ZF 4HP20 Autobox Strip
Introduction
This guide shows how to dismantle a ZF 4HP20 automatic gearbox as fitted to the Citroen Xantia V6, late V6 Citroen XMs, Citroen C5 and the Peugeot 406 V6 amongst others. I understand the 4HP20 is also used in some Mercedes, Fiat and Alfa Romeo vehicles.
These 'boxes have a habit of dying suddenly at high mileages. Despite being electronically controlled and fully auto-adaptive, it is conventianal mechanically and has the distinct advantage of not using brake bands. It is relatively easy to dismantle.
The 'box being stripped in this guide is believed to be good, however it is from an accident damaged vehicle and has a broken casting, making it suitable only for spares. The strip of this particular 'box is a prelude to stripping my own Xantia V6 gearbox which has failed with no drive.
Cautions
All information in this guide is presented as-is with no warranty whatsoever. The work is all my own and has been carried out with no reference to official ZF service data. No responsibility can be taken for any loss, damage or injury resulting from following this guide.
The 4HP20 is a very heavy gearbox, weighing in at nearly 90Kg. The risk of injury is very real and operations involving moving the gearbox around the workshop must be taken with care. The use of a hoist or block and tackle is highly recommended.
There is a lot of oil in circulation and not all of it can be drained. Therefore dismantling is a messy business. Waste oil and rags used should be disposed of in an environmentally friendly way.
All work must be undertaken in conditions of extreme clenliness.
Special Tools
Most of the disassembly and reassembly can be undertaken with normal workshop tools; however one retaining nut is special and needs a castellated socket for removal if the gearbox is to be totally dismantled.
In addition to normal workshop tools, a set of high quality "Torx" bits, in particular a T27, will be required.
Special tools are needed to ensure the final drive oil seals are not damaged upon reassembly. I've used official Citroen tools but it should be possible to improvise suitable protective sleeves.
A special torque convertor retaining tool is highly recommended but a large bolt will suffice.