Transmission movement - rear mount

[simon291971449]

Hi. Quick question. Suspect my rear gearbox mount on 91 4.0 needs replaced. I can push the whole gearbox up by hand easily. I know there is supposed to be movement, but that much? Not getting any clunk's or bangs on pulling away or braking hard. Should I be replacing the spring and the foam bush? Advice gratefully received. Thanks.

[ptjs1]

Simon,

It's also important you look for movement the other way. And you'll need an assistant to help. It's really critical that you establish if a drop of the gearbox down against the tension of the mount will cause the fan blade tips to touch the top of the fan shroud. In fact, before you do that test, check to see if there marks on the underside of the top of the fan shroud, and also if the tips of the blades have worn through scraping it.

If there has been any contact or if moving the gearbox can enable that to happen, you must change the mount. If a fan blade lets go through hitting that shroud, it can cause a huge amount of damage and potentially punch its way through the bonnet.

Good luck

Paul

[davidr]

Simon - there should be some movement. The mount has a spring for that reason but it shouldn't be excessive. As Paul suggests, check if the movement is sufficient for the fan to touch the shroud at the top. If so, change it immediately!
If you are not getting any noises that is a good sign.
Let us know how you get on

[PaulGover]

... I can push the whole gearbox up by hand easily. I know there is supposed to be movement, but that much? ...

In a nutshell, yes,the rear gearbox mount should allow a lot of movement, It's a design feature since the first days of the Jaguar front suspension being mounted on a subframe isolated from the body by bushes and that spring, to prevent vibration reaching the cabin.

That said, as davidr and ptjs1 say, if the foam rubber bush in the rear mounting deteriorates (and as it often gets coated with oil from leaks in the engine bay, that's very common), you can get too much of a good thing.

[JimMann]

The first time I became aware of the vagaries of this rear gearbox mount and its spring movement was when I was trading my 1991 XJS V12 for a New XK NA. Unknown to me, the trader I was dealing with was a bit of a 'shark' and during the test drive, with him driving and me in the passenger seat - which I thought unusual at the time - he slowed the car down and then floored the accelerator, resulting in a major thump at the rear of the transmission tunnel. This was his cue to talk me down from my expected trade-in value and him make me believe he was doing me a favour with all the work that the thump would involve for him.
I now know that doing such a harsh acceleration would make even a new spring and mount jump to the extent that it would make contact with the tunnel and I vowed to know much more about my prospective cars before dealing with the trade again.
So yes, there is a reasonable amount of movement as the other contributors have indicated, but the key test is to check if there has been any contact between the fan blades and the shroud as previously highlighted.
Jim

[PJAB]

Hi All

I think that this post is related to this thread - so rather than start a new one I have just added it here.

There is a very unpleasant sound on my 1990 V12 Coupe if I accelerate other than very gently from standstill or slow speed. I would describe the sound as “graunchy” - like two rotating components not lining up properly.

I believe that the problem is to do with the the gear box mount - which has a coil spring about 2.5” high and two rubber bushes. We ordered a new part from SNG Barratts but the new spring was not the same size as the old one - so we just replaced the bushes. However, this did not eliminate the problem.

Has anyone else experienced a problem such as this? Do you think that the diagnosis of the problem is correct? If it is, any advice about getting a correct replacement part. The idea of retempering the existing spring has been mentioned but it would obviously be quicker to buy a new part.

Thanks in advance for your wisdom.

Peter

[PaulGover]

I think the spring height is unlikely to cause the described noise - more likely is a misaligned ream mount. As far as I can recall, there's some play in the mountings before you tighten the bolts, and its possible for the mounting to leave the propshaft and universal joints too near the chassis rails, in which case they meet under high torque=acceleration. Another cause could be failed, loose or misaligned front engine mounts, for the same reasons.

As to the existing spring being a different height, check the part numbers for your replacement, but perhaps the old spring had been replaced with the wrong part. I don't know the right height, and I'm not taking mine apart to find out!

I vaguely recall I once reassembled the entire rear mount wrong, so the spring pushes everything up, removing the intended free motion before the bits hit the top of the transmission tunnel, which caused noise. I forget how I did it, possibly spacers in the wrong place, but I think it was to do with not having the various rubber bushes in the right places. The key component is the foam "cotton reel" that fits into the hole in the spring carrier, and the the rounded oversize washer which should be centralised vertically in the cotton reel by the tubular spacers. The big washer and the cotton reel form a friction damper to the low-rate spring support that's key to the rear mount's operation.

[PJAB]

Hi Paul

Thanks for the comprehensive and informed reply - great stuff. I will do some more digging - I definitely need to get this sorted properly.

Does anyone have any useful pictures of how this mechanism should be assembled and installed - I have the big green book but I don’t recall seeing any pictures of this when I looked last week.

Any other perspectives out there?

Thanks

Peter

[PaulGover]

Peter,
This page from the SNG Barratt web site shows the V12 rear engine mount - or at least one variation thereof.

Item number 3 is the "cotton reel" sponge that goes in the hole in item 1, the mounting bracket attached to the chassis; item 5 is the mount that's bolted to the gearbox; items 27 and 28 (my car has two spacers 27, one either side of the damper washer 28) form a tube round the stud in item 5, and should lie in the middle of the cotton reel, so that (a) it locates item 1 centrally around the stud, and (b) provides friction damping as the gearbox and engine move up and down, the mount 5 therefore following. AFAIR item 21, the spacer, will cause the mount to "lock up" if it's in the wrong place, and I'm not sure the diagram is clearly showing how it should be.

Here's another web page from SC parts with various XJ rear mounts, the bottom right one is what I think you should have, and I think its layout is more correct.

[ptjs1]

Peter,

It may or may not help you with assembly, but always start with the Jaguar parts website to see a layout of the relevant parts.

I presume you haven't found it yet, so here's the link of your transmission mount:

https://parts.jaguarlandroverclassic.co ... nd/jaguar/

Good luck

Paul

[PJAB]

Hi both Pauls

Wow - thanks to both of you for taking the time to contribute to this thread and help me out - it is massively appreciated.

I will follow your links and learn a lot more about this problem and how to resolve it.

Not only will I learn about this problem - I’ll also learn about resources that I can use to research problems like this in future. I am still very much at the start of my journey to learn how to maintain my car. I’m sure I will become more self-sufficient over time - but in the meantime I appreciate the help and support that people like you provide. One day I will hopefully be able to say something useful to someone on this forum!

All the best

Peter

[PJAB]

Hi Guys

Just posting back that this problem now seems to be sorted - yay!

As well as the need to replace the serviceable parts of the rear gearbox mount, there were a number of missing small components in the whole assembly. When these were replaced and the serviceable parts serviced, the whole thing worked like new - no “graunchy” noise even when accelerating briskly with steering lock - e.g. out of a T junction. It’s as if the factory knew what they were doing in the first place - and really emphasised to me how important it is to find people who actually know about these matters specifically - rather than just have a go at sorting problems based on their broad mechanical experience.

Anyway, I am really happy that this mechanical issue is now resolved - I hate the thought that driving the car is in any way damaging it.

Thanks to everyone who contributed to this thread.

Peter

[ptjs1]

Great news Peter, Well done!

It just goes to show that you can't trust the previous ownership of a car. At least now, you know it's complete and done properly!

Enjoy driving it!

Paul

[PJAB]

Paul

Indeed - thankfully I think that I am now pretty much at the end of the mechanical review / overhaul of the car - so can focus next on functionality issues and then - at the end - cosmetic issues.

The guy who did the work yesterday likes the car - as has every garage - he said it was a good one. It’s reassuring to know that it is a good “platform” to put all the time and effort into. It’s a “lifetime” car for me - so I want to gradually get it into excellent condition (not concours) so that it can be enjoyed for years and years.

As I said, I really appreciate people participating in my posts - it’s both encouraging and helpful.

Peter