Description

There may be several reasons why a short-circuit to 24 V is detected for a signal:

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

1. Turn ignition off.
2. Pull connector that carries the faulted signal out of the transmission control unit and check contacts.
3. Use a multimeter set to resistance mode to check whether the short-circuit in the cable is still present.
4. 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.
5. If the electrical component is intact, disconnect all electrical components from the cable. Measure resistance between the signal line and
   • all lines conducting 24 V
   • positive pole of battery
   • supply voltage of the transmission control unit
   • all other lines at unplugged connector
     .
6. Write down the number of contacts at which a resistance value was measured that does not comply with the correct value.
7. Disconnect one after the other all connectors on which the faulted signal is present, check contacts.
8. 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).
9. If the measured value is correct, the short-circuit is located in the previous line section.
10. Recheck all contacts of the faulted line segment and find defective spots.
11. If the fault can not 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.

See also:

		Aftersales service index