| Author | Message | ||||
     Chris Miller Prolific User Username: cjm51213 Post Number: 201 Registered: 5-2013 |
Hi Folks, Either the ACV does not work the way I think it should or mine is misbehaving. As a diagnostic effort, I want to mark four point on the level of brake fluid in the reservoir on the sight glass:
"Full", "Warning", and "Empty" are very easy points to find. Run the engine for a few minutes. That is presumptive "Full". Pedal cycles until the light goes on, that is "Warning". Continue to cycle the pedal until there is no change in brake fluid level and that must be empty. (Notice that you can calculate the pressure switch calibration point to a crude approximation with this technique.) So, the trick is to find "Recharge". I thought I could start at "Full", with the engine off, and deplete the accumulator by, say 16 cycles, turn on the engine and look to see if the ACV charges the sphere thereby lowering the brake fluid level. If so, then try again with half the cycles, and if not, add another set of cycles. (Anybody with a background in algorithms will recognize a binary search here.) I started at "Full" and I depleted by 16 cycles, which changed the fluid level by a perceptible amount approximately 1/8" out of a 7/8" range, and when I turned on the engine, the ACV recharged. I repeated with 8 cycles, 4 cycles, 2 cycles, and in all cases the ACV immediately recharged the accumulator. So, this violates my understanding of how the ACV works. Apparently it will recharge or by-pass, but there is no, "I'm happy and I have enough pressure for my needs, for now. Continue to by-pass until I am low enough to warrant a recharge." This thing has no hysteresis? I am pretty sure that during bench testing, under gradual depressurization, I could hear the ACV click over into a new state, which I assumed was, "O.K., pressure is low enough, now. Recharge." and I never tested to see if I could discharge a tiny amount of pressure and the ACV would immediately discontinue "By-pass". I just assumed that was how it worked. So, which is wrong -- my understanding of the ACV in general or the operation of this particular unit? Thanks for your help, Chris. | ||||
| Brian Vogel Grand Master Username: guyslp Post Number: 980 Registered: 6-2009 |
Chris, There is a period between max charge and min charge where applications of the brake will not trigger charging. I believe the PSI levels are documented in the Workshop Manual. The maximum pressure is commonly known as the cut-out pressure. The pressure slightly below that once the cut-out has occurred is called the fall-back pressure. And the minimum pressure, which causes the ACV to go into charging mode and bring the accumulator back up to the cut-out pressure is known as the, wait for it, cut-in pressure. I long ago typed out all of the details of this process in the second post on this thread on rollsroyceforums.com All pressure figures can be +/- 50 to 100 PSI or so. Brian | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 203 Registered: 5-2013 |
Hi Brian, > There is a period between max charge and min > charge where applications of the brake will > not trigger charging. Damn... I was afraid of that. This ACV is not working correctly. It is cutting out just fine, but he then thinks he is immediately eligible for recharge.And I haven't got the faintest idea how these damn things work... I think this next time, I need to calibrate the ACV. One is a respectable 2350 PSI cut-out but the other is a wimpy 2,100 PSI cut-out. I'm going to need some shims. Got any suggestions on your list of resources? ALbers, Flying Spares and IntorCar immediately leap to mind, but the truth is, a hydraulic accumulator control valve calibration shim is something I should be able to get at, like Grainger. Once more, unto the breech! Thanks again for the help, Chris. | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 204 Registered: 5-2013 |
Hi Folks, I have poured over the working diagram of the ACV, and I can see how it works, but I cannot find any mechanism for hysterisis, meaning my understanding is that it will charge the accumulator any time the pressure in the accumulator drops below by-pass (cut-out). That is not a good design and it will surprise me if that is in fact the designed behavior, because without hysteresis the system can oscillate which introduces all kinds of problems. I'm looking for confirmation that the ACV does in fact have hysteresis and mine is malfunctioning before I take it out again, although, it would be much better for me to learn that my understanding of its behavior was wrong; that is much easier to correct. (-: I'm asking for someone to volunteer to repeat my measurements in the original post and post the results. These are useful values for each owner to have in any case. If you are successful determining a brake fluid level for "recharge", then my ACVs are defective, and I will have to find someone to explain how the hysteresis is implemented, so I can diagnose why mine don't work. If you, like I, can't prove that it exists, then this will be helpful dispelling a myth. Thanks for the help, Chris. | ||||
| Brian Vogel Grand Master Username: guyslp Post Number: 981 Registered: 6-2009 |
Chris, Every, and I mean every, reference talks about the cycle I described in the post elsewhere that I pointed you to. I have seen it, live and in the flesh, during a Shadow Hydraulics Seminar I took at the Rolls-Royce Foundation. You should easily see the flick-up, cut-out, fall-back, and cut in pressures on your other ACV. If you don't then they're both not working correctly. I am attaching what is one of the definitive technical articles on the hydraulic system, authored by the late Cal West, and published in the official publication of the RROC-USA, The Flying Lady, in issue 1986-2. All of Cal West's technical articles have been collected in a publication called From The Shadow's Corner, which took its name from the article series. If you are, or become, a member of the RROC-US it can be purchased from the club store. It may also be available from the Rolls-Royce Foundation. I've also attached the Systematic Check Sequence Chart - Brake Hydraulic Circuit, that's from the fault diagnosis section of Chapter G of the Workshop Manual, in a format that makes it easy to print as a single sheet on legal size paper. Cal describes the how the system works and how to test it in his article, in great detail. If I am recalling correctly, the cut-in pressure is "tweaked" by using adjusting washers on the seating washer at the far end of the spring in the ACV. I think that "more washers is higher cut-in pressure" but I could have that backwards. I'm sure either someone else can confirm whether adding adjusting washers increases cut-in pressure or decreases it. The Crewe Original rebuild kit comes with adjusting washers but I never used them, but simply reassembled my ACV with everything precisely the same way as when I opened it up to overhaul the seals. You know that it's not essential to know the underlying "hows" in order to do an accurate diagnosis based on protocols that are decades old and very well documented? Brian
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| Chris Miller Prolific User Username: cjm51213 Post Number: 205 Registered: 5-2013 |
Hi Brian, > Every, and I mean every, reference talks about the cycle... I don't doubt that; I'm not claiming otherwise. What I'm claiming is the I don't see stability above "cut-in" pressure. My unit wants to be at cut-out pressure. And I didn't think I invented cut-in, because I also believe that the ACV has hysteresis. My assumption came from somewhere... But, I've been wrong before. I am often wrong two or three times before breakfast, just to stay in practise. (-: I wanted to be sure there was a malfunction before I undertook a project to correct it. I'm pretty sure there is. > You know that it's not essential to know the underlying > "hows" in order to do an accurate diagnosis based on > protocols that are decades old and very well documented? Right... Here's the problem. I have rebuilt these two ACVs recently, so if they are not working, (Well, truth is, I'm actually only talking about the forward unit..) I need to understand how they work to figure out why they are not working. Is there a blocked aperture that I didn't notice? Is there an o-ring leak? Is some sliding part (there are two that I can see) not sliding? Has the little spring on the one-way valve fatigued out of spec? Of course, I'll check these things again, but unless it is perfectly obvious, I'm not sure how I debug one of these guys. I know there is something wrong with the ACV; it is not submitting to the charged state and continues charging. I can see this with the fluid levels. From a full accumulator, I should be able to complete a small number of brake pedal cycles without triggering a recharge. I can see that the smallest noticeable raise of brake fluid level, two cycles, causes a recharge and the fluid drops right back to "accumulators full". Since I don't understand how the ACV works, I don't know where to look for the problem. I have looked at the fault diagnosis chart, and I don't see this condition on it. I'm looking forward to reading Cal West. On calibrating:I suspect the shims will determine the cut-out pressure. Then the only other variable is the tiny spring on the "one-way" valve, whatever that is. So the strength of that spring will probably determine the cut-in point relative to the cut-out point. These are guesses, but it looks like I'll be doing a bit of investigation and I'll know for sure soon... Thanks again for your help, Chris. | ||||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 1244 Registered: 6-2006 |
"As a diagnostic effort, I want to mark four point on the level of brake fluid in the reservoir on the sight glass: Full (ACV By-Pass) Recharge Warning (Dashboard Light On) Empty" WHY WHY WHY??? There is an idiot proof flow chart to check them. I thought it was at least. On review it doesn't say anything about NOT filling kittens with brake fluid and trying to squeeze it back through the return hoses to check the valves.  It does not say anything about NOT trying to guess the correct operation by measuring brake fluid levels and trying to press the pedal to test the break pressures. I guess the last laugh is on the technicians at Crewe for not foreseeing every possible eventuality. So basically - all the advice you need to seek is . . . JUST FOLLOW THE CHART CORRECTLY. I am not a stickler for sticking to the manual in every case BUT in this case YOU must. Here's one problem. You are inventing a way of testing accumulators which firstly is completely unnecessary because there is a perfectly documented method that works perfectly. Here's another. You have very limited knowledge when it comes to fluid flow systems. Soooo, you are inventing something that exists but doing it without knowledge. You are saying there is nothing in the flow chart that explains your findings. Consider this . .. There isn't a problem with the flow chart information. Don't ask why your method isn't working. Ask instead why you are trying to do it that way at all. Rolls Royce have invented an idiot proof test procedure. Use the flow chart and a pressure gauge. Remove the accumulator bleed nipple. Stick the gauge in and test away. If the results are unexpected, put the gauge in the output hose port instead. (put the bleed nipples back ) and retest. There may be a fault elsewhere. The no 1 and 2 systems may have been crossed over. But the flow chart tells you all this. With every single pressure release valve on your car you will have a hysteresis. Nearly everything in the world with a spring operated pressure valve will. It takes more pressure to open the valve than it does to keep it open. The pressure reaches 2500 and it will have enough force to open the valve. Once the valve is open the force needed to keep it open is less. The fluid is acting on the whole valve face now. The fluid is in the way of the valve closing. There is no 'clever' mechanisms, no one way valves involved. SO use a pressure gauge. Use the correct testing method. Test your valves correctly. Then go from there. Come back with a question about the results if you need to. Indeed it is helpful to understand the operation of a particular unit. But you need to learn the basics before trying to understand the advanced. Do all your pump tests with a gauge not by measuring fluid. Simple. | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 207 Registered: 5-2013 |
Hi Paul, > WHY WHY WHY??? Well, since you asked, because this can be done with careful observation without any other tools. Knowing these values can assist a diagnosis, if you happen to be not willing or maybe not able to crawl up in under the engine at the time or don't have any tools and gauges handy. There are lots of reasons. Your histrionic ranting and raving, ridicule, and ad hominem attacks are really tiresome. If you want to be helpful, be helpful; if you want to be abusive, go to hell. > It takes more pressure to open the valve than it does to keep it open. So, finally 75% through your post, you have calmed down sufficiently to be helpful, and I appreciate the help. Opening the valve is done with the central piston. Once open, you say that the face on the spring is what keeps it open? That makes very good sense and I almost believe it. I know that the face on the main spring is perforated so I disagree that pressure on the face of the spring can be maintained without a fluid flow across the face that is restricted by the perforations, maintaining a pressure difference at each end of the perforations, and there can be no flow, in this case, because the spring is in a sealed "canister". If there were a constant flow, then the perforations would certainly create a flow dependent equilibrium, and the spring would remain compressed until flow dropped below a certain rate. > Come back with a question about the results if you need to. I have posed a question, and it is a valid question. How does the ACV implement hysteresis? You have helped with a discussion and offered an insight, the face of the spring, that is certainly in the ballpark, but has its own mystery, the perforations that will prevent preserving pressure. As far as my technique is concerned, The pressure measurements is one way to do what I did. Any competent engineer knows that there may be more than one way to skin a cat. Fluid level is a function of fluid volume and the accumulator is powered by nitrogen gas pressure, so the Ideal Gas Law applies, which makes volume a perfectly good surrogate for pressure. Any competent engineer knows this, as well. I have diagnosed a failure of hysteresis, and I did it with fluid levels. Do you dispute my finding? Can you explain why my most imperceptible raises in fluid are immediately rectified as soon as I start the engine? Wouldn't correctly functioning hysteresis give me a region where fluid levels would remain constant? You would have a valid criticism, if I had used tea leaves, tarot cards, or the phase of the moon, but my use of fluid levels is a perfectly valid technique and one that doesn't require anywhere near the disruption of removing lines and inserting gauges. So, if you would like to be helpful, and I know you can be helpful, the outstanding questions I have asked are:
Thanks for your help, Chris. | ||||
| Brian Vogel Grand Master Username: guyslp Post Number: 984 Registered: 6-2009 |
Chris, I say this in an effort to be friendly and helpful: Stop trying to reinvent the friggin' wheel. You don't seem to pick up on the fact that more than one of us don't really understand this need in you and we also don't feel the urge to try to invent new ways of doing things that have been done, easily and effectively, for decades using tried and true techniques. Read the Cal West article. Stop getting so upset with people because they don't want to try to reinvent the wheel. We are trying to help. Some of that is by telling you, directly, to stop trying to force us to conform to the way you would like to do things. Sometimes you're going to have to listen to advice or you will very soon not be getting any. Brian | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 208 Registered: 5-2013 |
Hi Brian, > Stop trying to reinvent the friggin' wheel. I'm not "reinvent[ing] the friggin' wheel". I used a perfectly valid procedure and I diagnosed a defect in hysteresis. The chart does not evaluate cut-in at all, so, using the chart does not uncover this. The Cal West article does discuss it, but only talks about correct observable behavior and does not discuss what to do about it, nor does Cal West go into detail about how the ACV works (and I didn't expect he would, so that is not a criticism). I did read the Cal West article, and it was quite good -- very good in fact. I already knew what it had to say, but thanks for giving it to me just the same because it may have had new information. I'm not upset. I was rather pointed in my remarks to Paul, but I think they were justified. Maybe I just don't know Paul well enough and if I did I would recognize that his writing is just good nurtured fun and ribbing, but I repeat, I don't know Paul well enough and I don't see his comments as fun and good nurtured ribbing; I see them as derisive and pugnacious. I welcome his advice, but if it comes at the cost of ridicule, I don't think I need it. There is nothing ridiculous about my choice to measure brake fluid levels, and as a result I diagnosed a fault that the vaunted "Hydraulic Circuit Fault Diagnosis" chart missed, and I did it in about five minutes, in my suit and tie. A full sequence of the steps on the chart would take on the order of an hour, maybe an assistant, and I'd have to bleed the system. Rather than demonstrating ignorance, I think I have demonstrated a rather clear understanding of how this system works and a rather clever economy of effort. I ask for help with how this component works, because "System Faults" are in reality component faults, and system remedies advising "rebuild the ACV" are not entirely helpful beyond indicting the ACV. Because fluids are non-compressible (within the limits of their bulk modulus) and the potential energy of the system is stored in a gas, pressure and volume are dictated by the Ideal Gas Law, which means that brake fluid levels correspond exactly with pressure levels; they are not as easily measured owing to the accident of the relatively broad, flat brake fluid reservoir, but with a suitable graduated cylinder would give accuracy of measurement no less than the pressure gauge method and with considerably greater convenience. The theory is sound. This thread, (after the adjudication of fault between my understanding and my ACV was determined to be my ACV,) has been, presumably, the discussion of how the ACV works. Paul's initial remarks tried to hijack that and turn it into ridicule of my method, without actually discussing the method! If my method is faulty, then it should be so easy to show that fault, and wouldn't that be so much more rewarding anyway?. "Geeze, look at this obvious mistake! He didn't take atmospheric pressure into account! That Chris is a fool!" Paul has not indicated any fault with my method; just implied that it was there with ridicule. Ironically, Paul did talk about the face of the spring and entered into a discussion about how the ACV works. Pending the explanation for and behavior of the perforations in the face, Paul may have fully answered the question. This discussion, which I can only hope continues, looks like it may be productive, and I thank him for that part of his comment. For your consideration, Chris. | ||||
| Brian Vogel Grand Master Username: guyslp Post Number: 987 Registered: 6-2009 |
Chris, As I've already offered privately, I'll state in public as well: You need to consider, carefully, "how you read." While there have been a number of what I consider "overreactions" at times, you are not appreciating, at all, how your own style triggers the responses that it does. I consider the "mark the sight glass" bit a big version of "reinventing the friggin' wheel." I am quite sure that a very productive, and interesting, conversation could be had about exactly how the ACV functions as far as determining and adjusting the cut-in pressure goes. That, however, would be for a bit later after we all know a bit more about what's actually going on. I'd love to have that information just to satisfy my own curiosity, but it's not the "question that needs answering" at this point in the game. That's why there's resistance to "going there." Those who can answer it want to have a better idea of what's actually occurring to guide their responses. Topics like this tend to have layers, like an onion. Brian | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 211 Registered: 5-2013 |
Hi Brian, > reinventing the friggin' wheel. No, it is not. I was able to determine that the ACV is not submitting to the "Full" state, meaning it wants to reset the ACV from "By-Pass" to "Charge" with the minutest decline in pressure, with my brake fluid level observations. I was able to determine this in about five minutes, without an assistant, and in my suite and tie. The fault diagnosis chart requires a gauge, some wrenches, tubing, crawling under the car, an assistant, an hour of time, and a bleed of the hydraulic system when I am done, and it answers questions I wasn't asking, but more importantly, it does not answer this one question I am asking. > I'd love to have that information just to satisfy my own curiosity, > but it's not the "question that needs answering" at this point in the game. With all due respect, and I have a lot of respect for your contributions, it is exactly the question that needs to be answered. Having determined that the ACV is in fact misbehaving, it becomes important to know how it is supposed to behave, so I know how it implements hysteresis and I can debug it. Paul Yorke has offered the very real insight that the face on the spring probably has something to do with preserving the "Full" state until it reaches "cut-in", but it is still not completely clear how this happens with the perforations in the face, but if I can keep him engaged, I am sure he will completely explain it Thanks for the help, Chris. | ||||
| Bob uk Unregistered guest Posted From: 94.197.122.91 |
Not sure about this because I only have a brakeology picture from the good book. Usually valves like this are controlled by differential pressure either side of the spool. I think it behaves like a demand valve. These are fitted to pas pumps and are complex to understand without lots of info which is not in the manual. Marius Riker site has an animation of an avc. (Message approved by david_gore) | ||||
| Bob uk Unregistered guest Posted From: 94.197.122.85 |
Chris, The acv is a demand valve. I know that you don't understand hydraulic systems Because >perforations how does it hold pressure. This means you think that the pump is pumping all the time, the pump when 2500 is reached goes nominal pressure say about 4 psi. (Message approved by david_gore) | ||||
| David Gore Moderator Username: david_gore Post Number: 1445 Registered: 4-2003 |
I have returned from a short trip away to find this thread has become another problem where discussion has resorted to abuse rather than respectful vigorous expressions of opinion. I am very disappointed to see this and apologise for not being online to take remedial action before now. This situation has been exacerbated by the topic being one which is difficult if not impossible to understand by most members. It does not help when the main problem is due to someone "doing their own thing" instead of following the procedures detailed by the manufacturer. This problem is compounded where individuals have little or no experience in diagnosing and servicing hydraulic equipment. It is not appropriate let alone tolerated for a contributor to insult/abuse another contributor especially when one is an experienced professional who makes a living servicing R-R/B vehicles and the other is not. The secret of respect is to listen and try and understand what the other person is saying. If you do not understand, say so, and seek further information. If you want to explore an alternative, say so and ask for opinions as to whether it will work and, if not, why not so you can re-evaluate your ideas if necessary. Abusing someone who holds different views to yourself only serves to alienate possible future sources of advice and for others to form an unflattering opinion of you. Chris, I suggest you should take Brian's advice and go back to your post and review it in the light of his and other comments. You may consider a more conciliatory response would have been appropriate rather than reacting as if it was a personal put-down. This is an International forum and you must appreciate people in different countries have different forms of expression to your own and this may appear appropriate to them and inappropriate to you. I have no problems with Paul Yorke's expression being typical of a UK professonal person, forthright and descriptive but not insulting under the circumstances involved. I consider your response missed the point of Paul's comments for the reasons subsequently posted by Brian. Life is too short for acrimony and retaliation/pay back; better to agree to disagree and move on. Main problem here is you are probably alienating a valuable source of future helpful advice to help solve and/or fix an intractable problem you have encountered. P.S. I have a hide as thick as a rhinoceros and my philosophy is essentially "sticks and stones may break my bones but names will never hurt me". I accept the advice and opinions of those I respect; my respect is earnt by actions and behaviour and not conferred by position, title or alleged authority. | ||||
| Brian Vogel Grand Master Username: guyslp Post Number: 989 Registered: 6-2009 |
Bob, The pump is pumping all the time. It's always moving at the rate of the camshaft. Of course, when the ACV is in bypass mode, there's virtually no pressure in free flow mode since nothing's being "pushed against" but the fluid's simply going in an open loop between the reservoir, pumps, ACV, reservoir - lather, rinse repeat. My understanding is, though, that the pumps can easily keep up a pressure of 2500 PSI when needed if an accumulator's dead and the pump is the only thing delivering pressure in non-bypass mode, working in concert with the ACV. If that's incorrect, then I'd like to understand why. [And, no, I'm not proposing that one *should* run a car with a dead accumulator. I'm going on what I was told at the hydraulics seminar I took in early 2007 (I think).] Brian, who's always trying to get a better handle on how this whole system works | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 213 Registered: 5-2013 |
Hi Brian, I agree with your understanding. The pump is completely unaware of anything happening downstream and does not adjust its behavior accordingly. The pump can only maintain a pressure of 2,500 PSI if there is sufficient downstream "resistance". So, let's assume it takes a minute to fill the sphere, and it takes 30 cycles to empty it. That means the pump can do in sixty seconds what you can undo in thirty pedal cycles, or alternatively, the pump can produce one brake pedal cycle in two seconds. So, with a completely dead accumulator, the pumps can apply the brakes in two seconds, assuming that there is no latency in the ACV switching from "by-pass" to "charging". If the accumulator actually holds 60 pedal cycles, then the pumps can apply the brakes in one second. I suspect it is somewhere between. At 60 M.P.H., two seconds is 176 feet just to apply the brakes, and a really long time, so needless to say, slow and steady gets you home, but you can get home. For your consideration, Chris. | ||||
| Bob uk Unregistered guest Posted From: 94.197.122.78 |
The acv relief valve is held open by fall back pressure not by pump pressure. The fall back pressure comes from the pump originally. Hence the 100-150 pressure drop. When the pressure is.demanded the pressure drops and.when it gets to 1850-1900 the fall back pressure is to low to hold the relief open and it shuts. Then the pump recharges the sphere. This means that unlike an engine oil pump relief, which is held open directly by the pump and the pump is.there fore on duty all the time. The RR pumps go off duty and are controlled by the acv demand valve. The pumps are still pumping but with relief valve held.open by fall back, so the.pressure in the pump.is nominal. Otherwise the RR pumps would be pumping at 2500 all the teme '.And would probable wear out and wear the cam in very short.time.say 5000 miles. (Message approved by david_gore) | ||||
| Chris Miller Prolific User Username: cjm51213 Post Number: 214 Registered: 5-2013 |
Hi Brian, I've thought about this some more and it is not simple. Suppose your accumulators are dead. Your pump will provide pressure to operate your brakes, but it is not straight forward. Even before you apply the brakes, the pump will be applying pressure and the ACV will probably be oscillating as the downstream pressure meets "by-pass" and then immediately drops to zero, because there is no compressible element in the circuit. When you apply the brakes, there will be a short time while the pump fills the calipers, but once "by-pass" pressure is met, the ACV will cut-out and pressure to the calipers will drop to zero, followed immediately by cut-in and more pressure. It is not clear if you will have the equivalent of anti-lock brakes or anti-brake brakes, but in any case, I don't think it will behave gracefully. Farther thoughts, Chris. |
Cal West Hydraulics Article