Interesting fred. The way I see it…
The engine drives the gearbox. The gearbox drives the input to the ird. The ird sends this power to:
1. the front diff (inside the ird). The front diff connects to both front wheels.
2. rear pinion on the rear of the ird, which drives the front prop shaft. This drive passes through the vcu to the rear diff, which splits it to the rear wheels.
Both the front and rear diff allow the wheels connected to them to travel at the same or different speed (like when cornering).
HDC is for controlling speed by limiting it using the brakes. Level ground and downhill if yer going faster than the required speed and yer take yer foot oft the pedals.
TC is for reducing wheel spin so the power goes to the wheel with the grip, on an axle pair, to stop power escaping from the diff when it could be put to use driving wheels with grip. Power normally goes to the wheel with the least grip which is why it spins faster. TC will pulse brake 4 wheels individually as required (if they spin) in an attempt to gain traction by stopping them spinning. Floor it on a soft surface and all 4 wheels will spin, with TC fighting to gain control.
When hdc is active and yer FL wants to slow down, it will do so by using the bakes. This will be done by pulsing them. You will feel the brake pedal pulse, and hear the abs activating. LR allowed it to work like this to give feedback to the driver to let them know when it's active.
Switching on HDC doesn't change any of this. It does pulse brake wheels when needed, but that’s no different to a skid when braking when the abs tries to take control of yer FL to re-gain directional control. There are some slightly different forces applied when this all happens, but none that will be any different to normal driving circumstances. It’s just a case the abs will be used more often, and therefore all of this happens more often. Which is why it may be noticeable. It may pulse brake one front wheel and not the other, allowing an oscillating effect in the front diff… from turning both front wheels to turning only 1 front wheel… then back to turning both. This oscillating will happen at the speed of the brake pulses, which I think is 4 times per second on a FL1 if I remembers correctly.
The vcu is a dual use device. It:
1. connects the front and rear prop shafts to allow the engine power to drive the rear wheels. When the differing speeds of the 2x props increases to a certain point, the vcu's resistance across itself increases and more drive passes through it
2. it's a handy weight concealed in yer hand bag for clubbing tratterers
Regarding the OP's clunk… if it were mine I'd be looking at wobbling the props to see movement on the ird connection, vcu bearing and rear diff. Also wobble the rear diff. The cente mouth at the front of the rear diff is a good point of failure. Don't forget to wobble the eggsauce anorl. If yer FL won't clunk when rocking it to sleep when stationary, try wobbling it at 10mph by violently steering left to right repeatedly. But only while driving on yer private country estate away from the public where yer safe to do it, and not too much that it falls over.
what's inside the ird:
More ird pic's in this fred:
https://www.landyzone.co.uk/land-rover/freelander-1-ird-and-rear-diff-photos.280758/
How I test the FL1 transmission:
[q=Hippo]
With the engine oft, chock the rear wheels and put the handbrake on, disconnect the front prop from the IRD and:
With the auto in park (in gear for manuals) lift 1 front wheel and it won't turn. The IRD pinion won't turn when you turn the lifted front wheel. If you put the auto into neutral (neutral on manuals too) the lifted front wheel will now turn and so will the pinion if you turn the lifted front wheel.
If you put the lifted front wheel on the ground and lift the other front wheel the same will happen as above.
If you lift both front wheels and turn 1 the other will turn regardless of the auto being in neutral or park (neutral or in gear for manuals) in the opposite direction. The only difference is the pinion gear will turn if the gearbox is in neutral (doesn't matter if auto or manual). If you sharply turn or quickly slow down the wheel yer turning the pinion will turn more than the wheel yer turning. I assume it's due to the path of least resistance as the power escapes.
If lifting both front and rear wheels on the same side of the Freelander, with the prop/vcu connected, and the engine oft and the auto in park (in gear for manuals) the front wheel won't turn when you turn the rear wheel (assuming the vcu and rear diff work correctly as they should). The rear lifted wheel will only turn slightly which is down to the internal opposition of the vcu. If you do the same test with the auto in neutral (neutral for manuals too) the front wheel will turn in the same direction as the rear wheel.
Note: when I say won't turn, it will turn but only approx 1 inch circumference or a few degrees, as opposed to turning round fully. This is the movement between the gear teeth only.[/q]
