| Author | Message | ||
     Mark Taxis Experienced User Username: mark_taxis Post Number: 173 Registered: 04-2007 |
Up until recently the gear change in my mk6 Bentley has been perfect, the gear lever just glides into place, with little or no pressure. However recently I have experienced “resistance” when trying to engage 2nd gear. This only happens once the car has been running for a while and so I am assuming ii may be related to the oil temperature in the gear box. There is no graunching of gear just the reluctance and resistance to selecting 2nd gear, furthermore it is not consistent, sometimes it just drops in gear without any problem. This is starting to make me think that there may be some alignment problem between the gear selector and the gearbox. Has anyone else experienced this type of problem? Any info would be gratefully received | ||
| Hubert Kelly Frequent User Username: h_kelly Post Number: 370 Registered: 03-2012 |
Hey Mark, not familiar with your car, but from your description it sounds like a linkage problem ie worn bushing etc. I had trouble with an old 68 Van den Plas limo gear change, it turned out needed bushing sorted. If it's column change, with window down, try change gear and at same time support linkage if you can , this may help in diagnosing the problem?. | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 174 Registered: 04-2007 |
Hubert, it is not a column change, but someone has suggested that it could be caused by the rear rubber gearbox mount being compressed and causing a mis alignment, I will check this when I get home. | ||
| Chris Carnley Unregistered guest Posted From: 109.156.42.4 |
All other things considered, it may be that the large 2nd speed synchro cone is becoming jammed onto the 2nd speed driven gear hub. This then baulks the dog teeth on the 1st gear trying to make selection. I get this sometimes on overhauls if the 1st speed gear sliding keys push the 2nd speed synchro cone too close to the gear hub. As you suggest when the oil becomes hot and thin then sticking may occur. The solution is to move the 1st gear a bit forward by the top rear adjuster on the right hand side. It looks like a machine screw head with a big slot in it. Too much adjustment and the synchronising effect is delayed and you get graunching. About .020" between the ends of the keys and the cone lugs is about right. You should be using EP 89/90 gear oil and add a "dry" friction improver like Slick 50 which contains PTFE. (Message approved by david_gore) | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 175 Registered: 04-2007 |
Chris Many thanks for your reply. I have to admit that the workings of a gearbox is magic stuff to a simple sailor like myself. When I get home, I will check the oil grade that is in the gearbox, - I did an oil change about a year ago. Will also have a look out for some dry friction improver as you suggested. If this does not resolve the problem I will take the car to the Bentley specialist in Brisbane and get him to take a look. | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 79 Registered: 09-2004 |
I had a car with this problem, I think. Let's compare a bit to see if it is the same. When shifting from first to second, if the shift was not very quickly done, and the gearbox input slowed down so that the synchro had to speed up the input to engage second gear, it would be extremely stiff. For fear of breaking something with excessive force, I would go back to neutral, lift my foot from the clutch, and rev it a bit to speed up the input, and then quickly put down the clutch pedal and put it into second, which with the input sped up, would slip in very easily. This method commonly called "double-clutch," and a necessary technique for downshifting a non-synchro gearbox, like in a P-1 or a Ghost. This was an early Mk VI gearbox, before some mods to strengthen shifting mechanisms inside the gearbox, and I believe it was due to wear of some parts inside. I never did get it sorted, sold the car before I got around to it. Here is the key point I am trying to develop: I believe that if changing what is happening INSIDE the gearbox, i.e. relative speeds of the gears, affects the symptoms, the problem is inside the gearbox. However, if doing this has no effect on the problem, then something OUTSIDE, the gearbox, i.e. linkage could be the cause. Careful analysis of symptoms often leads to correct diagnosis of a problem cause. In your situation, try to associate exactly when and how the excessive resistance is occurring, and what is simultaneously occurring inside to make an association that will help you narrow down what might be wrong. Generally speaking, this approach requires knowledge of how the components function. | ||
| Christopher Carnley Unregistered guest Posted From: 109.156.42.23 |
There were never any modifications to strengthen shifting mechanisms. (Message approved by david_gore) | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 176 Registered: 04-2007 |
Gents Many thanks for your input. Chris, I checked the oil and found I have put 80 weight oil in the box at the last oil change. I have drained the oil and replaced with 85/90 I have only driven the car for about 60 kilometres and the improvement is remarkable. I have yet to add the Friction improver, I will do this in the next day or so. Bill, hopefully I have fixed the problem, but in any case I never use the first gear, have never found the need even on hill starts. | ||
| Christopher Carnley Unregistered guest Posted From: 217.44.228.67 |
Mark, With these old worn synchros, the Slick 50 Manual Gearbox treatment with PTFE makes them feel like new. The oil tension grooves in the PB linings wear grooves in the conical gear hubs. (Message approved by david_gore) | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 80 Registered: 09-2004 |
Chris, OK, I was wrong on that point. I was thinking of the thrust washers. I believe the new ones were thicker, and there was a bulletin that included some machining to accommodate the new washers. What was the purpose of this? What was the operation of the gearbox that would lead to problems without the modification? I agree this is not a shift mechanism, but I thought related to gear change operation. | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 81 Registered: 09-2004 |
Mark, Shifting from third to second was an even greater challenge. The input needs to be sped up in every case, and if the synchro cannot do it without application of excessive effort, double clutching is the only way. | ||
| Christopher Carnley Unregistered guest Posted From: 217.44.228.67 |
There were several modifications to the original 3rd motion shaft RG5469, the first being the splay grinding of the .175" groove to accommodate the .225" washer with the splayed inner as square grinding of the groove proved to be impracticable. The groove was ground .050" further back for the B3WG and on mods, this involved the deepening of the shoulder in the 3rd speed driven gear by .050", the shortening of the bearing sleeve by .050" and grinding .021" off the 3rd speed side of the synchro sliding piece. OK? (Message approved by david_gore) | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 177 Registered: 04-2007 |
Hi Bill, in my case double de clutching made no difference to the problem, but for now a change of oil has definately improved the shift, I will be adding the slick 50 today. | ||
| Christopher Carnley Unregistered guest Posted From: 86.182.105.203 |
As 2nd,3rd and 4th speeds are constant mesh gears, where the male engaging dogs on the sliding elements lock the gears on the splines by being pushed into the female dogs within the gear hubs. Therefore any double clutching is absolutely pointless and useless. The only double clutch gear is 1st, and you must not under any circumstances double -declutch this gear- ever, or you will cause expensive damage. (Message approved by david_gore) |
