I think I have done all I can to prevent failures of my convertale top. I have installed a voltage drop resistor and the hoses are not original. They failed once and have been replaced. Another possibility crept into my mind. When hoses burst, is it because the pump is running when it should not be? Is it running against a dead-head? If a switch has failed our any other of a hundred sequences are screwed up that could happen. The pump running against a dead-head might burst a hose even if it does have a voltage drop resistor. Can anyone tell me what mode there top was in when they got the leak. Was the top operating normally and the leak occured? Or had the top failed to move, or the latch jammed, or was something abnormal happening when the leak occured. My thought is that the resistor is a good precaution but maybe there is still a possibility of bursting a hose. I was much happier before I thought of this. Can anyone tell me what mode their top was in when the hose burst?

 

If you have the pressure relief valve, it spills pressure above 1000psi. So, if there is "deadheat" pressure - 1000 psi is 1000 psi. It will dump the pressure.

With the resistor solution, I don't know. If the motor is turning slower because of the resistor and there is a "deadhead" period, would the pressure go over 1000 psi? I suppose that was thought of when the resistor solution was designed. When it was designed, was the pressure measured, or was it a theoretical design?

Jack

 

Martin,

The pump does indeed dead-head briefly, a few 10th of a second probably. This has been understood from the get-go, and is in fact not unusual for convertible top hydraulic systems.

At dead-head, the pump creates the max pressure of which it is capable. That presure depends on the motor torque available to drive the pump, which in turn depends on the voltage supply to the motor.

This max pressure has been measured at ~ 1600 PSI when the pump is driven by alternator voltage. Also by measurement, the max pressure is reduced to something under 1000 PSI when the resistor is placed in series with the voltage supply. That's what the resistor is there to do. The resistance value of 0.2 ohms was chosen because that is the value that was found to limit max pressure to the desired value.

 

Very good! I thought that would be the case. One more precaution I take, is to not have the engine running when I put the top up or down. Thank you much.

 

It usually bursts when opening.

 

I had my hose failure upon opening. The top opened in normal fashion and I noticed a darkening stain on the screen at the light console. I guess I am lucky because I didn`t have a shower, just barely a drip. I did close the top again right away with just a little more leakage. I consider myself lucky that I have a relatively slow leak and not the shower described by other forum members...gotta fix it soon, the weather in SC is getting ripe for top down enjoyment. If anyone knows a good independent shop in the Charlotte NC area I`d appreciate a post. Still love these cars....

 

You can go to my page and read the information I have on the hose failure or you can go directly to my information on the hose. A lot of information I have on it was posted as people gave me information. That is when I realized I needed to come up with real information on the failures. I hope this helps!

Link: Hyd Hose Evaluation

 

Gus: Thanks for sharing all of your work. I didn't own my car when she blew, but it still had a lot of oil under the back seat, so I don't think the leak was at the latch. Dennis07 made a good statement. With this system some normal short term dead heads are normal. If there is a mis-que in any of the controls, that could cause another dead head, but if the pump has limited pressure (caused by the resistor) the hose is not apt to burst. I am satisified to stop with what I have. That is: good quality hoses, a 0.2 ohm dropping resistor, don't operate the top on alternator voltage, and try to only operate the top when ambient temperatures is not extremely high. If anyone ever has a leak after installing the resistor it would be great to let us all know. I don't expect it to happen. Thanks eveyone!

 

I need to tell you that I never experienced a Dead Head situation with my roof prior to and after the install of the pressure relief valve. However Dead head is common with the resister system. Hose and seal failures were due to excessive pressure. You say you have oil under the rear seat that is from the lift seal or lift hose. Both are expensive items. Make no mistake if your hoses are already stressed they may fail with either system you have installed and yes it has already happened several times.

 

Gus:
Wow! You are saying the cars that have the resistor installed have failed? Also... my hydraulics are new. They just failed to clean up under the seat when they were done. Thanks some more.

 

New is a good thing. Old only means that it is a matter of time and with a reduction installed it will prolong and in some cases eliminate the possibility of failure with no guarantee. As Cigar Guy mentioned earlier everything wears out eventually. It is a matter of when that we are concerned with.

 

I need to straighten out a potential misunderstanding here with respect to the voltage reduction system.

The dead-head condition does exist in a stock system. Even the manufacturer or the relief valve notes this. Without attaching measurement equipment, a car's owner has no way to detect a dead-head and so cannot say if it did or did not occur in his car; it's not as though the pump stops turning or anything like that. So: any inference that voltage reduction brings about the dead-head condition is simply wrong.

A "dead-head" just means the pump is working against a load it cannot make enough pressure to move. That's all it is, and it is not in and of itself a problem, unless it goes on too long (in which case heat can be an issue.) The dead-head in question here lasts a few 10th of a second.

A dead-head condition is unrelated to excess pressure: it says only that the pump cannot move the load it is working against. This could occur at higher or lower pressure, and in fact the whole point of voltage reduction is to have it occur at lower pressure.

I'm repeating myself from an earlier post, but the dead-head condition is common in convertible top hydraulic systems. It is not the problem; excess pressure is the problem.


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p.s.
Martin,

There have been a few failures with a relief valve in place; one or two with voltage reduction in place. Certainly no data has been presented here to show that one system or the other is better at preventing failure. Both seem to be reducing failure rate significantly. (In theory, they should be about the same.)

 

And that may be so, however I stand by my statement that I never experienced a dead head situation with my roof prior to and after the install of the pressure relief valve. And my “however” is based on my testing the resister system that resulted in dead head. Maybe I am not understanding “Dead Head” it is when the roof pump runs and the roof does not move, or when the pump is unable to move the flow.

 

Here's where I'm stuck:

The dead-head we're talking about is at latch-closure time, right? The actuator goes to the limit of its travel, no more flow is possible, and until the pump stops pushing, we have a dead-head and pressure builds. That, as I understand it, was the motivation for the relief valve project.

But I don't see how you could have determined that this dead-head did not occur in your car before installing a relief valve (with the valve in place, I do see why it would not occur). If you did some pressure measurements, I did not know that.

With voltage reduction, not only is this dead-head "common", it is business as usual. I doubt any system out there functions without it. Compared to a stock system though, the dead-head results in pressure building not to ~ 1600 PSI, but to something less than 1000 PSI. That's what the voltage reduction is there to do.

Or ... do you mean that you saw a dead-head with a resistor installed while raising or lowering the top (i.e. a failure to complete that) as opposed to at latch-closure time? I have not heard of any of those with a properly sized resistor in place. (Has anyone?)

These are just questions. I do not wish to disturb the "Pax Jaguarum".

 

Dennis,

No That is not dead head that is hose rupture from the excessive pressure. Keep in mind that the hose that fails is the CLOSED hose not the open. That pressure was measured at 1600psi + when closing hence the reason for the pressure relief valve that bleeds off the excess pressure as the pump is turning. In this situation the pump continued to run until the micro switches and body processer told it to stop as it does with your resister application. I guess you could call that a dead head.

The Dead Head I am talking about is not excessive pressure it is the result of the pump running at low speeds and unable to produce hyd pressure resulting in a stall making the mechanical (motion) part to stop moving and the motor continues to run until you turn the system off and turn it back on. I should mention that the proper resister was installed buy your recommendations on my test. I guess you could call that a dead head as well.
Definitions of Dead Head

Very briefly, pump dead head/shutoff head is where all power is delivered to no net flow out of the pump, so all power is being input only to the fluid contained within the pump itself.

Dead head is the head produced by a pump at zero flow. You don't want to size your pump such that the system curve intersects the pump curve "near" the pump's dead head value. Otherwise, the pump will deliver no flow (which is usually the point of utilizing a pump) and the pump will fail prematurely.

 

Since the pressure relief valves and resistors have started being installed, I'm aware of one failure with a resistor installed and two failures with the relief valve installed. But don't use that data to make a decision about either method. There simply aren't enough data points. We don't know how many units have been installed, and we don't know if any failures went unreported (not everyone who installs one of the devices posts on jaguarforums). We don't even have any data which would show us the expected failure rate, so we don't know if the three failures are actually a decrease or if we would have expected three failures without the devices installed.

Basically, we only have partial data on which to make a decision. But, we know that installing a relief valve or a resistor will reduce peak pressures. I'm more comfortable knowing that my system is operating at a lower pressure.

 

I guess all this dead head stuff comes down to semantics. As a layperson, what I was envisioning dead head meaning pump continuing to run past the end point of the time when the pump really needs to run. In other words, the top closes and the latch sets so at this point there is nothing more to move but the pump continues to run. Obviously this might not be the correct interpretation.

However it seems sort of moot since the curves that Gus presents on his website show the very highest spike in pressure for the stock system occurring during the latch closure period (about 1650 psi) which then drops sharply off. Apparently all that is needed for satisfactory top and latch operation is about 900 psi. My understanding is that the valve comes set to bleed off any pressure above about 900 psi no matter what the circumstances. If you could force the pump to keep running past the time the top is fully opened or closed it is obvious to me that the pressure will still never exceed 900 psi since the valve continues to allow bleeding of pressure. The resistor solution, under this condition, is less obvious to me. Even though you are running at lower rpms if the pump keeps running can't the pressure still build up since there is no bleed-off mechanism? Of course it is possible that the microswitches are cutting off the pump when the roof is fully opened or closed even if you hold the up/down switch too long and, if that's the case, great. But I am not even certain of that.


Doug

 

OK, I want to be sure all is clear here, as the idea was introduced that a dead-head is "common" in a voltage reduced system, but not elsewhere (which is not the case).

"Dead-head" has a precise meaning. The pump is turning, trying to pump fluid, but it cannot. Zero flow. An example would be if you just capped the output line of the pump. At dead-head, the pump creates the maximum pressure of which it is capable.

In a stock system: this condition occurs briefly as the latch is closed or opened (I always forget to mention the "open" part). The latch actuator has hit the end of its travel and cannot move. Until the electronics figure this out and tell the pump to stop pushing, we have a dead-head. The pressure quickly rises to the max that the pump can produce, ~ 1600 PSI.

In a voltage reduced system: everything is identical except the pump can no longer produce 1600 PSI, but only ~ 950 PSI. The pressure rises to this value.


Often misunderstood: pressure reduction does not result, at least not directly, from how fast the pump is turning. Nor would pressure rise to higher than 1600 PSI (stock) or 950 PSI (voltage reduced) if given more time. The pressure produced is just the result of vanes or impellers or something inside the pump pushing against the fluid, and the force with which they push is derermined by the torque applied to the pump, and that torque is determined by the voltage driving the motor. For any applied voltage there is a corresponding steady-state maximum pressure. For this particular pump, we know by experiment that 14 volts makes about 1600 PSI, 8 volts makes about 900 PSI. The pump could sit there all day maintaining that max (dead-head) pressure, were it not for the heat problem if it went on too long.


Raising/lowering the top: far as I know, while the top is going up or down -- as opposed to at the time of latch open/close -- there has never been a dead-head condition observed either in a stock system or in one with voltage reduction installed. (Such a dead-head would be easy to detect; the top would get stuck, and stop moving somewhere midway between the open and closed positions.) If anyone has seen this, please let us know.


And that's what I wanted to say about that.

 

That is 2 failures on the resister and 2 on the valve but who is counting 1 or 10 does it matter? And by the way they all were original equipment and the life was extended by either system.

We have statistics on the pressure relief system that is on my page, I do not see any on the resister other that theory and that was where I have a problem.

My interpretation of dead head came from 3 hydraulic experts not my interpretation.

The curve change on my page is due to the valve doing the job and bleeding the excess pressure back into the system as the switches and computer tell it to stop. On the resister system what tells it to stop? The pump will continue to pump until the switches and computer tell it to stop. The question is when and at what pressure? In the factory system the pump continues without bleeding the pressure resulting in the 1600+psi when the switches and computer tells it to turn off. Remember it is the closed pressure hose that fails.

As for the stall (dead head) on the closing of the roof, I used the same resister you used and it stalled! Also I have spoken to a few that experienced the stall on the open and closed operation. I can say it was not every time the roof was operated but it did take place.

I am not condemning any situation and deal with facts, anything less does not satisfy me in doing the right thing. I have always said “Don’t ask the question if you do not want to hear the answer” and I live by that. I make every effort to provide accurate information and that is why I tested the system myself.

 

Really. Perhaps if you'd look in the paper which has been around since day one. In fact, way back when I sent you one for comments. www.scorekeeper.com/jaguar/jaguar01.htm


So far, we have only your say-so on this. 200+ other users, none has reported it. Can you identify another case? Perhaps you did not install the resistor properly. But you would not have asked for help at the time, would you?