4HP20 Disassembly Preparation
Removing the Torque Convertor
The torque convertor simply pulls off. Be careful to pull it straight off. Immediately rest the convertor spigot-down in a bowl to darin as much oil out of it as possible. The convertor is heavy.
Draining the oil
As the 4HP20 is "sealed for life" it may well be still full of oil and this must be drained before commencing work. If the gearbox is off the vehicle it will be necessary to raise it off the ground and the easiest way is to lift it on a hoist from the ground and pop a pan underneath.
Allow the gearbox to drain for as long as possible and tilt it to ensure as much as possible is drained out from all the nooks and crannies.
Cleaning
The entire outside of the gearbox should be thoroughly cleaned to remove every last reace of dirt, especially around the valve block cover. Any ingress of dirt can be fatal.
Removing Ancillaries
Once the oil has drained and the gearbox is externally spotlessly clean, all the ancillaries should be removed. This includes the cooler, vehicle speed sensor and any bracketry still attached to the gearbox.
The cooler is attached to the top of the gearbox with two Allen bolts. They are captive and must be unscrewed a bit at a time equally whilst alllowing the cooler to rise up. DO NOT undo one followed by the other.
Remove the cooler and plug the two threaded holes with lint-free rag to avoid the ingress of dirt.
Remove the Vehicle Speed Sensor from the Final Drive Housing. Begin by removing its securing bolt.
It is likely the top will come free and leave the gear drive in place. Simply lift it out.
Re-hoist the gearbox so that it is a few inches clear of the ground.