Hidden cause

Andy Crook works through a test plan to get to the bottom of an immobiliser fault

Published:  25 January, 2013

Vehicles with network faults can pose a real challenge for workshops - the symptoms are obvious but the cause can be much more challenging to establish.

As we know, vehicle networks consist of modules communicating with each other via a bus network, saving on wiring, fuses and relays. Workshops need to devise a test to tackle network communication faults. I always start with the global scan or 'class register' to establish what is and is not communicating. This stage is vital and most scan tools can provide this in the diagnostic menu.

Once we know what is willing to talk, fault codes are the next step. Again many scan tools show the presence of diagnostic trouble codes during the global scan. If the codes help pinpoint a likely cause, this can be investigated. Often this involves the use of functional tests, or the process continues with bus and module checks.

The final check is the actual messages themselves. This sounds simple but the whole process depends on system knowledge or information. Recently, a Vectra C was towed into the workshop and it appeared to have an immobiliser fault. The dash indicated that the key had not been recognised. The Immobiliser MIL (Malfunction Indicator Lamp) was illuminated even though the spare key did not work. However, both keys were in frequent use. A global scan was performed and this is what came out of it:

B3601 Key Capture Relay Circuit Malfunction

B3926 Wrong Environment Identifier Received DDM (Driver Door Module)

B3927 Wrong Environment Identifier Received PDM (Passenger Door Module)

U2105 CAN Bus No Communication with ECM (Engine Control Module)

U2106 CAN Bus No Communication with TCM (Transmission Control Module)

U2111 CAN Bus No Communication with EHPS (Electro-Hydraulic Power Steering)

At first glance, the fault appears to be the key capture relay circuit. However, the only other faults present were the driver and passenger door modules. Could they be part of the problem?

After doing some research, I discovered the immobiliser on receipt of a wrong environment identifier from the DDM and PDM which would not authorise a start command to the ECM (Engine Control Module). The door modules are part of the body bus. The bus itself must be OK as we could communicate; in fact, both modules appeared on the global search. The next step was to check the modules. Function checks can be performed in a number of ways - pressing the window switch resulted in no movement so I tried to perform the functional tests via the scan tool but they couldn't be performed due to the wrong environment id. The mirrors could be adjusted, the boot lock release worked so the modules appeared to perform some of their functions.

Figure 1

This sort of problem is typical of network faults. The immobiliser is a function of the CIM (Column Integrated Module) unit but it shares information with the Instrument, Rear Electrical Centre, Body Control Module, Engine Control Module, Passenger Door and Drivers Door modules. A fault with any two of the modules or the communication lines could result in a non-start and the immobiliser MIL. Having a test plan makes tackling this sort of puzzle possible but you need information, knowledge and the correct tools... What I call the diagnostic triangle.

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