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Not sure were to post this one, I seem to have a strange knocking noise
around the gearbox tunnel. It does not do It all the time, some times it
does it going over pot holes but it really dose it accelerating hard 70 -
80 MPH +

I have checked the gearbox mounts and more or less all of the front
suspension. The only thing I have not looked at yet is the prop shaft. Can
anyone tell me if this could be the cause of the problem? Or does anyone
have any other ideas as what the problem could be?


Thanks

Hi Damien,
From your description it doesn't sound like a propshaft problem, more likely a gearbox mount. What model do you have?
My XJR-S has a similar probem, a heavy thump as the front nearside wheel drops into a pothole or descends from a speedbump. A Jag specialist has diagnosed a failing gearbox rear mount allowing too much movement.
He's going to replace it in a couple of weeks time.
Are you sure your mounts are OK?

Cheers,
Glenn.

Yes I think so but I could be wrong. My car is a V12 HE XJS. Does yours also make a vibrating knoking noise at speed? It sounds like a bloke with a hammer is in the garbox tunnel when the car is pulling under load at arround 70mph +.

The rear gearbox mount seems to have some strange spring contraption, maybe a solid mount would be better? The spring seems fine but I can push the box up slightly with my hands. Im not sure how much play there should be in it so im assuming its ok.

Ta

Damian

Hi Damian (sorry for the mis-spelling on my last reply),

I don't get the vibration or knocking at speed or under load so we may have different problems.
Yours sounds like it may be the gearbox or even an exhaust pipe touching the body. Unless you have a mechanic with a hammer trapped in there from the last service
You'll want to stick with the Jag mounts to keep the quietness and magic carpet ride, but I'd check the mounts for engine, front subframe and gearbox for splits or damage.

Do you get the same noises if you put the transmission under load when stationary? You could try the old torque converter stall speed test - left foot firmly on the brake, right foot on the go pedal. It'll put the transmission under maximum load and with the bonnet up you'll see just how much the engine and gearbox move. Three notes of caution, check the engine mounts before doing this test, don't put it under load for more than a few seconds at a time or you'll overheat the oil in the torque converter, and make sure there's enough space in front of the car in case your left foot slips...

Regards,
Glenn.

Yes I think so but I could be wrong. My car is a V12 HE XJS. Does yours also make a vibrating knoking noise at speed? It sounds like a bloke with a hammer is in the garbox tunnel when the car is pulling under load at arround 70mph +.

The rear gearbox mount seems to have some strange spring contraption, maybe a solid mount would be better? The spring seems fine but I can push the box up slightly with my hands. Im not sure how much play there should be in it so im assuming its ok.

Ta

Damian

If your rear (only?) gearbox mount is u/s-usually due to oil contamination of the rubber damping components the gearbox will drop, pulling down the rear of the engine due to torque under hard acceleration..........this will allow the cooling fan to chew its way through the top of the shroud
Regards
DH Dove

If your rear (only?) gearbox mount is u/s-usually due to oil contamination of the rubber damping components the gearbox will drop, pulling down the rear of the engine due to torque under hard acceleration..........this will allow the cooling fan to chew its way through the top of the shroud
Regards
DH Dove

Aha, it sounds like this might be happening to mine. I get a brief scraping noise on hard acceleration from a standstill (so I don't do it often). Also, the tips of the cooling fan look as though they have been scraping the housing. Worth my looking into. I'd assumed it was a rear exhaust pipe fouling a drive shaft as hard acceleration would throw the car back on its haunches.

Is it an easy job to do the gearbox mount ?

Regards, Julian

Hi Julian - There's a lot more info on this particular matter on the 'Pride and Joy' thread on the JWM Forum. Might be worth a read. Cheers, Archie.

Hi Archie,
Thanks for letting me know
Regards, Julian

If your rear (only?) gearbox mount is u/s-usually due to oil contamination of the rubber damping components the gearbox will drop, pulling down the rear of the engine due to torque under hard acceleration..........this will allow the cooling fan to chew its way through the top of the shroud
Regards
DH Dove

Aha, it sounds like this might be happening to mine. I get a brief scraping noise on hard acceleration from a standstill (so I don't do it often). Also, the tips of the cooling fan look as though they have been scraping the housing. Worth my looking into. I'd assumed it was a rear exhaust pipe fouling a drive shaft as hard acceleration would throw the car back on its haunches.

Is it an easy job to do the gearbox mount ?

Regards, Julian

Julian,
Sounds like you've identified at least part if not all your problem. I haven't refurbished the mount myself....the garage did it.....I would suggest you consult the parts cat. and ident and price the bits. Youll almost certainly need the rubber bits, then theres a big washer (or two I cant remember) which should be ok, but the springs might have gone weak. If you have access to a ramp and gearbox stand of sorts I dont think its a hard job, access, remove and substitute then replace. But mega dirty if youve got oil leaks which have probably caused the problem in the first place
Good luck
DH Dove

Hi DH,
Like you I'd get someone to do the work, but I always like to know what's involved (I've invented the term 'theory mechanic').
Regards, Julian

Another possibility is the rear radius arms. I know I had much the same sort of problem. Changed the gearbox mount, a little better since clearly it was past it's best though not the whole story. The real problem were the bushes in the arms and since I changed them, no problem, no knocks, rattles or vibrations. I can enjoy myself again. A qucik look underneath with a pry bar at least will give you a clue, so worth a look.

Nick

Well I have taken the gearbox mount to bits and indeed the rubber is like a spounge so I guess It needs replacing anyhow. The spring looks ok and isnt broken but could this still be in need of replacing?

Also the main mount that bolts under the car has like a welded in bearing type thing. This seems to have two possible machined grooves in the side. These look a bit scratched and although there is no sign of breaks in this area in the spoungy rubber bush, I was wondering if these groves should be there ?