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Diagnostic communication

Diagnostic communication

The transmission control module is not connected to the bus. Diagnostic communication takes place on leads from the transmission control module pin 21 to the data link connector pin 7, and from there directly to the diagnostic tool.

The diagnostic tool communicates digitally and serially with the TCM system. Digitally means that the difference in voltage between the lead and ground takes two values, in this case about 0 V and B+. The information is coded so that different combinations of 0 V and B+ pulses have different meanings.

Serial means that the information is sent in “packets” which are transmitted one after the other in rapid succession.

With the ignition key in the ON position and the diagnostic tool connected, the voltage on pin 23 will be 9-11 V.

Diagnostics

If the system cannot be contacted, the fault may lie either in the car or in the diagnostic tool. If the diagnostic tool can make contact with the same system in another car, this will mean that the fault is in the car.

First, check that the control module is awake, the ABS warning lamps are tested by the control module each time the ignition is turned on. If the control module is awake, the fault can probably be localized to the lead between the data link connector and the control module.