"Hard" clunk when changing gearbox direction

Antharro · December 19, 2019

Hi forum,

Another question about my LS430. I had a new rad and auto box fitted a couple of years ago. (Changing the rad was on the list but I forgot about it and it failed... usual consequences). Recently, it's developed a seemingly minor, but annoying problem.

Changing the direction of travel can cause an unpleasant jolt when power is applied. This is tricky to explain, so please bear with me.

If I've reversed into a parking spot, when I come back to the car and put it in drive, I will feel the transmission engage the gear, but if I don't wait for a couple of seconds before applying power, it will jolt. It feels like it's engaging the gear heavily, (equivalent to dropping the clutch in a manual), except I've already felt the box engage the gear and felt the car "lean" in the direction I've selected and felt the engine take the load.

If I then put it in drive, let the engine take the load, put it back into neutral then back into park, and the car hasn't moved at all, there will not be a jolt. If it's moved, even slightly, there will be a jolt. The severity of the jolt depends on how much the car has moved in the opposite direction to the gear engaged. So if it's rolled backwards only a couple of cm in neutral, and I engage drive, the jolt will be minimal. The larger the distance, the larger the jolt.

I can reproduce the problem worse by shifting from D to R, backing up a little way, braking to a stop, shifting to park then accelerating. It will jolt in whichever gear I select as long as it's the opposite direction to the one it was in previously.

I can exacerbate the problem by reversing, coming to a stop, engaging drive and accelerating very lightly (maybe up to 900-1000rpm) band NOT giving it an extra couple of seconds after feeling the engine take the load. I'll feel the car engage the gear but there'll be an almighty jolt. Works from R to D and D to R.

So to get a completely smooth shift, I have to engage the gear, then wait 2-3 seconds before moving (with my foot on the brake). This is fine if I'm at home but if I'm holding up other people in a car park while I try and get into a space, it's damned inconvenient for everyone.

I had a local garage look at this, and they noted that the "slide joint in the propshaft" was completely dry. They disassembled it, lubricated and reassembled it, which helped a little, but the fault is still present.

My hunch on this is that there's no fault on the gearbox. It shifts smoothly for 98% of the time and hasn't given me any indication that there's anything wrong with it - no hard shifts between gears, it kicks down fine, and it's happy downshifting using the gear stick, although occasionally lazy when forcing it from 3rd to 2nd, especially if I'm going down a hill - but it does it smoothly. The gearbox oil was changed last month I feel the problem is either something to do with the prop shaft joint or the rear diff. The gearbox engages the gear, forcing the prop to turn slightly in one direction as it engages. (If the car was brand new then I wouldn't expect any movement of the prop until the brake is released as everything would be new and within tolerances). If the prop is changing direction (i.e., R -> D or D -> R) then there's play in something which results in the jolt. The jolt is exacerbated by light acceleration because it makes the prop change direction faster and therefore the play is felt more strongly as whatever it is moves and then stops suddenly.My other feeling backing this up is that my mechanics tried to change the oil in the diff but the bolt was so seized that they felt they would snap it - so it hasn't been changed in a very long time.

Edit just to clarify - the jolt is significantly worse when I shift gear with my foot on the brake. If I shift without my foot on the brake, I'll feel both "stages" - the gearbox engaging/engine taking the load, and what would be the jolt - except it isn't because the brakes aren't holding the car. Hope that clarifies and this isn't too long - it's a tricky thing to explain!

Does anyone have any experience with this, or any ideas?

Thanks!

The-Acre · December 19, 2019

A really difficult one to comment on but again, if I remember correctly, the diff oil should be changed every 20k and at 160k and a seized plug is it possible its never been done?