Possible reasons for the lack of reverse gear after automatic transmission repair:
Mechanical problems:
* Incorrect torque converter installation
* Faulty reverse planetary gearbox
*Damage to reverse shaft
* Incorrect adjustment of the reverse belt
Hydraulic problems:
* Faulty reverse gear selector valve
* Blocking of hydraulic channels
*Low transmission fluid level
* Contaminated transmission fluid
Electrical problems:
* Faulty reverse gear selection solenoid
* Faulty output shaft speed sensor
* Damage to wiring
Other problems:
* Incorrect automatic transmission installation
* Faulty transmission ECU
* Damage to the gear selector mechanism
Diagnosis and elimination:
1. Check the level and condition of the transmission fluid.
2. Check electrical connections, including solenoids and sensors.
3. Disconnect the gear selector cable and make sure that the automatic transmission shift lever moves to the reverse position.
4. If the gear shift lever does not move, check the gear selector mechanism and wiring.
5. If the shift lever moves, remove the transmission and further inspect the mechanical, hydraulic and electrical components.
6. Eliminate any detected faults and assemble the automatic transmission.
7. Add new transmission fluid and check its level.
8. Connect the battery and start the engine.
9. Check the operation of the reverse gear.
If the problem persists, more in-depth diagnosis and repair may be required. It is recommended that you consult a qualified mechanic.