Depends. Is it a 3.6 litre TDV8 (= ZF 6HP26/ZF Lifeguard Fluid 6) or a 4.4 litre TDV8 (= ZF 8HP70/ZF Lifeguard Fluid 8)? Phil

Or the output shaft spline has failed through fretting corrosion. Hopefully just your transfer box, though…

That diagram shows the straight-forward 5HP24 system from BMW. The circuit for the 6HP26 in the Disco 3 is a good deal more complicated :) Phil

I’m sure that you are already aware but, just in case, it might be worth highlighting that the BMW-engined petrol and diesel L322 final drives have a different ratio. M62 4.4 V8 = 3.727 (41T/11T) M57 3.0 TD6 = 4.100 (41T/10T) Phil

From JPAT: Shaft, Direct clutch hub and shaft 191mm long, JPAT p/n N224 29 04 = £93.75 +VAT Phil P.S. We should just keep in mind that we don't know for certain that this component has failed and caused the issue with 4th & 5th gears so I certainly wouldn't recommend shelling out for one...

I have an account with JPAT and I’m ordering some (ZF) parts from them tomorrow so I’ll ask them to quote for a 5L40-E direct clutch hub/shaft at the same time. Phil

Well that didn’t take a lot of persuading! Hopefully you will have received the repair manual(s) by now. 100% agree regarding the feeling you’ll have after fixing your own transmission – you’ll never worry about it again. Sonnax produce a large selection of replacement valves for the 5L40-E...

If you message me with your email address I can send you the official repair manual. The cheapest method of dealing with the piston seals is probably to use the Seal-e-Zee cones and freezing spray. https://www.hydratest.co.uk/transmission-tools Phil

If you’re capable of undertaking a manual gearbox conversion then you’re more than capable of re-manufacturing a 5L40-E transmission. Why don’t you rebuild your transmission yourself to ensure that, this time, it’s done properly? If you get the valve body assembly fitted with Sonnax valves this...

The bolt pattern for the E53’s NV125 transfer box is not the same as the NV225 (which was unique to the L322). I think, along with the electrical/electronic/control issues mentioned by holidaychicken, such a conversion would be extremely challenging. Phil

https://www.landyzone.co.uk/land-rover/is-it-the-dreaded-box.222192/page-2#post-2525905 Phil

Have you lost 4th AND 5th gears? It's highly significant if your transmission can't select 5th either (using manual mode). The torque converter lock-up clutch is automatically disengaged if failsafe mode is flagged. Phil

For the ZF 5HP24 transmission, the relevant dimension from the bellhousing face to the four mounting pads is 25mm, when the torque converter is installed correctly. To prevent damage to the input seal, caused by converter movement during shipping, my transmissions are all supplied with the...

Does this happen in Sport mode as well as ‘normal’ mode? Are all the brake lights working? Phil

Not sure if this is of any use : Phil

Possibly a problem with the Manual/Sport switch on the gear selector lever? Phil

Possibly number 5 on the list of standard 6HP26 failures? https://www.landyzone.co.uk/land-rover/tdv8-56-plate-gearbox-knackered-options.330242/#post-4260591 Phil

I once re-manufactured a transmission for a guy who had been driving round with his Range Rover transmission in failsafe mode (i.e. stuck in 4th gear) for 9 months ! If any parts inside the inhibitor switch are broken then you can’t get replacements. However, as a new switch costs £600 from...

You could first check that the pins look okay in the connector but, failing that, you could drill out the seven rivets and check that the five wiper fingers are all in place and the copper strips are clean enough to allow a good electrical contact Phil

It’s important to distinguish between the aerodynamic resistance due to drag, which is a force, and the power required to overcome that drag Power = Force x Velocity So, the power required at the wheels to overcome the aerodynamic drag force is directly proportional to the drag coefficient and...