Description There may be several reasons why a short-circuit to ground is detected for a signal: - The signal line has contact to the chassis or to other elements linked to the battery ground.
- The signal line has contact to a ground cable (e.g. in the cable harness).
- An electrical component is internally defective (e.g. switching contact is jammed).
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Short-circuits can occur in different places, for instance: - in plug connections due to warped contacts,
- at vibrating parts,
- in places where there is a lot of cable movement, e.g. in the joint of an articulation bus,
- where the cable passes close to sharp-edged parts
- by foreign bodies inside plug connections or cables, e.g. dirt, moisture...
- inside the transmission control unit
Localising the short-circuit - Turn ignition off.
- Pull connector that carries the faulted signal out of the transmission control unit and check contacts.
- Use a multimeter set to resistance mode to check whether the short-circuit in the cable is still present.
- Refer to circuit diagram to find out whether the short-circuit may have been caused by a connected electrical component. Fault possibilities are thus narrowed down.
- If the electrical component is intact, disconnect all electrical components from the cable. Measure resistance between the signal line and
- all ground lines
- battery ground and ground contact at transmission control unit
- all other lines at unplugged connector.
- Write down the number of contacts at which a resistance value was measured that does not comply with the correct value.
- Disconnect one after the other all connectors on which the faulted signal is present, check contacts.
- Measure into each opened connection with the multimeter before disconnecting the next plug connection and track the short-circuit to the connecting point of the electrical component (e.g. kick-down switch).
- If the measured value is correct, the short-circuit is located in the previous line section.
- Recheck all contacts of the faulted line segment and look for defective spots.
- If the fault cannot be localised, there may be a loose contact. The fault may be reproduced by shaking at the cables.
Eliminating the short-circuit - If abrasions, cable breaks, defective contacts or other defects have been detected, the affected cable or the electrical component has to be replaced.
- Eliminate the defect causing the problem.
- If you are certain that the short-circuit is not located outside the transmission control unit, replace the control unit.
- Reinstall the original transmission control unit and if the fault reoccurs then, the transmission control unit is probably defective. Order a control unit with the same item number and install it in the vehicle.
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