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Thread: OTR - Prevost Nashville

  1. #21
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    Nov 2006
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    Wilsonville, OR 97070
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    852

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    Jim and Chris is there by chance anything worry with your cooling system on the gen set?

  2. #22
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    Jun 2006
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    Indian Hills
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    NO Coolant was fine, Oil was fine and radiator fans were turning, air blowing out from the bottom from the fans. Its just a hot one. enclosed in that bay/box. Seems OK. Only change to make, it we discovered we need to use Straight 30 weight oiil in it instead of multi wiehgt. A tech pointed that out since our venset is non-turbo'ed
    Jim and Chris
    2001 Featherlite Vogue XLV 2 slide with Rivets-current coach, 1999 shell
    Previous 22 years,
    We have owned every kind of Prevost shell but an H3-40

  3. #23
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    As a former aviator I would guess that every other pilot on here is very sensitive to the equipment we all flew. I was never able to prevent every equipment failure in-flight, but I probably was able to correct a lot by paying attention to what the plane was telling me. If a gauge was different by as little as a needle width I wanted to know why because it might be the first sign something is starting to fail.

    I try to be as aware of clues that are forecasting a problem in the bus in the same way, but despite my best efforts there is so much going on in the bus I am not always succeessful. I posted earlier that if we take the time to measure our AC discharge and return air readings and record them we can probably predict when our AC systems need attention. Unfortunately we can spend hours gathering and recording data and stuff will still happen. This summer appears to have been a true test of our coach's AC systems. And when our AC systems are really needed none of us wants to find out they need service. Ditto with our generators, engines, etc.

    But how many problems with AC systems as an example were predictable? My plane like a lot of others had a scanner or engine monitor. It had sensors which made it possible to read the cylinder head temperatures, exhaust gas temperatures, turbine inlet temperatures, etc. That information was available as the plane was flying and it could be downloiaded to a computer and displayed as raw data or as a graph. What was very important about the ability to have such data was that once a baseline was established, deviations from the norm could be used to diagnose or predict problems. There wasn't much going on in the plane that I did not know or have access to.

    I am not going to outfit my coach with sensors and track the data from all the things in the coach. It would be information overload. But what everybody seems to be experiencing this summer with unanticipated AC problems might be avoided if I start making AC system data logging part of my routine maintenance. I will not be able to predict a compressor failure, but I should be able to identify a loss of refrigerant, dirty filters, and other changes that might be signs of an impending issue. Most of the problems we encounter with our AC systems seem to be related to loss of refrigerant or dirty filters, with motor problems lagging somewhere behind in terms of frequency.

    So as soon as I can I am going to create a data logging sheet to include with my regular service to see if AC system failures are predictable to a certain extent. I am not worried about data tracking on my engine or generator as much because most if not all the data is already available, including oil pressures, engine temps, voltage output, etc.

    Who knows? Maybe if this turns out to be worthwhile, future coaches can be set up with displays or data logging capability to make this information available so we all can predict low refrigerant levels and get them adjusted when we don't need our AC systems.

  4. #24
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    Jan 2006
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    Jasper
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    Aviation habits are hard to break, such as gauge scan and can immediately identify a problem. Yesterday when my A/C compressor locked up, I picked up on the problem even before the cooling stopped. My engine and tranny gauges always read 15 -20 degree's less when the A/C is running due to the engine fan running on high. As soon as I saw the temps change, I knew there was a problem and soon after, things started getting warm in the bus. I stopped shortly after and the A/C belt was smoking since the compressor was locked up. Cut off the belt, start the gen and went on our way.

    So, scan those gauges often. What about installing a digital thermostat on the intake and output of your A/C and then you can watch the difference in temps and learn what is normal. I have one in my inverter bay and generator bay so I can see the temps on one gauge. Is that overkill, maybe, but nice to know whats normal.

  5. #25
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    That's exactly the kind of data point that is needed, but a snapshot is not sufficient. We need all the data on a continuing basis so we can see variations. This type of information is available to a very limited extent on multiplex coaches because with the multiplex message center the OTR high side and low side system pressures can be monitored.

    I presume there are devices which will enable numerous inputs to be monitored and even downloaded for analysis. It is not much of a stretch to envision this on future multiplex coaches. I do not know if the multiplex system can or will store data for analysis, but I do know once a benchmark is established it would be possible to generate a fault code if some parameters are not met.

    I suspect if a person was willing to go to the expense such a capability could easily be retrofitted to all existing coaches and there is probably no limit to the number of things such a system could monitor. In case this sounds too futuristic it is not. Our cars can display a lot of information now and tell us when all sorts of things require attention from tire pressures to engine temps to oil pressure. I am not familiar with Silver leaf but it sounds like that is the exact type of platform that is needed to pull all this together. If we can warn our holding tanks are full or our engine parameters are exceeded we already have the tools.

  6. #26
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    Sep 2010
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    Mt Baldy, CA. and Nashville, TN.
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    Jon has pointed out what is currently being done on many mission critical systems for military and commercial aircraft applications. While it would be nice to have real time data with air conditioning logging on the bus, it probably would become data overload for the majority of users. Just look at the Pro Driver on you coach; it does virtually the same thing but is dedicated to the engine and makes adjustments or shuts down the engine for you.

    Gulfstream aircraft systems currently in use monitor critical and non-critical system data and will send an automatic signal to the closest MRO or maintenance repair organization recommending adjustments, repairs or replacement of components. Gulfstream even goes as far as dispatching the required part before the aircraft has landed. Prevost would love this but you might not.

    In the bus world only a little data needs to be collected to predict with a high degree of probability that a failure or problem will soon occur. Using the example Jon pointed out; taking differential temperature across the intake and supply ducts will give you an idea of system performance. Adding a few more readings will give you additional predictive data. Once you know the typical differential temperatures you will be able to see a trend indicating on going performance to be used as a base line. Just because the delta is increasing doesn’t mean you have more cooling capacity. The likely problem is a dirty evaporator air filter reducing the air flow etc.

    In addition to Jon’s recommendation to look at the liquid receiver to see the refrigerant level on the sight glass, use your infrared thermometer to record the refrigerant temperature in the receiver. It will change with the ambient temperature but the delta between the OAT and the refrigerant should stay roughly the same. If the delta increases, there is a high probability that you have a dirty condenser coil etc.

  7. #27
    Join Date
    Nov 2009
    Location
    Vero Beach
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    Question

    Since we had our OTR overhauled we have not used it - today with the temp of 75 out and 85 in we turned it on while driving for about a half hour - it got cool but certainly not cold.

    How long sholuld it take to get the temps you all talk about?

  8. #28
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    OTR brings the inside temps down fairly quick which means absolutely nothing because the answer is not definitive. If I were to question the effectiveness of my OTR one of the first things I would do is get my temperature gun and measure the temperature at a discharge point such as where the air blows from the window sills. I would then find where the air return for the coach is and measure the return air temperature.

    The difference between those two temperatures is a good indication of the performance of the AC and this applies to any AC unit.

    If I saw a temperature spread of 15 degrees I would consider the AC to be working. Keep in mind that is not necessarily the correct temperature spread for a well performing AC system or unit. In order to determine using the IR temperature gun as my only tool if the system is working properly I would have to have a series of temperature readings spread over time to know if that is indicating the system works well or if it is showing signs of reduced performance.

    There are a lot of things that have to be looked at to determine the health of your OTR, but most of them require gauges or the knowledge of the system a professional would possess.

    A few general comments however. First, if the entire coach is heat soaked it is going to take a while for the OTR system or the Cruise Airs to absorb that heat. You can help by making sure the return air filters are clean so you are getting the maximum air flow. A second thing to do is to verify the valve (that is used to mix the heat from the coolant with the cooled air to temper the air when you adjust the temperature knob up) is closing and you are not getting hot coolant flowing through the heat exchanger offsetting the efforts of the AC system. That valve is in the evaporator bay and you should not feel any heat on the piping.

    I assume you are driving the bus or at least running on high idle when you express concern about the performance. A well performing OTR system is going to get a coach so cold you will be wearing sweaters and the outside of the windows will be sweating if it is working. But how long it takes to get to that point is dependent upon a lot of factors so I doubt if any one can give an accurate estimate of how long that will be.

  9. #29
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    May 2007
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    Port St. Lucie, FL
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    As Jon mentioned, a well performing OTR system can make your teeth chatter.

    Another tip is to check the outside air intake flap valve located in the evaporator bay. The OTR system draws air from both inside the bus as well as outside via flexible ducting attached to the evaporator bay door. There is a manual flap valve in that ducting which can be closed, thereby limiting the amount of "outside" air being drawn in. By maximizing the return air supply from inside the bus, I have noticed that my OTR will cool the interior at a somewhat faster pace, but ultimately it depends on how heat soaked the bus is to start with.

    Once the bus has cooled down, it seems to continually decrease in temperature as you continue to cool primarily interior air which is now cool to start with.

    At least that's how it worked for me...

  10. #30
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    Sep 2010
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    Mt Baldy, CA. and Nashville, TN.
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    I’m not sure exactly what overhauled means. At the minimum it may have been a major service with belt and seal replacement. Whatever the case may be its important to run an open drive compressor at least monthly for 15 min or so. The reason is periodic lubrication of the shaft seal located behind the clutch on the compressor. If the compressor is properly cared for with all of the high and low pressure safeties operational your open drive compressor air conditioning systems should run several years without problems.
    Compressors should be just as dependable as they are in your home air conditioning system or your late model car. A compressor is similar to an engine with cylinders and an oil sump. Rooftop electric compressors will typically be rotary rather than reciprocating. The bottom line is they are dependable and 99% of the time fail for external causes that could have or should have been preventable.

    Typical preventable compressor failures include the following:

    Los of oil and refrigerant. If you have a refrigerant leak you also have an oil leak since the oil is entrained with the refrigerant and circulates thru the entire system. Many leaks are found by locating oil at connection points or shaft seals. The oil may not be apparent but the collection of dust on the oil leak is a dead giveaway.

    Oil needs to be added carefully. The open drive compressor on OTR air will have a sight glass were you can see the oil level. Just because the oil is at the bottom of the sight glass doesn’t mean its low. The oil needs to be checked after the system has operated half an hour or more at high load. The reason is that the compressor has unloaders that reduce the refrigerant mass flow at light loads. Oil will tend to collect in some areas of the system and lower the oil level in the compressor. When the load returns to high, the additional mass flow of the refrigerant will return the missing oil to the compressor. If someone has added oil to a system that is not low, the cooling capacity will be reduced. The reason is the additional percentage of oil in the entrained refrigerant will insulate the refrigerant from the coils and reduce heat transfer. I’m probably getting close to information overload so I will leave it at that.

    Florescent dye is commonly used by technicians in the search for leaks. It is easily added to an operational system and allows a technician with an ultraviolet flashlight to see tiny leaks that now glow in the flashlight beam. You can purchase a portable florescent leak detection kit made by “yellow jacket” at most refrigerant supply stores.

    High discharge pressure caused by a dirty condenser coil will increase the compressor discharge pressure. In the short term the affect will be reduction in cooling capacity and a substantial increase in power to drive or run the compressor.

    If the compressor is for OTR air, the unloaders seals life will be reduced. If a technician ever indicates your compressor is worn out, ask him to do maintenance on it. The unloaders seals, shaft seals, cylinder liners etc. are all replicable. If he doesn’t have the knowledge, purchase a rebuilt unit from one of many companies that remanufacture the Carrier open drive bus compressors. You should be able to get one complete with a new clutch for less than $2000.

    If the compressor is an electrically driven roof mount or bay mount, your compressor can not be rebuilt. The damage that will occur is a breakdown of the motor windings. Eventually the motor will fail.

    Dirty evaporator filters will cause the refrigerant to have an incomplete phase change from liquid to gas. If it’s bad enough some liquid refrigerants will get back to the compressor. Since liquid doesn’t compress, the compressors valves will break.

    Eclectic compressor short cycling will cause motor winding failure due to lack of cooling. You electric compressor should have a short cycle time delay. If it’s been removed or the time interval has been changed overheating and premature failure will result. The reason is your electric compressor is cooled by return refrigerant gas. Each time a compressor is started a large amount of power is used to overcome static friction and then increase the motor to operational speed. This puts residual heat into the motor that takes around 5 minutes of normal operation to remove. No or very low cooling is taking place while the compressor is off. Repetitive short cycling of the compressor will progressively heat the motor winding until a failure occurs.

    This represents a partial summary of the preventable failures that your air conditioners are experiencing.

    Its time for an adult beverage
    Last edited by johnklopp; 08-07-2011 at 12:30 PM. Reason: Engineers can't spell

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