Hi folks,

Looking for a bit of help or shared experience on this one.

I’ve got a 2009 facelift TDV8 (digital dash) running the ZF 6HP26X box. Gearbox was always smooth apart from a slight judder in 3rd, so I decided to give it a full service — fresh ZF Lifeguard 6, a genuine Filtran plastic pan/filter, and new mechatronic seals.

When I bought the car, it already had the metal sump and separate filter fitted, so this was my first time switching it back to the proper Filtran setup.

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The issue

After the service it drives spot-on for about ten minutes, then I get:

“Gearbox Fault” and it limits to 1st 2nd and 3rd Gear.

The only code showing now is P0735 – Incorrect Gear Ratio (5th Gear).

If I restart the car, the fault clears instantly and it drives fine again until it warms up and shifts into 5th.
If I keep it in manual mode (1–4) it’s flawless.

Level was set correctly — hot fill at about 38 °C, engine running, steady dribble — added roughly 5.5 L total.

Has anyone had 5th-gear ratio faults that clear on restart after a fluid and filter change?
Wondering if this points to something like pressure loss, air in the system, or possibly an internal seal issue that’s only showing up once hot.

Would love to hear from anyone who’s had similar behaviour or ideas on what to check before I drop the pan again.

Thanks
Liam

Certainly classic symptoms of a worn rear stator bush, but starting immediately after a fluid & filter change seems unlikely. Did you change both the bridge seal and the four jump tubes?

Are you able to read the clutch adaptions at all? The measurements for the E-clutch might provide a clue as to whether the stator bush is an issue, otherwise it would mean removing the Mechatronic unit again and testing with air pressure.

Phil

Don’t think I have any way to read the clutch adaptations, I was thinking it could be the rear stator bush but was also baffled as to why it’s started after a fluid and filter change. I changed the four tube seals the double square looking one and the connector sleeve.

That along with it being absolutely fine in 4th or does it not affect 4th as much?

Traditionally, a worn rear stator bush manifests itself by the transmission refusing to engage gears 4, 5 & 6 i.e. the three ratios which use the E-clutch.

Generally, therefore, the transmission shifts through 1st, 2nd & 3rd gears but then refuses to upshift into 4th.

If you calculate the torque being transmitted by the E-clutch, in 4th gear it’s 72.6% of input torque, in 5th gear it’s 125.5% and in 6th 100%.

Theoretically, therefore, it will begin to slip in 5th gear first.

I agree that this is far from conclusive, though, and still doesn’t explain the relevance of the oil & filter change. All of the pressure solenoids are electrically ‘off’ in 5th gear, so it doesn’t really sound like a solenoid issue, either. Pressure loss combined with temperature dependency does point towards a leaking internal seal.

Phil

Thanks for the responses Phil, much appreciated and I know your are considered the all knowing of these ZF boxes so I really appreciate it.

I took it for another drive last night after leaving it for a few days to think about its actions and was solid for about 30 mins. Then the inevitable happened again. This time as I was at motorway speeds I noticed it slipping in 5th and 6th which kind of goes with what you’re it slammed its way into 3rd gear and wouldn’t let me have anything higher. No noticeable slip in 4th at all. Could it be a badly placed seal when I replaced the seals? Just seems so odd to me that it’s only occurring after a service. Albeit the existing fluid in there was very dark and red so unsure if it was genuine lifeguard 6 or not. I guess if it was old thicker fluid it could have been masking it slightly?

I think it may be time to drop the sump pan off again and take another look.

Although I keep banging on about the E-clutch, despite the fact that your 4th gear appears to be working fine, the four jump tubes seal the hydraulic feeds to the B-clutch, the C-brake and the D-brake (an explanation as to why there are two feeds for the D-brake appears below, for those dull enough to be interested).



The A-clutch and E-clutch don’t have a flexible seal between the Mechatronic unit and the main case, they just rely on the quality of the machined surface to provide a seal. I don’t think, therefore, that it’s related to the seals that you replaced. I do, however, think it might be worth removing the Mechatronic unit again to test the E-clutch feed with air pressure to see whether it’s leaking or not.





Maybe the relevance of the oil and filter change is, as you say, that the new fluid has a lower viscosity than the fluid you removed, making it more prone to leak at higher temperatures. If the fluid you took out was red in colour then it certainly wasn't Lifeguard Fluid 6. The genuine fluid is stupidly expensive, so it’s easy to imagine that a previous owner might have used a cheaper alternative.

Phil



Why does the D-brake have two feeds, D1 & D2?

The D-brake is used in Reverse and 1st gears. In Reverse it has to react 4.924 x engine torque. It is therefore designed with a high torque capacity.

This is a problem when the transmission down-sequences through the gears as the vehicle comes to a halt. The 2-1 downshift can come in with a bang when the D-brake engages because its torque capacity is so high.

In the 5-speed 5HP24 transmission, ZF got around this issue by using a sprag clutch, which automatically disengages the drive in 1st gear on the overrun i.e. the vehicle freewheels to a halt after the 2-1 rollout shift.

ZF were keen to phase out the use of sprag clutches due to their cost so, for the 6-speed transmission, they got around the 2-1 downshift issue by having a second hydraulic feed (D2) to the D-brake which pushes in the opposite direction i.e. holds the clutch piston off. Feeding pressure to D1 & D2 at the same time (rather than to just D1) reduces the brake’s torque capacity by 35%.