| Author | Message | ||
     C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Hello, The problem I am having is with the almost violent CLUNK when moving from Park through to Reverse. All other gears mesh in smoothly. I have checked the control rod linkage to the Actuator and at each gear position the grooved pin slides easily into the jaw/lever. The car is a 1988 Bentley Turbo. Thanks in anticipation, Clive (Message approved by david_gore) | ||
| David Gore Moderator Username: david_gore Post Number: 976 Registered: 4-2003 |
Clive, What is your engine idle speed? A CLUNK is usually due to excessive idle speed speeding up the fluid flow in the transmission. Another cause could be worn universal joints or a failed engine/transmission mount. (Message edited by david_gore on 16 September 2010) | ||
| Peter Talbot Experienced User Username: squerryes Post Number: 45 Registered: 7-2010 |
Clive Agree with David, check idle revs as first priority. Query: do you get this CLUNK only when starting engine from cold or does it still occur when engine hot? Suggest next step is to examine all mounts - easily done and non-invasive. Peter | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 651 Registered: 6-2006 |
Clive , is it always happening? ie if you go from park to reverse you get the clunk - if you then go from reverse to park to reverse without taking your foot off the brake do you still get the clonk? what about drive to reverse and back? | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
David,Peter & Paul, Many thanks for your very quick response. The rev. counter on the car is showing just under 600. I once slowed the engine speed down but the CLUNK was still apparent. Paul, when I get back this afternoon I will try the various gear changes as you suggest and also check if it is worse cold/hot. Checking the engine/gearbox mounts will probably have to wait until the weekend when I can get the car up off the ground. All your advice is very welcome, thank you. Clive (Message approved by david_gore) | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Hello All Again, Following on from your suggestion Paul as it was the first thing I could do I have tried the car cold & hot. The clunk or thump is worse when first starting from cold. I took the car for a short run of about 12 miles and the it was a lot less. Going from P to D slowly you can feel the transmission engage quite noticably. Going from P to D and D to P quickly there is no 'thump'. I have checked the engine speed using an external rev. counter and it is running a fraction below 600rpm say about 580rpm. Any further thoughts would be welcome as I hoped to use the car next week on holiday. Thanks again in anticipation. Clive (Message approved by david_gore) | ||
| David Gore Moderator Username: david_gore Post Number: 977 Registered: 4-2003 |
Clive, Have you checked the transmission oil level? If it is low, the transmission could be sucking air with the oil. Top up, take the car for a drive then try the selector test. If the oil level is OK and the car is fitted with a T400 transmission, a possible cause could be the O-ring seal at the top of the oil intake tube has failed allowing the transmission to suck air in. This is a simple repair and you need to purchase an after-market T400 transmission oil pan gasket and filter set as well as an O ring replacement set with the common sizes included. | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
David, I checked the fluid levels when I took the car out to test the gear change hot/cold. The fluid was on Max hot and I checked my records and I had changed the transmission fluid and filter 3,978 miles ago. I have taken your point on board and will get a filter and new fluid and try and at least eliminate one more possible cause for this aggresive gear change. Thanks for giving more thought to the problem. Clive (Message approved by david_gore) | ||
| David Gore Moderator Username: david_gore Post Number: 979 Registered: 4-2003 |
Clive, what transmission fluid are you using? Dexion II or Dexion III. I have a lot more information at home when I get back on Sunday night and will check and advise. | ||
| Neville Davies Experienced User Username: nev_davies Post Number: 22 Registered: 9-2006 |
Clive One of the main controls for the oil pressure in the autobox is the vacuum unit on the side of the box.Normally a loss of vacuum or the modulator itself being faulty causes late bumpy upshifts but can cause the idle pressure to be too high and thus bumpy engagements.The valve it controls being stuck gives the same symptom.One thing to check if all else is ok. Nev | ||
| Bill Coburn Moderator Username: bill_coburn Post Number: 1276 Registered: 4-2003 |
I wonder whether the problem is not in the transmission but instead in the mounts for the rear sub-frame. If they are very worn, the whole rear assembly is flopping back and forth between Park and Drive. Is the 'thump' detectable when you pump the accelerator up and down while driving? The rear wheel centres may also move back and forth while stationary and the transm ission changed from forward to reverse (not sure there with the later system but that was a clear indication of the problem on the Shadow setup.) | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
David & Nev, I am learning a bit more each day about this gearbox. David, I am using ATF-Q3 Dexron III fluid. The label said it is recommended for Jaguar, Rolls Royce & Volvo. I went on to a Jaguar info website and again Dexron III recomended, That is how I came to be using it, so I hope I have the right transmission fluid. Nev,is it possible to alter the pressure on the modulator? I will look at that after checking out everything else. All contributions to resolving my 'aggresive' gear change have been most welcome. Clive (Message approved by david_gore) | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Good morning all from the UK, A quick note to say I am on holiday next week and therefore will not be in a position to reply to the advice you have given me to resolving the 'aggresive ' gearchange I am experiencing. On my return I will have time to check mounts front and rear, check if the modulator is working correctly etc. and will report in due course. Your advice has been most helpful. Clive (Message approved by david_gore) | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2211 Registered: 4-2003 |
Clive, A few issues: Governor, Modulator, One-way valve in vacuum line, Fluids, Rear Hubs. Sorry for the length of this reply. It may just be that your non-return valve in the vacuum line to the modulator, located underbonnet, is sticky or gummed up. A leaky modulator would not help, and can also suck fluid into the intake (smoke, loss of transmission fluid). Only the Turbo cars have this valve. It stops the intake pressure from blowing the modulator with turbo boost. I have been tuning my transmission of late, and it is more to my liking now. I used B&M bits: their governor weights and spring kit and their double-red-stripe adjustable modulator. It had always upshifted Wide Open Throttle WOT at just 3’900RPM which was annoying. That is especially bad as we can only buy 98 RON unleaded here, and on full-throttle upshifts at 3900 the knock sensors cut the power as the engine loads to drop RPM. It was better in Europe with 100 RON (100ROZ). I set it up to upshift at 4’400RPM WOT, then adjusted the modulator not to give late or bumpy upshifts. That way, WOT it now rides sufficiently past the full-torque knee point to avoid the knock sensors taking over. Incidentally, the major UK Crewe outlets sell this exact modulator, or the near-identical B&M Red Stripe, for these cars, but preset them to be about right bolt-in and multiply the price a few times. All the B&M modulators are fine in our cars, and have more than enough range of adjustment for any road car. Only in extreme race motors do they run out of range, so they fit cable-operated throttle position control instead for those applications. Incidentally, I use Elf G3 Synth transmission fluid. G3 = Dexron III as required for these transmissions, and it is a fully synthetic fluid. It adds about A$250 to the bill, but is worth it. The type of transmission fluid will not affect the takeup in reverse, but will affect upshifts slightly if you use a Dexron IID or a Ford ATF. Having noted that, has the CLUNK just started ? Did it come suddenly, assuming that the engine behaviour has not changed (idle speed) ? If so, then the rear band servo may be malfunctioning, have a broken spring or the servo piston may be cracked. There may also simply be some muck in the valve body. There is a separator plate sandwiched in the middle of the valve body. The checkvalve (ball bearing) seats do wear, and can cause problems once they become badly worn. A new separator plate costs not much, but great care must be taken to ensure that all the tiny orifices in the plate are redrilled to match the original otherwise the transmission may misbehave. Reverse has: Forward clutch – released Direct clutch – applied Intermediate clutch – ereleased Roller clutch – freewheeling/ineffective Front band – released Intermediate roller clutch (sprag clutch on a very early THM400) – freewheeling/ineffective Rear Band – applied Worst case, the valve body is easily removed in-situ – it takes 20 minutes – and secondhand ones are a dime a dozen in Australia and the USA. If you go for a fixed modulator, you take pot luck. The adjustable ones have only one drawback, and that is if you dive through a large range of altitudes. The original ones are altitude-compensated, and the B&M ones are not. If you don’t go between seal level and 2500m regularly, then the non-compensated one is preferable anyhow. On the CLUNK, and referring to Bill’s comment in a slightly different manner, Turbo Rs habitually have loose splines at the hub. I fitted a new hub and driveshaft pair just after the full transmission overhaul a few weeks ago, but that was OTT and expensive. The symptom is that you can see the hub lift when you engage reverse, and it all comes tight with a clunk. You can also lift the hub drive with the vehicle in neutral by lifting by hand at the outer Lobro – maybe even as much a 10mm, where ˝ mm is acceptable. It all looks and sounds quite alarming, but it’s not that bad really. It can usually be corrected by removing the hub drive flange where the Lobro bolts on – the nut is inside the flange behind the Lobro flange, then reassembling it all properly with the nut to 80lb-ft well lubricated with Nickel Antiseize or Silver Antiseize. That worked just fine on a friend’s Turbo R just recently. If that doesn’t work, then a little Loctite 638 on the splines will fix it http://tds.loctite.com/tds5/docs/638-EN.PDF If you want to remove the flange later, the Loctite will melt with a cheap heat gun. The original design is adequate but I am working on an improvement. That will be a fitted sleeve, with the end result the same as the splined flanges everywhere else, such that the splines are not left fully floating, as the slightest spline wear causes alarming free play. You can see the existing arrangement in the manuals. The retaining nut tightens the shaft onto a washer, and the washer is only located by a circlip within the flange. The shaft and hub flange are free to spin over the spline backlash as the washer is not rotationally fixed to the flange. Every time the shaft torque rotation changes, the splines move across their backlash and wear a little over time. At least it’s originally infinitely better than on the naturally-aspirated cars with their Woodruff key and a massive and ludicrous 500lb-ft tightened nut until the Turbo hubs were thankfully standardised at Chassis 30’000. Even with the alarming free play, unlike the cars with Woodruff keys they never vibrate. RT. | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Hello All, Just got back from my hols on the Gower, Wales. Richard thank you for a most detailed analysis, the deciding factor will be if I am technically competent to undertake all you advise. Now I am back home over the weekend I will put the Bentley up on the ramps and start checking over all that everyone has suggested. I took the car with me last week and the gear change was a lot smoother after the fluids were hot. I will look at all the mountings but may well change the fluids and filter as a precaution and starting point. Once again thanks to everyone for their very welcome advice. I will report back when I have some news. Regards, Clive (Message approved by david_gore) | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 666 Registered: 6-2006 |
Clive - you should have popped in for a cuppa - I'm only down the road from the Gower.  | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Hello Paul, I was at Middleton,Rhossili and I did think about calling in, just to meet you and say hello. As the car was not running as well as it might I kept its use to a minimum. Did come into Swansea on the bus, what a scenic journey on the 118, visiting just about every village on the Gower. I do come down quite frequently so possibly come and visit you then. I hope to have the car on the ramps over the next week and then I will check off all that everyone has advised as could be the possible cause of the sometimes aggresive gear change. Regards, Clive (Message approved by david_gore) | ||
| C Lungmuss Unregistered guest Posted From: 195.93.21.34 |
Hello again everyone, Just a quick update. Got the car up on the ramps and had a good look at the various mountings. As best as I could see nothing out of the ordinary there. Checked the actuator lever and tweaked it a bit to make sure it was spot on. Checked the engine speed again and settled for 580 rpm. Drained the gearbox, put a new filter in and 'O'ring. New transmission fluid although I had changed it 4,000 mile back. While it was up in the air checked foot/handbrake and all levers etc. Took it out for a test and the change from Park to reverse and on to drive was a lot better and compared to the RR Spirit. Something cured the problem, so thanks to you all for the quick advice it helps so much. All the best & happy motoring (Message approved by david_gore) | ||
| michael vass Grand Master Username: mikebentleyturbo2 Post Number: 405 Registered: 7-2015 |
Thanks Clive. Mine is the same, must get it changed soon. Cheers Mike |
