P38 'jerking/jolting' on lift-off

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Diehard

New Member
Posts
205
Location
Darlington (County Durham)
Alright,

Another day another problem when owning a P38, this time, in my drive-train.

Following a replacement*4-pin diff install 2 weeks ago (due to horrendous noise being a fooked pinion bearing) I continued to get a 'rumbling' on over-run from the rear at speeds over 50MPH then vanishes below this speed, however, I started to get a 'wind-down' noise (the old hairdryer noise) from 30-0 which I initially thought was the rear diff starting to strain, gradually getting louder.

Anyways, due to loosing a rear diff on my old P38 due to a seized VCU, I asked the garage fitting the diff to check the VCU (I was there at the time) and they jacked up both N/S wheels and with hand break off, they rotated the rear prop back and forth, showing a fair bit of slack and clunking also the guy tried to turn the rear wheel by hand (one in the air) with no look and he stated that the VCU was on it's way out, but should be ok until end of month, when I said I would replace it (I wasn't too sure about this test mind as I had checked the VCU by the turn-circle test and no skipping/scrubbing etc).

Anyways, like I said, the noise from 30-0 was getting louder so had it back to the garage and they stated that the VCU was causing wind-up and if I am lucky, the rear diff may not be shagged (noting the noise I was hearing was from the rear), so I panic bought another VCU on next day delivery to fit, hoping to save any further damage.

So, VCU arrived, went to fit it but thought I'd test it first as still was not totally convinced. Jacked front N/S wheel up, transfer in Neutral, Handbreak on and applied a normal sized wrench and the wheel turned nicely with my weight on it at a stable speed, so VCU was fine (the prop was moving so diff wasn't taking the turn).

Removed front prop and tried the RR, rumbling still there, but winding noise from 30-0 gone, however, now on lift-off the RR lightly jerks twice and you can hear the rear diff engaging at these times (imaging strong engine breaking/a sticking break etc). Now I'm thinking this is excentuated due to the removal of load as only 2WD but with the slack/clunk when tested with both N/S wheels off the ground, could this be the Transfer box chain?

Both the original garage and a second opinion guy have looked at the RR with front prop removed and believe the front diff is on it's way out (input bearing) due to extremely hot flange but cold diff housing (after a drive) I have checked/changed the diff oil and it was full/pretty clean. The 'rumble' from the rear is pretty likely to be a bearing and the second opinion man confirmed this.

Now question is, should I be worried about the jerkiness on lift-off and would this cause two diff failures?*

For reference, the RR has done 150,000, gearbox seems ok apart from a clunk from 'P' to 'R' or 'D' to 'R' which I hear is 'normal' but changes up and down no issues.

Any help/ideas would be great, Im currently running on the rear prop only and not sure whether to reconnect or leave as is until such time that I get the diff changed.

Regards,

Diehard :cool:
 
Datatek, go get another cuppa inside yourself. Running with no front prop, will not put any strain on the VC. The front and rear outputs will be running at same speed, so the VC will be doing no work. The VC will only be working when front and rear are at different speeds.
 
Diehard, try running (not too far!) with just front prop on and rear off? This will put all the drive thro the front prop, and, if it is shot will increase noise etc. And take strain off rear.
Do you mean this noise started from installation of new rear diff two weeks ago? If so surely this is the problem??
 
Many thanks for the replies people.

With regards to the whine from 30-0 this started a few days after the replacement diff was fitted and I would have swore it was from the rear, the first garage (who fitted it) said it was rear winding up due to VCU when they road tested, but as I've noted I tested the VCU and all seemed ok, so took it back (no whine without front prop) and they diagnosed front diff... Hence why I sought a second opinion as loosing faith in first garage and although he had no experience of the RR with both props fitted due to the front diff flange being hot and feeling a bit 'notchy' by hand, he believes front diff too.

Wont get a chance to do rear prop today but gonna stick on front prop later as I am sure the 'jerking' on lift off was slot less when both axles were in play... Any ideas if that would be the tranny chain?
 
Datatek, go get another cuppa inside yourself. Running with no front prop, will not put any strain on the VC. The front and rear outputs will be running at same speed, so the VC will be doing no work. The VC will only be working when front and rear are at different speeds.

If you lift one wheel off the ground and apply power via the prop, the wheel will spin and you will get no traction to the other wheel. The centre diff works in exactly the same way, the VCU locks to prevent all drive being lost if one end loses traction, ergo if you remove the front prop it is the same as loss of front end traction so the VCU will have to lock the diff to provide traction to the rear. With no front prop, the front drive is free to rotate just like a wheel off the ground, so without the VCU it would do just that.
I reckon you need something stronger than tea and I suggest you go find the detailed drawings in RAVE that show how the centre diff and VCU work:rolleyes:
 
Was replacement diff a 2nd hand unit? And how sure are you it`s a good`un?
The Morse chain seems to go on forever. (Cue posts from people who have had failures). Provided oil is there it has a relatively easy life.
Are U.Js and prop sliders all ok?
 
Many thanks for the replies people.

With regards to the whine from 30-0 this started a few days after the replacement diff was fitted and I would have swore it was from the rear, the first garage (who fitted it) said it was rear winding up due to VCU when they road tested, but as I've noted I tested the VCU and all seemed ok, so took it back (no whine without front prop) and they diagnosed front diff... Hence why I sought a second opinion as loosing faith in first garage and although he had no experience of the RR with both props fitted due to the front diff flange being hot and feeling a bit 'notchy' by hand, he believes front diff too.

Wont get a chance to do rear prop today but gonna stick on front prop later as I am sure the 'jerking' on lift off was slot less when both axles were in play... Any ideas if that would be the tranny chain?

Sounds to me like your replacement Diff is not healthy:eek:
 
Datatek with one prop disconnected the free out-put shaft will have no load, so will turn at the same speed as the shaft driving the wheels. You are describing having one shaft moving and one being stopped. With one prop off there is load to stop or resist turning. Since both front and rear outputs are spinning at the same speed there will be no load on the VC.
 
Datatek with one prop disconnected the free out-put shaft will have no load, so will turn at the same speed as the shaft driving the wheels. You are describing having one shaft moving and one being stopped. With one prop off there is load to stop or resist turning. Since both front and rear outputs are spinning at the same speed there will be no load on the VC.
Sorry you are wrong, if one diff output has no load it will spin and the other side will remain stationary, Remove the front prop and without the VCU the center diff front output would spin and the rear output would remain stationary. The function of the VCU is to lock the concentric shafts of the front and rear outputs of the centre diff when either of them has no load either due to a wheel spinning or a prop being removed. As I said look at the detail drawings in RAVE and all will become clear:)
 
Hmm, well ill be sticking the front prop back on regardless so after ive done it ill double check the VCU again to make sure still ok and go from there.

With regards to rear diff, it was supplied/fitted by garage as 2nd and they maintain it was a good one, hence why I'm apprehensive about diagnosing other issues instead of the rear diff.

All U/J's on front prop are sound, and I'm assured rear prop is same but not checked myself.
 
Well prop back on, tested VCU again for good measure and all ok, also had a play with ye VCU flange when handbreak off/in neutral and a fair bit of backlash but noise seemed to be very 'chain sound' from the transferbox.

Have video'd the VCU test and that described above (badly) ill try post on YouTube to see what propel think.
 
P.S. the whine is back but now I think the front diff is shot I'm enclined to think the noise is at the front, also the jolting on lift-off is gone so assume that was a consiquence of being in 2WD.
 
Hi Chris,

Good video. Your wheel turns much more easily than mine - mebbe you're right about me replacing mine. Hmm.

Did the first garage supply and fit the diff? This is the garage that seemed to think you tested the VCU by rotating a rear wheel?

Hmm.

You might be best off doing what I ended up doing and finding a garage that can stick microphones over everything - it's by far the best way of diagnosing this kind of problem. The downside (on mine, anyway) was that they diagnosed pretty much everything as worn - but they can distinguish individual noise sources.

Have you checked the oil in the rear diff since they fitted it? If the primary pinion bearing is breaking up then it should show in the oil. Seems a bit unlikely though - they'd have to be total numpties to fit an obviously knackered diff.

I've never been able to get a straight answer about how much backlash is acceptable in the drivetrain on these. I would have thought that ideally there shouldn't be any...

Keep it up - you'll get there in the end!

Guy
 
Cheers for the comments and yep same garage, hence why I went to another to double check.

Came to work today on both probs and noise not as prominent as before (the whine) but defo there.

Incidently I have a VCU in its housing for sale at £75 delivered (what I paid) and has a 30 warrenty, either that or I send it back.
 
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