Possible reasons for the lack of fourth gear in the Nissan Terrano automatic transmission:
Electrical problems:
*Speed sensor malfunction
* Problems with wiring to the automatic transmission selector
* Error in the automatic transmission control unit
Mechanical problems:
* Wear of clutches or steel discs
* Biting or damage to the fourth gear clutch pack
* Contaminated or low transmission fluid level
* Valve unit malfunction
Other possible reasons:
* Problems with the automatic transmission valve body
* Damage to the automatic transmission pump
* Wear of planetary gearbox or final drive
Diagnosis and elimination:
1. Checking the transmission fluid level and condition: Check the fluid level and make sure it is not cloudy or dark.
2. Reading Automatic Transmission Trouble Codes: Connect a scan tool to the vehicle to read any stored trouble codes that may indicate a problem.
3. Checking the speed sensor: Make sure that the speed sensor is working correctly and that it accurately measures the vehicle speed.
4. Wiring Inspection: Inspect the wiring leading to the automatic transmission selector for damage or corrosion.
5. Reset the automatic transmission system: Reset the automatic transmission system by following the procedure specified in the vehicle’s owner’s manual.
6. Disassembling the automatic transmission: If other troubleshooting has failed, it may be necessary to disassemble the automatic transmission to determine the exact reason for the lack of fourth gear.
Note: Do not attempt to repair an automatic transmission yourself unless you have the necessary skills and tools. Contact an authorized service center for professional diagnosis and repair.