Diagnostic Trouble Codes and Freeze Frames

Diagnostic Trouble Codes and Freeze Frames

While the ignition is on, the TCM diagnoses the system for faults. The extent of the diagnostic capability at any particular time depends on the prevailing operating conditions, e.g. it is not possible to check for a short circuit to earth if the circuit concerned is already at a low potential. If the TCM detects a fault with the transmission system, it will enter a default mode to prevent further damage to the transmission and allow the vehicle to be driven. When a fault is detected a Controller Area Network (CAN) message is sent from the TCM and is received by the instrument pack. The instrument pack illuminates the Malfunction Indicator Lamp (MIL) and displays 'TRANS. FAILSAFE' in the message centre. Some transmission faults may not illuminate the MIL or display a fault message, but the driver may notice a reduction in shift quality.

The diagnostic socket allows the exchange of information between the various control modules on the bus systems and a diagnostic tool using Keyword 2000 protocol. The information is communicated to the socket via a diagnostic ISO9141 K Line. This allows the retrieval of diagnostic information.

The TCM uses a P code strategy, which stores industry standard and Land Rover specific Diagnostic Trouble Codes (DTC) relating to faults.