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Dons Prevost OTR ac/heat ramblings(w/pics)
I recently did my summertime 2000 trip mile thru hot TX, NM, CO and back. I do this trip every summer. I’ve been doing this trip in various vehicles in the summer and winter for the past 8 yrs. I’ve seen temps as low as -26 to as high as 106. Good ac and heat system(s) are a must. The 80k+ btu Prevost OTR ac/heat system is awesome, and the 80k btu Webasto heat system ain’t to shabby either. This summer, out of curiosity, I shut off the OTR system, ran all 3 Cruisairs while driving in the heat, just to see how they would handle the hot road conditions (I tuned them up in June). Would they would keep functioning and/or keep the bus cool? To my surprise, they all stayed running(no HI PS) and blowing cold air, but not as cold as what the Prevost OTR system provides. But still, redundant ac and heat systems is a wonderful thing.
This summer, after driving in the heat and humidity with the OTR ac on for an hour or so, I had to add a little cabin heat to keep the red OTR ac light from cycling on/off occasionally. Also, when climbing Raton Pass, the red light will come on for 5 seconds, then turn off for 15 seconds. Of course to climb this pass, the gas pedal is on the floor, the tranny has down shifted to fourth, and the rpm is around 1500. Then going down the other side the light never comes on. My foots off the gas pedal, I’m manually down shifting, keeping the rpm up, and using the Jake brake for speed control. Weird! Has anybody else had this happen?Even after adding a little heat, the OTR ac system keeps the entire interior of the bus at 70-71 degrees while driving in 95-100 heat. My guess as to why I need to add a little heat, to keep the red light off, is because the system may be a little under serviced(low on Freon). Its been my experience that this can cause the evaporator to freeze up. The OTR heater core is sandwiched next to the OTR ac evaporator, so turning on a little heat keeps the evaporator thawed out, which keeps the red OTR compressor light off. Prevost Service? Anybody? what do you think?
Its been a while since I put my gauges on the OTR ac system, so I did just that. It was a humid pre-hurricane Harvey 85 degrees. The initial static ambient pressure was 90 psi. I have a pressure/temp table that says R134a pressure should be 95 psi at 85 degrees. Next, I hooked the gauges up to my Toyota, that has an great working ac system, and it read 95 psi. Again, I thought the bus was probably a little under serviced? Then I started the bus up, put it on hi idle, and turned on the OTR ac and added one 12oz can of R134a. After about 5-10 mins, the pressures stabilized at 32/195(see pic) and the red ball was floating midway in the lower site glass of the accumulator. Then, after about 15-20 mins, the pressures read 34/160(see pic) and the red ball was all the way at the top of the lower site glass. Had the compressor UNLOADED a cylinder while I wasn’t watching? As you may or may not know, when the compressor reaches its happy place, it starts UNLOADING cylinders to save fuel. So, I suppose the lower pressures and the red ball not floating after 15-20 mins was because the compressor had UNLOADED cylinder(s). Both dash and cabin circuits/vents were blowing 45 degree “frost the windows” air(see pic). Interior air temp was 73 degrees and dropping.
I’ve owned the bus for 3 1/2 yrs and I’ve never needed to add Freon. I also keep my heater core circuit valves in the engine compartment open all yr round.
Always keep good O-rings on the caps of the king and schrader valves.
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5 Replies
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Don, Sounds like your OTR is in top condition. Mine never got quite that cold. I have been following your OTR posts which have encouraged me to give charging the OTR a try. Keep them coming
Gerald
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We can talk about pressures, where the red ball is and any other technical description of a great OTR, but I know mine is working right when I have to run my wipers to clear the condensation.
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You are correct as to the changing pressures when you have added a little heat. As the system brings down the low side pressure we can get to a conditon where the evaporator becomes very cold. Ideally the unloaders should have “unloaded” to try to prevent this condition. Whether you have the mechaical unloader that reacts to the compressor pressures or the newer system with pressure switches to control whether the unloader is off or on, the unloaders are trying to prevent the cycling of the compressor clutch.
Normally with the heated air you are pushing past the evaporator you have enough heat load to prevent cycling. But once the air becomes cold then the system gets very efficient and the temperatures at the evaporator start fo drop. Ideally one unloader will activate and drop the compressor to 4 cylinders to try to prevent low pressure. If the pressure (temperature) continue to go down, the other inloader activates and yoiu are now running on two cylinders. If you continue to drop in pressure/temperature then you hit the low pressure switch and the clutch is unloaded. On the XL a/c system the red light indicates a high pressure or low pressure issue as the light is turned on in response to one of those switches.
That is why is is good to have the system set for a temperature around 70 degrees. That way when the thermistor in the return air duct sees the temperaure dropping below 70, it opens the hot water valve for a brief time and then closes it and tests the change it may see. It may need to do this several times to get the temperature adjusted. When you are climbing a hill under load at high rpm, the compressor is also at high rpm and if the system cannot keep up due to being slightly undercharged, you will get the red a/c light in the dash. From your desription and because you really did not lose cooling in the vehicle I would wait and observe to see if it becomes more frequent and you are then losing your good a/c in the vehicle.
With all mobile a/c systems with rubber/teflon hoses there is a migration rate of the refrigerant molecules through the hoses. Thus you may need some additional refrigerant added when you note a loss of cooling effect. This varies by vehicle or so it seems,
In general it seems that your sytem is working correctly and still providing good cooling on high temperatures.
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Excellent explaination Prevost, thanks.
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