Defender 90 Diff Repair

Ever since we bought the 90, there has been a fault with the rear diff.  It wasn’t noticed for a long time because it was only apparent on a long run when the axle’s oil was fully warmed up.  Initially, once the rattling sound was noticed, we had thought it to be a rattle on the rear door latch, so it was ignored for a short while, seen as a niggle rather than something important, but after trying to adjust the latch and finding it to be perfect, more investigation followed.  Several long drives allowed us to tie the rattly noise down to road speed, only noticeable when on the over-run with a little bit of throttle; accelerating, driving at steady speed or full engine braking stopped it, but coasting would bring the noise whenever the axle was warm.  This pointed at the rear axle, and jacking one wheel off the ground to spin freely identified the diff as the source.  There was one knock for every turn of the diff (two turns of the road wheel), highlighting the diff carrier, its bearings or the crown wheel as the problem.

We took the 90 to our local franchise, but it turned out that our warranty was only six months, not the full Approved scheme twelve months we had understood – we had misunderstood the contractual details when we bought the car from Stratstone, and I don’t think the confusion was accidental.  One more reason I’ll never buy anything from a Stratstone dealer ever again.

I contacted Dave Ashcroft (Ashcroft Transmissions) for some advice.  Obviously, it’s not possible to do fault diagnosis over the phone, but he suggested it was a high spot on the crown wheel, and that it would eventually settle down.  So, with minds set a little more at ease, we left it a while.

The noise did not settle, though neither did it worsen.  Another contact suggested it might be the diff carrier cross pin moving freely and hitting the diff pinion head.  I jacked a wheel up and inspected the pin ends through the diff filler plug.  It’s not particularly easy to see because of the oblique angles, but I was immediately able to see that this diagnosis was spot on – the shaft was offset, protruding at one end all the way to the inner diameter of the crown gear.  There was a small point of contact damage on one edge of the pin, through there didn’t seem to be any other marking.  The pin could be slid with one finger back into its hole, but not quite evenly – it was still offset a little and reluctant to go further.

I spoke to Dave Ashcroft again (very kind of him to spend a few minutes giving advice that effectively costs him a job).  His comment was that as long as the pin wasn’t wobbly, then the diff was OK and the pin just needed resecuring, and that such a failure only resulted in damage to other parts if the pin breaks, and that would have resulted in an undriveable vehicle, so would already have been obvious.  He also suggested that if for any reason I couldn’t fit the correct retainers, a tack weld from my arc welder would be quite sufficient to hold the pin in place.  All very encouraging stuff.  So, in preparation, I went to my local supplier to check which type of securing system this diff should have.  The computerised parts system showed that this chassis number was fitted with diffs with circlips on each end of the pin.  There is an alternative with a single roll-pin that goes through a cross drilling in one end of the pin.  The clips were duly ordered and arrived after a few days.

The diff was removed in a self explanatory manner, but simply: chock the front wheels, jack up the rear and remove the rear wheels, remove the half shafts, prop shaft and the ring of bolts securing the diff to the axle, use a hammer and chisel to break the seal of the silicone between the two, and then lower the diff out.  Once on the bench, the problems were two-fold.  One, the retainer shad broken.  Two, the retainer was a roll-pin, not circlips!  There being little alternative on a Sunday afternoon, I had to take a quick drive to Halfords to buy a box of roll-pins to finish the job.  Damage checks of the diff were good.  The cross pin has a small chip on the end where it has been striking the pinion head, and there is a corresponding impact pattern on the pinion head itself (sorry about the poor focus of the photo, but the camera kept trying to focus on the wrong part of the diff).  Both are immaterial, and the parts are 100% serviceable.

The cross pin was pressed to one side and rotated to allow me to drift the remnants of the original roll-pin out.  There was no sign of the piece that would have been in the diff carrier drilling.  Thankfully, the roll-pin only passes through the open side of the carrier and through the thickness of the cross pin, but not any deeper (so the roll-pin only emanates from one side of the cross pin), as the only way to have removed any remnants from the other side of the carrier’s cross pin hole would have been to drill it out, and the shape of the carrier would have made that nigh impossible.  With the old pin parts cleared, the cross pin was carefully aligned to take the new roll-pin (4mm diameter when fitted, but a little wider before compression), which was tapped in with a hammer.  The Halfords box had a wide selection of roll-pins, and just one of them was the right length and diameter for the job, so care had to be taken to get it right first time.

With the diff sorted out, it had to be refitted.  Removing all traces of silicone from the mating face of the diff casing was easy, but doing the same to the axle was much more difficult, partly because of having to lay under the vehicle, partly because of the studs, and largely because some idiot had used an entire tube of sealant in the factory on the one diff – it was everywhere.  Once cleaned off, with the studs all wire brushed to allow easier nut fitting and to give better adhesion to the Locktite thread lock, fresh RTV sealant was applied to the flange and the diff installed.  Did I forget to mention how heavy they are when you’re lifting them into place while laying on your side?  All was going well, my son helping out and learning by usingt he electric wrench (like a windy-gun, but electrically operated) to do up the diff mounting nuts, when one of the later ones stuck on its stud and stripped the locating splines in the axle case.  The nut hadn’t even got half way along the thread.  So, the diff had to come back off again (at which point the poor lad though he’d done something wrong to have broken something and was a bit worried, so I had to tell him it was just bad luck and not his fault – the same would have happened regardless of who was tightening the nut and whatever tool they used to do it).  The stud and its hole were meticulously cleaned and JB Weld (the strongest metal epoxy I have yet found) was used to bond the stud back in place.  The diff was fitted again, and by crafty use of long nosed pliers and a ring spanner, the nut on the offending stud wound down and tightened (gripping the base of the stud until enough of the tip was exposed past the nut to grip that end) – as the nut nipped up against the diff casing, the stud was pulled tight enough that it gripped.  The JB Weld cured over night, so hopefully the stud will behave normally again, but if it slips again, I’ll just have to use the same technique with the pliers and spanner.

The half shafts were refitted after the boy had done a spotless job of cleaning away the old gaskets and muck, using new gaskets smeared with grease (seals as well as silicone, but future disassembly is so much easier and cleaner).  I did make one small mod, though – on removal, the right hand drive flange and shaft were nicely fill of oil, as were the wheel bearings, the stub axle’s seal on the shaft apparently having failed.  On the left side, though, the drive flange was bone dry and rust powder was emitted from the splines as I checked the small amount of wear on the drive flange, this side’s seal being in good order.  I consider this a design flaw.  the earlier axles with the thick drive flanges and longer plastic cones don’t have this internal seal, and that is a major factor in why their drive flanges and wheel bearings last longer.   So, with the aid of a sharp chisel, I set about wrecking both stub axle seals.  Both hubs and drive flanges will now have good lubrication from the axle’s oil.  I did clean the outside of the flange nose and the inside of the plastic cap with brake cleaner and then bonded them on with more RTV sealant, just to make sure that the oil migrating along the splines into the cap doesn’t seep out and make a mess, though.

The job was finished with the replacement of the rear brake pads.  There was plenty of material on the old pads, more than 3/4, but despite previous cttempts by stripping and cleaning, they still squealed a bit as they were released (so just with slow rotation and light pressure).  I hope the new Mintex ones are quieter.