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 [emoji848] . .

Sent from my SM-G930F using Tapatalk

[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. . .[emoji6]

Sent from my SM-G930F using Tapatalk

[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

Sent from my SM-G986B using Tapatalk

[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. .

Sent from my SM-G930F using Tapatalk

[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.

Sent from my SM-G986B using Tapatalk

[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.

[MudRunnerTD]

I would put my money on Hubs too. Much more likely than a transfer case failure. There ain't much in a hub and a little front diff wind up and they will jump a tooth.

[PeeBee]

I can't see this happening with chains jumping cogs or gears jumping out of mesh, you would have serious issues and it would happen more often. I am thinking its transfer case wind up and its releasing as a reaction thru the wheels/suspension. I had a pinion bearing fail on a landcruiser and it went bang in an instant, however didn't manifest into a total failure until 200klm down the road. I would also revisit the pinion bearings for movement, however would think the gear set would be well and truelly gone by then, as mine was.

[Winnie]

I would put my money on Hubs too. Much more likely than a transfer case failure. There ain't much in a hub and a little front diff wind up and they will jump a tooth.

I have put a new set of hubs in it and it didn't make a difference. If it was a hub surely they would show some wear? I'll pull them off to confirm.


I can't see this happening with chains jumping cogs or gears jumping out of mesh, you would have serious issues and it would happen more often. I am thinking its transfer case wind up and its releasing as a reaction thru the wheels/suspension. I had a pinion bearing fail on a landcruiser and it went bang in an instant, however didn't manifest into a total failure until 200klm down the road. I would also revisit the pinion bearings for movement, however would think the gear set would be well and truelly gone by then, as mine was.

Yeah I've been thinking wind up, but why would this be happening? It has been hapenning for about 3 years randomly, I've done a lot of 4wding since and other than when it skips, it drives great.

Sent from my SM-G986B using Tapatalk

[PeeBee]

You don't need much difference in tyre diameter from wear to have this happen, and if you are crawling around on grippy surfaces the wind up will build and release on the next bit of slippery stuff. I used to get this climbing out of burgoynes with my tyres that had worn more on the rears than the fronts, didn't get the release bang when driving , but needed to reverse back and forth a few times and then sometimes it was a significant clunk as the transfer case was moved into 2wd. I would check the tread depth front to rear.

[mudski]

When you remove the hubs take a pic of the gear in the selector part in both lock and unlock positions.
The hubs you bought were new? Genuine? I have pics somewhere of the differences between oem and the Chinese hubs. The gears were noticeably different.


Sent from my iPhone using Tapatalk

[Winnie]

You don't need much difference in tyre diameter from wear to have this happen, and if you are crawling around on grippy surfaces the wind up will build and release on the next bit of slippery stuff. I used to get this climbing out of burgoynes with my tyres that had worn more on the rears than the fronts, didn't get the release bang when driving , but needed to reverse back and forth a few times and then sometimes it was a significant clunk as the transfer case was moved into 2wd. I would check the tread depth front to rear.That would have made sense a couple of years ago as the tyres were definitely unevenly worn. But I have brand new tyres now and it hasn't made a difference.

Sent from my SM-G986B using Tapatalk

[mudski]

Found them.

Check out the teeth size differences between Chinese and OEM. No wonder the Chinese knock offs were jumping. The last pic you can see the wear on the teeth from it. So also make sure the outer gear is throwing in enough to properly mess with the gear in the axle. Which as said this can be adjusted if its not. Or you want to make hem throw that little bit more.

[Winnie]

Thanks Mudski, I didn't know they were adjustable.
I replaced the old ones with 2nd hand genuine.

Sent from my SM-G986B using Tapatalk

[mudski]

Thanks Mudski, I didn't know they were adjustable.
I replaced the old ones with 2nd hand genuine.

Sent from my SM-G986B using Tapatalk

Yeah those six ( i think there is ) pins that hold the gear in the hub, come out and then that gear winds out, its on a thread. So all you need to do is wind it out bit and put the pins back in. Your issue might be a simple as not enough throw in that gear to mesh with the other gear on the axle.