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Rather elusive vibration issue


rqsall
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Hello all,

I have a rather elusive vibration issue on a 1991 RR (3.9 EFI) with a LT77 gearbox and the infamous BorgWarner transfer case. It is not felt through the steering.

It happens at 80km/hr(50 mph) in 5th gear, but not in 4th. Accelerating in 4th to 100km/hr brings it back around 90 -100km/hr and shifting to 5th doesn't bring it back. It also happens in 3rd and 4th at about 55km/hr to 65 km/hr. Removing the front propshaft doesn't change anything substantially.

Done so far (also in relation to fixing other problems):

- replaced u-joints in both propshafts. Can't feel any play in the sliding joints.

- changed oil in transfer case.

- changed oil in diffs.

- ball joints in drag link/drop arm changed/overhauled (track rod ones seem ok)

- replaced rear wheel bearings and hub seals

- full electrical tune up (new genuine rotor and cap, alternator, coil, ignition module, plugs, leads)

- Head gasket blew 18 months ago and was fixed, but the vibration was there before it went.

- swivel preload checked, is within spec.

- panhard rod bushings replaced.

- rear trailing arms bushings replaced.

- oil level in gearbox is ok, and oil looks ok. (been planning on changing it, but haven't gotten around to it yet).

- overhauled rear brake calipers.

Things to suspect:

- when I changed the u-joints, I found out that the front propshaft was mounted the wrong way around, the manual specifies the sliding joint nearest the transfer case, it was mounted at the diff flange.

- There's noticable play in the bearing of the front output shaft of the BW transfer case.

- I think I can feel some play on the swivel bearings.

- compression imbalance in engine? Injector or two not giving as much fuel as the others, or too much?

- LT77 mainshaft problem, even though the box shifts fine and doesn't jump out of any gear.

Things I think I can rule out:

- front propshaft: even with it removed, the vibration is still there

- wheel balance: same speed, different gear makes it go away.

- alignment: not felt through steering

- front diff

I realize the combination of V8, LT77 and BW is not a very common one :unsure:

Thank you for any input and ideas,

Ian.

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I'm experiencing the same problem - vibration at around 50MPH in 5th gear. i also during the past year have replaced knackered prop U/Js and checked wheel bearings etc. during normal servicing and not turned my attention to the problem until lately. I am suspecting the source is in the transfer box but have not as yet checked it for play in the output bearing. Mileage is 170k so i suspect worn chain also.

If anyone out there has experience of this problem then we have 2 avid listeners!!

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Things to suspect:

- when I changed the u-joints, I found out that the front propshaft was mounted the wrong way around, the manual specifies the sliding joint nearest the transfer case, it was mounted at the diff flange.

- There's noticable play in the bearing of the front output shaft of the BW transfer case.

- I think I can feel some play on the swivel bearings.

- compression imbalance in engine? Injector or two not giving as much fuel as the others, or too much?

- LT77 mainshaft problem, even though the box shifts fine and doesn't jump out of any gear.

Because you don't feel it through the steering I don't think its the front prop, swivels or transfer case front output. If you feel it through your bum I'd look at the rear prop and rear diff.

I'd consider that the engine mounts might be a bit soft?

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I'd consider that the engine mounts might be a bit soft?

They've crossed my mind too, but escaped me when I was compiling my "suspects list". They've certainly never been changed at 130 k miles. They're certainly cheap enough, so I suppose I'll give that a go, but the mounts just lessen vibration, it would be nice to know what's causing it :)

The vibration is definitely felt through the bum.

Ian.

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Have you thought about rear wheel balance?

yes, thought about it, but I don't see how changing from 4th to 5th would change the vibration at the same speed. If it was wheel balance, or propshaft, why would it matter in what gear the box is in? Anyone ever had this and had it solved by balancing?

Thank you,

Ian.

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That points towards the gearbox then surely if the vibration changes with gear?

With mine its more about engine RPM that affects the vibration

*sigh* just what I didn't want to hear... ;-) seems to keep coming back to that though. Drove a bit more with front prop removed today, perhaps wishful thinking, but it seems more mellow. What I also forgot to mention earlier (and points to gearbox as well) is that it gets worse as the engine/drivetrain warms up.

Regarding RPM based vibration, have you checked compression? It shouldn't differ too much among the cylinders. Also a bad injector, perhaps check plugs... engine mounts

Good luck, with fixing yours! :)

Ian.

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