Dash1
Active Member
- Joined
- Nov 9, 2010
- Messages
- 227
- Location
- Merseyside/Staffordshire
- Car
- 2013-Audi A8L (W12) Exclusive (D4) 1998 H-D FLHTCUi and FLSTS 95th Anniversary Models.
Hi Paul/
Thanks for the question and some very good points that you have raised. This post is going to seem very much like a lesson on 5-spped transmissions, and it pretty much is, the electronic control anyway, but worthy of reading to understand the complexities that I previously stated in my last post. My comments here are for reference purposes only to your questions. I do not really want it to detract from the OP’s original opening questions and concerns – “drive on S or C”. I’ve already covered that area.
Apologies to forum members for the lengthy post, but it may be of interest to some of you like Paul/ who genuinely seem to have an interest in this area of a vehicle.
Firstly, lets look at transmissions.
A 5-speed gearbox as opposed to a 7-speed gearbox.
I haven’t got any statistical data regarding their respective gearing ratios in front of me, so I can’t directly compare and calculate the dynamics of the drivetrain/powerflow. However, the electronic shift strategy for up/down gear change is going to be somewhat different on a 7-speed compared to that of a 5-speed. The gearing system must be so as to achieve the end output speed, however, on the 7-speed, one would expect at the very least that the gear changes would be a much smoother - sooner - and a refined operational flow, in other words - seamless. There are actually 8 different hydraulic pressures involved in the transmission, and all have their own respective roles to play. The phasing pressures involved and solenoid duty-cycles on a 7-speed transmission would be totally different to that of a 5-speed, even though they may actually be the same parts used in the valve block. It’s all about the electronic strategy controlling the hydraulics and their respective line and system pressures. The mechanical aspects of the transmission won’t change, how can they, they are “fixed” ratios, whether it be 5-speed of 7-speed – This is a fact.
If you were to have a look at a typical gearing sequence of an automatic transmission, even at a glance, you would be able to see what gears are interconnected throughout the whole drive train process. All of the parts of the transmission are interconnected and lubricated, and at all times. It’s just a mechanical set of components that so happens to use a very efficient lubricating system and oil. You can’t alter the mechanical ratio mechanism unless you specifically include alternative gear ratios, as said above; the electronics are the key factor in all this, and they alone solely control those pressure line solenoids, which in turn allow for the appropriate gear selection given any demands placed upon the engine. Many other factors have to also to be considered, such as vehicle mass, tyre size etc., pretty complex as I have already said.
I use the terminology (TCM) specifically because it belongs to the manufacturer that I work for and only ever came to use when a particular vehicle came in to production in August 2000, of which, I personally was heavily involved in. Other manufacturers call it something else, but that’s of little importance here. The TCM (Transmission Control Module) uses various inputs signals to achieve the correct phase shift frequency and shift-feel under extreme and diverse driving conditions, which takes into account, four very clever, but yet very basic, in principle anyway, adaptation - (a due process of adapting to something such as environmental or external prevailing conditions) programs: the drivers style, the phase shifting time, the valve block and line fill pressures and the valve block fill time frequency. I’ll briefly explain to you what these represent in terms of their overall input/output control.
Drivers Style of Driving. This specific program is “alert-to-adapt” to the overall driving conditions as they occur. The TCM module constantly monitors the vehicles speed; throttle opening positions and the rate and/or change of the throttle position from its potentiometer with reference to time-frame structures. It also makes the provision and looks specifically at any lateral acceleration performance through a series of YAW rate sensors - lateral accelerometers, or floating variable capacitors, which is a specific engineering term for “deviation/curve-recognition”. There are other sophisticated ways, but way beyond the scope of this thread. It also monitors and calculates the rotational speeds of the wheels so as to determine when the vehicle is actually maneuvering/turning, its so accurate as to within 0.5 of 1 mph through the use of anisotropy – anisotropic magneto-resistive wheels speed sensors, known as - AMRWSS’s. In addition to these inputs, it also monitors the frequency of any gear changes. The overall outcome, a system that can and does in fact adapt extremely well and quickly to a shift pattern time and feel appropriate for the present conditions at that time. This particular adaptation, even though it is never ever written-to-memory within the TCM, is consequently referred to as an adaptation that is “live-for-the-moment.”
Shift Time Strategy and Shift Phase-overlap. This particular strategy does in fact focus primarily on the refinement of any gear upshifts/downshifts under any loaded/unload conditions. Shift-time adaptation predominantly gives the TCM’s microprocessor the ability to electronically alter the time – (in milliseconds) it takes to go from one gear to another; in other words, the actual time frame it takes to disengage one clutch while at the same time applying another further clutch. Once the TCM has calculated the type of shift that needs to addressed, the TCM uses the following fill and time strategies to accomplish that very task.
Valve Block and Fill Pressures - sometimes known as - Fill or Applied Pressure. This electronic/hydraulic fill strategy once again gives the TCM the ability overall to control and/or modify - increase/decrease the pressure used to engage a particular clutch engagement scenario, which inevitably results in the type of shift feel that will consequently occur, hence, some of your exact comments in the way you explained them in your other post which I read on the various symptoms - smoother and slower.
Preload Pressure or Fill Time. Once again, this is particular strategy that gives the TCM its ability to control the time it actually takes to fill a particular clutch drum, which in turn, consequently bringing the clutch pack to a zero-clearance but not actively applying or exerting pressure to the clutch pack. This adaptation does actually compensate for the wear characteristics of the friction clutch linings – longevity.
With regards to the “C” and “S” on your vehicle, then it’s possible that they are as you say. As said, I can’t directly compare the two, because I haven’t driven a 7-speed, but would agree entirely with what you said above. My vehicle, to me anyway, performs flawlessly, if there is such a word, and you really can’t feel the gears change, it’s that smooth, you can only detect and notice any change with the rpm fluctuations. I assume yours would perform much better, only more refined.
My view on the OP being told what he was is pretty much what the general consensus was. There are a few ways of looking at this. Lets just assume for the moment that what the OP was told was in fact correct, then the transmission would have inevitably self-destructed by now or at the very least be experiencing some form of mechanical failure. There are several regimes of lubrication principles evident and at work here - directly or indirectly - in a transmission, one is referred to as “hydrodynamic” another is referred to as “full film” the third is referred to as “splash film” and the fourth is referred to as “boundary film”, there are others, if you really wanted to bring them into the equation, all differ in their respective principles of lubricating, but all equally serve the same purpose, to provide a film or lubrication barrier between the “metal-to-metal” or “surface-to-surface” contact. If MB thought for one moment that issues were to arise from a driver just adopting to drive in a particular gear mode, then don’t you think for one moment that would pose or be of concern. They would have clearly outlined and included this in the owner’s manual if that was the case. These 5-speed transmissions do suffer from various types of failure, as to my mind and recollection that has never been one of them.
In answer and finally to your third question, I would definitely not say that “Comfort” mode is regarded as “abnormal”, why on earth would you think or say that, how can that be so? Comfort mode has a specific function within the overall transmission program for the purpose as I outlined in my previous post – winter or adverse driving conditions. Paul, don’t lose sight of the fact that the mechanical process and their respective mechanism do not change; only the electronic control over the mechanics. The names that manufacturers apply to some of these functionality features, is as said, name only, those names do not alter the principles of operation of any system. This is something that you will always encounter in the automotive industry, different names for the same component or system. Granted, your transmission will undoubtedly differ in many respects to that of a 5-speed, but you would expect it to do just that, why on earth would they have changed to a 7-speed transmission if it didn’t benefit the product they were selling - refinement and improvement? To be honest, I have read quite a few horror stories regarding the 7-speed, but not so much regarding the five speeds, although 5-speed do have their issues and problems. You could probably ask other forum members on here who have had vehicles with both transmissions and maybe get their opinions and feedback as to what they prefer in terms of their experiences.
I hope this information is of assistance in helping you to understand a little better how complex a system you are actually trying to analyse, if so, then its been worth my while in writing it.
Regards,
Dash1
