Diagnosing my drivetrain clonk

[sim303]

Hi

My Cruiser makes a disinct clonk when coming on and off the throttle. It's not grossly offensive, but does detract from the ride. I had a quick look in to it a few years ago (thread here: Driveline Clonk (rear diff?) - [Leaving Land Cruiser Club]). I later found that the front hub splines were badly worn and replacing these certainly helped a bit. However it didn't eliminate the problem. So, as things like this have the potential to get expensive quickly, I am keen to isolate the culprit as best I can before replacing more parts.

Today, I went for a test drive to try to identify the specific conditions that give rise to the clonk. I wanted to check whether it was specific to a certain gear etc, so went through various steps, disabling O/D, limiting the gearshift to "2", driving fast and slow etc. The clonk occurs regardless, when the load comes on and off the drivetrain.

Now for the more interesting discovery.... I found that if I engage the centre diff lock, the problem goes away! It's so smooth! I'm sure that's how it is supposed to be!

So, what does this finding tell me? To my thinking, one of the following:

1. There's excessive lash in the centre diff itself which gets locked out by engaging the diff lock
2. There's excessive lash in either the front drive train OR the rear drive train, but not both. By locking the two props together, the good one inhibits the bad one from slopping back and forth as the power is transmitted through the locked centre diff.

If these propositions are correct, I guess my next experiment might be to remove one prop entirely and drive (carefully) with the centre locked. If the problem disappears, then the problem is in the disconnected end of the drivetrain. If it remains, that's the errant end, and at least points me at a diff or prop. That's my thinking anyway. Other suggestions welcome!

I wonder if this sounds like a reasonable next line of investigation...

(From memory I think it is the rear that showed the most slop when turning its prop by hand in neutral, but I'd need to get under there and check again.)

Cheers

PS I should add, all the grease points were pumped full to no great effect

 

[uHu]

Hoist the car up, or put it on 4 jack stands, get under there and turn things. Should be evident then.

When you lock the centre, there will always be a slight binding in the drive train (as long as you have 4 wheels on the road). That will always eliminate any slop.

Another issue seen on the rear prop.shaft of 100s, is hydro-lock of the splines. That happens when the seal for the splines is i perfect condition, not letting any pressure out. When you then pump grease into the shaft, it will act as a hydraulic ram, pushing the rear diff. The rear diff (axle housing) will then be forced into one of 2 positions where the prop.shaft is pushing it up or down, as far as the rubber mounts will allow it, depending on the forces between engine and road wheels. The way to check if this is the case, is to undo the grease zerk for the splines to let any pressure out.
The splines need very little grease, like 2 pumps on a simple manual lever pump every 5000 km. (like the UJs btw)

 

[sim303]

Thanks for the suggestion uHu. I've just emerged from under the car and, unfortunately, it's definitely not the prop or UJs. With the car on the ground (i.e. wheels unable to move) and the transfer box in N, centre diff unlocked, there is a marked difference in the amount of free rotation when rotating the front and rear props by hand. The rear has a small amount of movement, whilst the front has much more, and is accompanied by a very clear clonk each way. I'm trying to upload a video to Google Drive but so far it's refusing to upload

Assuming this amount of play is not normal, the next task is to try to identify where in the front drive train the slop is coming from. My sense is that it is somewhere in the front axle / diff, but it's hard to say at the moment.

Thanks

 

[sim303]

I managed to upload one of the two videos at least. This one shows the movement in the front prop when turning it back and forth by hand: https://drive.google.com/file/d/1fYOGjAaxOH6ouPbrC-IeaVK5omtmoJPv/view?usp=sharing