ZF6HP21 No forward drive, only reverse.

[Rhys01 0]

Hi guys, I'm after a bit of advice from the transmission folks. I have a ZF6HP21 in my 335i with 100,000km on it. I have just given the valve body an upgrade with a Sonnax zip kit, new ZF solenoids, new separator plate and replacement mechatronic seals. As well as new ZF Lifeguard 6 Fluid and transmission pan, as I was having a few harsh shifts and delayed engagement in gears 5 and 6. 

The car is all assembled again and the transmission has been filled using the ZF procedure, adaptations have been reset and there are no fault codes in the DME or EGS. I backed the car out of the driveway and selected drive and there is no drive at all in foward, it seems to engage in foward momentarily then disengages and goes back to park. However reverse still works normally.

After reversing it down the road to turn around and back into the driveway, Ista D and INPA both show no faults in any of the Ecu's, transmission fluid level is normal, all the solenoids function normally on INPA, the valve body passed all of its vacuum tests before reinstallation. The Air test of the clutches was satisfactory. I am currently suspecting an electronic issue with the TCU however with no faults logged it's proving to be challenging to diagnose. 

If anyone has had a similar issue I'd love to know what your fix was, or just some guidance on where to go next, as I'm at a bit of an end now, if I can't find a fix soon it may have to go the gearbox specialists for a complete strip down and rebuild. 

[KwS 2425]

You seem well sorted, but ill ask anyway, did you fill the trans, run it to temp and then fill it again when running at temp? If you filled it with the engine off, it'll be low by a couple of litres.

I hate to say it but it sounds like something you have done isnt working right and its leaking off pressure in the trans (especially since it didnt happen beforehand). Might be worth catching the fluid, dropping the pan and valvebody and making sure everything is how it should be?

[Rhys01 0]

Hi thanks for the reply, yeah I have done the fluid level three times before pulling it apart again. I have had the valve body apart again today. Everything looks fine and tests satisfactory, all the part numbers of the seals etc all match up and seat perfect, I then reassembled the car again with still the same issue. The car is going to go to the transmission shop soon, I'll pull the valve body again tomorrow reinspect a third time, and make sure nothing is incorrect again and its all as it should be. Before reassembling again and taking it in on a tow truck.  Thanks. 

[XP93 0]

Hi. Did you manage to sort out your problem? I seem to be having the same issue. 

[smiddy 76]

Some of the Asian produced oil pans have/had, smaller oil pickup tube diameter, not allowing enough fluid flow for forward drive.