Strange goings on when driving downhill under load

[Winnie]

Hi all,

I've been chasing this problem for a few years now in my TD42 GQ, when driving downhill under load with no acceleration, ie driving under engine braking, occasionally there will be a loud clunk and it feels like the car slips out of gear for a split second before going back to normal. There is no warning that it is going to happen and it scares the shit out of me every time. It happens in both low range and high range, but only when in 4WD, usually the steeper the hill the more frequently it will happen. Worst case once every 5 mins or so.
I first thought it may have been a free wheeling hub and because I had some other issues with one, I replaced the pair and the problem persisted. I then thought possibly a CV. I did a GU diff upgrade with new CVs and axles and the problem has not changed. I did reuse the original diff gears but inspected them and they looked brand new. The ARB air locker in the front is about 6 years old.
I've got no idea where to look next, has anybody ever experienced this or have any idea what it could be? I am at a complete loss.

[Rossco]

Hmm sounds weird, only a theory but maybe chain drive in transfer is slipping? I know they are usually bullet proof but could be culprit . .

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[Winnie]

I've never seen inside the transfer case so no idea what's going on in there.
Could that explain why it happens in both 4H and 4L? Are you telling me that the transfer case will have to be opened up, at which point it would be madness to not install some reduction gears while there?

[Rossco]

Certainly no expert on gearboxes but think would make sense only in 4H and 4L. . . I've never heard of the chain drive giving trouble but guess it definatly could fail or give trouble. . They are a bloody big wide chain tho. . And yeah be crazy not to put in reductions damit. . .

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[mudnut]

The chains in both transfer cases I've had in the Patrol slap like buggery when going down hill, but it always feels solid. The back lash is considerable, but I've never experience a feeling of disconnection in the drive train.

Does the selection lever move at all when it occurs, Winnie?

[Winnie]

Not that I have noticed

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[Rossco]

Doing a quick bit of research looks like the chain drives the front wheels so I guess going downhill and slips a tooth or two back wheels would slide down and feel like no drive? Also looks like they can stretch and give trouble only other cause can think of is damaged gears in transfer slipping every now and then maybe. .

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[Plasnart]

Hey Winnie. I've had this exact symptom happen in mine (2001 TD42T) some years ago. Yeah it's really crunchy and not nice. It worried me a lot but hasn't done it for maybe 7 years now and I havent looked into it.

Something else mine does though is pop completely out of 1st gear downhill under engine braking load. I have to hold my 1st gear in downhill which is not really recommended and can be dangerous driving one handed in dicey downhill situations. Has done this since I bought it at 6 years old. Haven't fixed it as I'm still waiting for 5th to sh!t itself then I'll do both but 5th keeps on keeping on! I wonder if it's a synchro problem as my 1st is difficult to engage unless going REALLY slow, like 2kmh or less. I would have thought it'd slip in below 10kmh? Not sure but I've lived with it as it's only an issue downhill, not uphill or under acceletation.

[Winnie]

Hmm it really feels like it's only the front wheels that slip, the rear feels solid as.

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[mudski]

My first thought was hubs. I had the exact same thing with my Patrol, exactly...only in 4wd and during a decent. The sound is horrible.

The issue with mine was the chinese knock off free wheeling hubs. The gears in the hubs weren't meshing right and jumping when under load. Put some genuine hubs in and all was good. Seems not the case with you.

The only thing I can suggest is maybe seeing if the gear thats in the hub is throwing far enough to lock properly onto the gear on the axle. That gear can be adjusted by removing the pins in the hub then turning the gear as its kind of on a thread.

Other than that, you are welcome to borrow my hubs and see if it makes a difference if you like. Just to 100% knocked out its the hubs. My Patrol doesn't get driven a lot so its no biggie to me.