Mercedes-Benz W126 automatic missing fourth gear
Why does the W126 automatic not go into fourth gear?
The most common cause is too little automatic transmission fluid. Low fluid can first take away fourth gear and then third as well. Check the fluid level and the condition of the filter first, before suspecting the internals of the transmission.
Does adding oil help when fourth gear has disappeared?
Sometimes it helps: the transmission was short of oil, and after topping up the forward gears returned and the level rose above the mark on the dipstick. Drivability improves, but the condition of the transmission remains unclear, so keep an eye on how it works.
What is found in the transmission if oil is not the cause?
The fourth-gear bands may be worn out. A worn seal can also let pressure leak, so the transmission does not shift into fourth or downshift on throttle. When cold the shift may slip and improve as it warms up. The repair requires opening the transmission.