Diff Difference

Ihad a phone call from a chap who said that his Series III started jumping when four-wheel drive was engaged. Trying to put visions of Skippy the Bush Kangaroo out of my mind I arranged for him to drop it off for a look. The selectable four-wheel drive system on Series vehicles is a primitive, but robust affair which mechanically locks the front and rear axles together when engaged. It has no means of compensating for any differences in rotational speed between the two axles, which is why it should never be engaged on tarmac or other hard, grippy surfaces. If you select four-wheel drive on tarmac in a Series Land Rover you will probably notice after a short time that the vehicle will stop rolling freely and start to feel as though the brakes are dragging slightly. This is transmission wind-up, caused by slight differences in tyre outside diameter between front and rear: the greater the difference in tread depth between the front and rear tyres, the more quickly the transmission will load up. Driving on tarmac in this state puts an abnormal load on the transfer box internals, propshafts, differentials and halfshafts, and eventually the weakest component will let go.

However, this degree of front-rear imbalance should not cause a Series III to jump, or hop or skip for that matter. If transmission wind-up is serious enough to manifest itself as a tendency for one wheel to momentarily lock up and then release as the transmission unwinds (which is what gives the jumping effect) there are two possible causes: either the vehicle has two or more different sizes of tyre fitted (unusual but not impossible, especially on ex-farm Land Rovers) or the front and rear differentials are of different ratios.

Ignoring the very early long-nose Series I differentials (4.88:1) and the Rover P5 units (4.1:1), the differentials to be found on Series Land Rovers come in two ratios. The standard ratio, used on pretty much everything except the 109 V8, was 4.7:1. The 109 V8 was 3.54:1, which is the same as all the coil-sprung vehicles spawned by the original Range Rover (Range Rover Classic, Ninety/one Ten/defender, Discovery 1). Up to 1970 the 4.7 Series differential had a separate seal carrier secured by a ring of six bolts which makes it easily identifiable, but following the launch of the Range Rover, the Series differential was redesigned to use the same casing as the Range Rover, complete with two threaded holes underneath for the Range Rover track rod guard. The two types of differential are externally identical and this can result in differentials being replaced by those of the wrong ratio.

To establish which ratio you have is simple enough. Chock the wheels at one end, then jack up one wheel at the other, leaving the fourth wheel in contact with the ground. Make a chalk mark at the top of the tyre on that wheel. Now rotate that wheel through one complete revolution, while counting the number of turns of the propshaft. Take that number, double it and you will have the approximate ratio. It isn’t scientifically exact, but the difference between 4.7 ratio (two and a third turns)

“It was great on road but tried to tear itself in two whenever four-wheel drive was selected”

and 3.54 (one and three quarter turns) is large enough that you are unlikely to confuse the two.

In the case of this particular Series III I didn’t even have to go as far as counting turns to get a pretty big clue as to the cause of the problem. The rear differential still had the track rod guard bolted to it, which meant it had to be a 3.54 unit from the front of something coil-sprung. Checking the ratios confirmed this. Changing the rear differential on a short wheelbase Series vehicle is the work of minutes as the halfshafts can be withdrawn without disturbing anything else. To change a front differential is rather trickier, involving either completely dismantling both hub assemblies, or wrestling with the rusty, inaccessible ring of bolts that attach the swivels to the axle casing, then withdrawing the hub, swivel and halfshaft as a single lump. On this vehicle it was obvious that the front differential had not been disturbed for a long time, with all the fasteners thickly encrusted with mud and oily gunk.

The vehicle was fitted with a 300Tdi engine, so 3.54 differentials made sense as a low-cost way of raising the gearing to take advantage of the power increase. But the job had only been half-done, leaving the owner with a vehicle that was great on road but tried to tear itself in two whenever four wheel drive was selected. I dug out a good 3.54 differential from the parts store, treated it to a new pinion seal, then pulled both hub/swivel assemblies, unbolted the 4.7 differential, removed it from the casing by the usual method (lump hammer and prybars while trying not to drop it on my chest as it comes free), shoved in the 3.54 replacement and in a fairly short time I had everything back together and working fine.

Three points to be aware of if you are contemplating a 3.54 diff swap on a Series vehicle. Firstly it will put your speedometer reading around 30 per cent out, and decrease the effectiveness of your handbrake (which acts on the transmission upstream of the differential) by a similar factor. Secondly, Series rear axle casings (for Rover-type differentials) up to the mid 1970s did not have an external oil filler/level plug. The filler was incorporated into the differential casing. 3.54 differentials with a built-in filler plug exist but are very rare. Usually you will have to drill a large hole in the back of your axle casing and weld in a threaded boss to take a filler plug. Thirdly, 109 inch Series IIIS, and some earlier vehicles, have Salisbury rear axles which need special tools to remove the differential assembly. A Salisbury differential is not interchangeable with the Rover type. 3.54 ratio Salisbury rear axles were fitted to the 109 V8 but these are very hard to find second-hand now.

It may be possible to fit a 3.54 Salisbury differential from a pre-1993 One Ten to a Series III axle casing, but this is not something I have ever tried.

You can swap the Salisbury axle on a Series III for a Rover type axle from a Series II or IIA 109 inch, but the propshaft will need to be changed as the Rover differential is about three inches shorter than the Salisbury. In any case the Rover axle is a bit marginal for strength on a laden 109 even with standard 2.25 petrol or diesel power, which is why Land Rover went over to the Salisbury axle in the first place.

When modifying old Land Rovers, nothing is as simple as it initially appears.