The gearbox in the Peugeot 806 is the "AL4".
This is supposedly a "Sealed for Life" gearbox and the manufacturer does not even supply a "Dip Stick" on these gearboxes as they state it will last a life time. (the problem is, how long is a "Life time")
The gearbox is controlled by a set of electric valves (as opposed to the usual hydraulic ones).
Gearbox Oil pressure is governed by these valves and when they become worn, they cannot supply the correct amount of pressure. Thus the gearbox computer senses that there is a pressure fault and imposes "Limp Home Mode".
A worn Torque Convertor can also cause the same symptoms as it fails to deliver the fluid drive.
The torque convertor "Slip" is noticeable in 1st and 2nd gear. If you watch your engine temperature gauge as you drive up a steep hill, you will notice an increase in temperature during 1st and 2nd gear travel which is due to the convertor slipping.
Once the gearbox gets into 3rd gear, a "Pawl" in the gearbox engages and the gearbox goes into direct drive and eliminates the torque convertor slipping, the result being that if the steep hill was long enough, you would then see the engine temperature start to drop when 3rd gear was reached.
So the faulty gearbox is either down to Torque Convertor wear or Electric Valve wear.
The valve wear can be remedied by buying a refurbished valve block (aprox £350 exchange) which is possible to do if you are handy with the tools. (there is also a kit available whoch allows you to change the pressure valve only, but requires a bit of skill with a reaming kit and a little more know how).
The Torque Convertor slip can only be remedied by exchanging the unit which means a Gearbox Off job.
Either way it is an expensive remedy and you have to consider whether to just cut your losses and get rid of the vehicle.
The Pegeot/Citroen AL4 gearboxes are notoriously poor and should be avoided at all costs.
Stephen H, July 2012
