Possible reasons for speed loss on an automatic transmission:
Mechanical faults:
* Wear of friction discs
* Damage to the torque converter
* Belt or chain slippage
* Shift solenoids malfunction
*Damage to gearbox housing
Electrical faults:
* Damage to speed sensors
*Transmission control module (TCM) malfunction
* Short circuit in electrical wiring
Fluid problems:
*Low fluid level in gearbox
* Contaminated or degraded fluid
* Fluid leak
Other reasons:
* Vehicle electronics control
* Engine problems (eg lack of power)
* Improper maintenance or operation
Diagnostics and repair:
1. Check the level and condition of the fluid in the gearbox.
2. Scan trouble codes (DTC) using a diagnostic scanner. DTCs may indicate a specific component or system that is causing the problem.
3. Check speed sensors and wiring. Make sure the speed sensors are working correctly and that the wiring is not damaged.
4. Check shift solenoids and transmission control module (TCM). Replace faulty components.
5. Disassemble the transmission and inspect the friction discs, torque converter, band/chain and housing. Replace worn or damaged parts.
6. Replace the transmission fluid. Use the fluid recommended by the manufacturer.
7. Reprogram the Transmission Control Module (TCM). This may be necessary after replacing components.
Important: Do not attempt to repair the automatic transmission yourself unless you have the appropriate experience and tools. Have your vehicle diagnosed and repaired by a qualified mechanic.