search as simple and effective as possible, trial and error should again be used to find out which systems are not working. On the basis of vehicle-specific data about the linking and installation locations of the individual control units, the faulty system can be found. By separating the data bus plug-type connection at the control unit and connecting the plug-type connection to the potential distributor it can be established whether the fault is located within the cable connection or in the control unit. If signals can be seen on the oscilloscope, the data bus is working and the cable connection is OK. If no signals can be seen after the control unit has been connected, the control unit is faulty. If a faulty cable connec-tion is found, resistance and voltage measurement can be used to detect a ground or plus connection or a connection between the lines.

Systems:

Systems: The CAN data bus

In vehicles which do not have a potential distributor, troubleshooting is significantly more complex. The oscilloscope has to be connected to the data bus line at a suitable spot (e.g. on a plug-type connection at the con-trol unit). Then all the concon-trol units present must be removed and the data bus plug-type connections disconnected directly at the control unit.

Vehicle-specific data are necessary for this to determine which control units are installed and where in which vehicle. The oscilloscope must be monitored again before and after disconnecting the plug-type connec-tions. The further procedure does not differ from that of our example vehi-cle.

To test the termination resistors, the data bus has to be in sleep mode.

The control units must be connected up during measurement. The total resistance resulting from the two 120 ohm resistors connected in parallel is 60 ohm. This is measured between the CAN-High and CAN-Low lines.

Systems:

During troubleshooting with the diagnosis unit, testing depth is a deciding factor.

Always start by reading out the fault code. If there are faults in the CAN bus system, first indications of these could be found here.

Further functions can be tested by reading out the measured value blocks.

If a fault is established using the diagnosis unit, tests using the oscillo-scope are once again required to narrow down the fault even further. One frequently occurring problem is that control units have not been recoded / adapted following replacement or having been disconnected from the volt-age supply (e.g. if battery has been replaced).

Troubleshooting with the dia-gnosis unit:

Systems: The CAN data bus

In this case the control units are installed in the vehicle and connected up but do not carry out a function. This can also lead to faults in other systems in individual cases. In order to exclude these faults, make sure that control unit(s) is/are coded correctly and adapted to the vehicle follo-wing replacement or an interruption to the voltage supply.

The installation of auxiliary devices e.g. navigation systems, which also require signals from the data bus, can be extremely difficult. The problem of finding a suitable location to tap the speed signal, for example, is extre-mely difficult without vehicle-specific documents.

There are some sites on the Internet which provide information and possi-bilities about connections and their installation locations. This information is always subject to change so that garages always have to bear the risk of the correctness of this information. The safest method is always to take the vehicle manufacturer's instructions into consideration.

In order to become familiar with all the possible data bus systems, find out how data transfer, structure, function and troubleshooting work, how any auxiliary devices can be installed, we strongly advise visiting a training workshop.

Installing auxiliary devices

Systems: