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A leaking transfer case on our Dmax

Not long after returning from our 3 month trip up north, I noticed a bit of oil under the Dmax, near the back of the transfer case. Asides from the turbo replacement when I first got it, its the first thing that’s actually gone wrong with our new 4WD. Granted it had only done about 27,000km and to have an oil leak so early is pretty poor.

We do everything possible to care for our gear, and so to have this happen was a bit of a surprise. There’s 12 ways to wear your 4WD out faster, and we ensure none of them happen!

A quick look made it pretty obvious that it was leaking out of the rear transfer case seal, where the tail shaft bolts on. I checked the oil level, and monitored it for a few weeks, noticing that it was leaking extremely slowly – under 50ml in 1000km. This was the second, and most annoying contributor to our list of Dmax problems.

Dmax oil leak

The typical oil droplet, forming at the back of the transfer case where the tail shaft starts

Isuzu Dmax transfer case and tail shaft design

The Dmax runs a different tail shaft than what I am used to. I’ve always had two piece tail shafts with the spline in the middle, but the automatic Dmax’s have a one piece tail shaft that has a spline going into the transfer case.

As your rear diff goes up and down it slides in and out of the transfer case, which means not only is the seal taking rotating pressure, but its taking pressure in and out as the shaft slides back and forth. As a fitter this seems awfully daft to me, as every single corrugation you drive on is going to cause the shaft to move slightly, and over time will wear the seal.

Normally the practice is to have an oil seal rotating on the same piece of shaft all the time, with no axial movement. I believe this is common amongst other modern 4WD’s, but there are still plenty that don’t have this setup, and they’d be better off for it.

We use our Isuzu Dmax for touring and as a result need a reliable 4WD. The idea of it breaking down in the middle of no where just wouldn’t cut it!

Isuzu warranty (second seal; first replacement)

The first point of call was Isuzu, and they replaced the seal free of charge in its 30,000km service. I had a good discussion to the foreman there, who said it was leaking because the tail shaft was sticking out further than normal, due to the GVM upgrade.

Initially I was a bit sceptical of his conclusion, but after some of my own research I found lifts did seem to contribute to the likely hood of a leak. It’s easy for a company to blame aftermarket work on a problem to weasel out of claims.

I was shown a number of other Dmax’s, running different lifts and even factory ones. It was obvious the tail shaft sat about 10 – 15mm in further than mine, so I didn’t have much of a leg to stand on. Aftermarket modifications hugely affect your ability to claim warranty when something goes wrong!

The weird thing though, was it had a drop of oil already formed when I picked the car up from its service, and it kept leaking. In hindsight, I should have left it there to be done again!

Seal number 3

Not being happy with the seal still leaking, I spoke to Isuzu again, who had another look at the vehicle, and essentially refused to work on the seal again. They blamed ARB for the lift. Ironically, I couldn’t find anyone else who had issues with the seal leaking due to lifts, and mine was a tiny lift compared to others.

After speaking to a well reputed mobile mechanic, I purchased two brand new seals from Isuzu and got it replaced, on my driveway. Again, it still leaked.

Interestingly though, the mechanic found a fair bit of metal fragments (some up to 3mm long) on the drain magnet. I asked Isuzu about it, and they said it was pretty normal. I suspect its probably a bit beyond normal, but again, given the lift I had no leg to stand on.

By now, I’d gotten pretty tired of topping the transfer case up, and having oil stains all over my driveway.

Dmax transfer case seal

The offending transfer case seal

Transfer case metal

Chunks of metal from the transfer case

Approaching ARB

Of course, the only other business that could help was ARB, who installed the GVM upgrade in the first place, and changed the tail shaft position.

They’d never seen the issue before, and wanted to see the vehicle. In the mean time, I did some research and found a lot of people were running spacers for bigger lifts. This pushes the tail shaft in further, and they are marketed at stopping the seal from weeping, so it was worth a shot.

ARB purchased a 15mm spacer (from Ironman ironically), and I went in one morning and installed it with one of their mechanics. This pushed the tail shaft into the transfer case 15mm, and we measured that it had a further 25mm that it could slide in before bottoming out. I was especially concerned about this, as I didn’t want the tail shaft to bottom out on a big bump.

Interestingly, when you lifted the vehicle up and let the diff hang down as low as it would, the tail shaft position didn’t change by more than 2mm. As the diff drops, it goes forward as well, maintaining the same position.

It would seem very unlikely that its ever going to bottom out then, but I will monitor it with more weight when we are fully loaded up, and hooked up to the trailer.

Dmax oil leak

The tail shaft position with the spacer in place

Worn transfer case bush

The one thing I was a little concerned about was whether the bush locating the tail shaft into the transfer case was worn, as this would allow the tail shaft to sag and the weight applied to the seal, causing it to stretch or wear and then leak. The spacer meant that the tail shaft was sitting 15mm in further, and theoretically had 15mm more bush to run on, but I wasn’t sure if the seal was already damaged, or whether the bush would still need replacing.

I went through all of the spares list, and drawings from Isuzu, and it makes no mention of this white metal bush at all. I wondered if all the corrugations up north might have worn it down given the tail shaft wasn’t in all the way.

Corrugations in the Dmax

We’ve done lots of corrugations in the Dmax, and I can only imagine how much the shaft moves in and out

Seal #4 and #5

At the 40k service (34k on the clock, but due by time not km) I got the same mobile mechanic to replace the seal once more, thinking it may have been damaged from not having the spacer in. I got home from work, and saw a small puddle of oil under the transfer case leak point, but assumed it was the same as usual.

The day after the service, we hooked the camper trailer up, and headed south to Nannup for the long weekend. I stopped to check on a noise after about 15 minutes as I could hear a tinging (turned out to be a mud flap washer loose), and then we motored onto Bunbury. I’d been busting for a pee, and stopped at the Shell fuel station 5km out of Bunbury. To my absolute horror, I saw dripping from under the car on the way back. 

I stuck my head under the car, and was shocked. Oil was everywhere. Literally everywhere. My rear diff was coated, the suspension, fuel tank, exhaust muffler, spare tyre and even the whole draw bar and front of my camper trailer was coated.

It was so bad that I initially started looking for leaks elsewhere. With only a 100ml syringe available to transfer oil, it took nearly 10 back and forths to fill the transfer case up. It was pretty much bone dry, and I have no idea for how long.

Oil absolutely everywhere

Oil sprayed everywhere underneath

At 5:08PM on Friday afternoon, and the start of a long weekend, I googled Bunbury Isuzu, with no idea if there was one or not. Picton Isuzu popped up, only 6km, away, and despite showing closed on Google, someone answered. I got put through to parts, who said there was no way they’d have a seal. However, my luck continued; he had 3 available, and agreed to stay open for me despite it being past their normal closed time.

I arrived a few minutes later, and had a chat. One of their mechanics was still there, and even better agreed to replace it then and there. Seriously. How’s that for service. At 6:30PM he’d finished, and told me to go for a 15km drive and come back to ensure it wasn’t leaking. Even better, he suggested the seal was pushed too far in on installation, and that was why it was leaking.

Dmax transfer case

Looking into the transfer case with the seal removed

Dmax tail shaft

The tail shaft removed, in good condition

Picton Isuzu Review

Getting the seal replaced by Picton Isuzu well after hours on a Friday before a long weekend. Absolute legends

15km down, and we arrived to zero oil dripping out. So far so good. We continued on to Nannup, arriving at nearly 8PM with two tired and cranky children. Again, no obvious sign of oil under the car.

No more oil leak

Since then, I’ve done about 10,000km and only noticed one small drop on the bottom of the transfer case, which never returned. Normally you’d see a drop every time you finished driving. I’ve checked the level, and its all good. Fingers crossed it continues this way, and that the 2nd, 3rd and 4th seal put in were just not done correctly as the guys at Picton Isuzu suggested.

I also dearly hope no damage has been done internally to the transfer case, as I have no idea how many km’s it had done with little oil inside. 

Plenty of trips in the Dmax

So far, no further leak and lots of fun with the Dmax

So, if you have a Dmax with a rear transfer case leak, you’ll have some ideas to pursue from reading this!

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12 comments… add one
  • Joshua December 28, 2019, 2:24 PM

    Hey mate. I’m having the exact same problems with my dmax. Currently on the 4th seal and started leaking again today. How is yours going now?


  • Aaron Schubert December 28, 2019, 6:56 PM

    Hey mate,

    Such a pain in the backside isn’t it! Mines been good as gold since the spacer was put in and the seal was done properly. Are you doing it yourself? As long as the faces are clean, there’s no issues with the breather, no excess play in the shaft and the seal is put in the correct location it should be fine. Try and install the seal a bit further out and see how you go. I’ve done about 4000km trouble free

    All the best

  • HARRY February 27, 2020, 2:35 PM

    I’ve just got the same prob mate. Where do you check oil for the transfer case? Cheers


  • Aaron Schubert February 27, 2020, 6:32 PM

    Hey Harry,

    There are two bolts, about 24mm diameter. The lower one at the bottom of the transfer case is the drain one, and the upper is the full. Basically keep filling until it runs out the plug level. From memory it only takes about 1 litre of oil

    All the best

  • Paul February 28, 2020, 8:43 AM

    Same problem… replaced seal about 4 times. Replaced the casing on the transfer case. Replaced the yolk on the tail shaft. All under warranty under Isuzu. Told me it’s the lift kit -and they put shins on the diff to change the pin angle and I still have the same problems and Isuzu does not want to cover any more under warranty and consider it’s not their problem?

  • Aaron Schubert February 28, 2020, 8:03 PM

    Hey Paul,

    What a pain. How far in is the tailshaft? Did you do the seals at all? Mine is resolved so far by not installing the seal in so far.

    All the best mate

  • Daniel Mcpherson May 27, 2020, 12:49 PM

    Hi would you have the seal size as I have a customer with a leaking seal as well and cant find a part number without going to Isuzu.

  • Aaron Schubert May 27, 2020, 6:22 PM

    Hi Daniel,

    You have to go to Isuzu. It’s not a normal oil seal – it has weird grooves in it. I tried getting one through the various bearing suppliers and had no luck.

    All the best

  • Nathan June 22, 2020, 6:40 PM

    Howdy guys – very interesting reading as i am having the same issue…….
    Replaced seal 5+ times placing dead square in housing (evan tried putting two in at once), 15mm ironman tailshaft spacer (has 2″ lift), evan tried thicker (heavier weight) oil.
    Have just spent $300+ on new tailshaft yoke and unijoint (and another new seal!) to try and get new seal surface, but having read one of you guys replaced the housing and yoke under warrenty im afraid to evan try it……..
    This will sound silly but where is the transfer case breather? I have found the 3/16″ hose from plastic 4wd select box on back of transfer case which vents back into top of automatic transmission but i cannot get air in either direction back into the transfer case through this hose.
    I am going to drill and tap the fill plug and fit a breather pipe in a last effort before fitting the new tailshaft yoke in case it is just a pressure venting issue…..

  • Aaron Schubert June 22, 2020, 7:22 PM

    Hey Nathan,

    Sorry to hear you are having issues. It’s such a pain. The breather is on top of the transfer case. Breathing into the plastic box pipe shouldn’t do anything – I confirmed this with my dealer.

    When you put the seals in, just put them a tiny bit in, and that might do the trick

    I still think its a dumb design having a seal that has to work in both directions

    All the best mate

  • Peter July 31, 2020, 12:41 PM

    Just for interest, refer “slip yoke eliminator” on new process/venture transfer cases. NP231.

  • Aaron Schubert July 31, 2020, 3:44 PM

    Hey Peter,

    This looks interesting. Have you fitted one?

    All the best

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