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Prevost Dash Air – High Pressure
Looking for thoughts from the HVAC experts here. Jeff and I have pulled the hair out of our heads trying to figure out why I get “high pressure” light that comes and goes from the dash air. The compressor, dryer and expansion valve are less than a year old and the pressures with the gauges is dead on. I also get “low pressure” from time to time, but now it is mostly the high pressure light. PIA, everytime the light comes on it kills the compressor, then starts all over again.
Of course it won’t do it while the gauges are hooked up.:(
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20 Replies
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Tom, make sure your cooling fan is coming on. If your engine fan does not engage with the A/C, or if your fan clutch is going that could cause the problem. Usually the low pressure light is the result of running the A/C in cooler ambient temperatures. It could also be a bad pressure switch.
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Agree with Steve. High pressure is usually associated with an expansion valve not opening or only partially opening, so that also might be the issue regardless of age. Here is the order for checking if it was on my bus…..
Condenser fan
Pressure switch
Expansion valve.
I do the easy stuff first.
Where is your condenser? I assume it is stacked on the radiator so when the system is “on” the fan should be locked. You can test this by running with the A/C on. Note the temps. Then turn the A/C off. The coolant temp should go up a little because the fan clutch is cycling.
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Thanks.. The fan goes to high speed for sure. I had the same thought on the high pressure switch, but Jeff said they usually just fail and mine is intermittent. I guess a new switch and another expansion valve is in my future.
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Before replacing the expansion valve I would suggest you go to Harbor Freight and get a cheap set of refrigeration gauges and when the light comes on check with the gauges to see what the real pressures are.
It depends on how long the high pressure light stays on, but with practice you can get the gauges on the coach in a few minutes and crack the valves to get a reading.
I’ve seen the gauges for as cheap as $50. If the light is on and the gauges are within the normal range (which varies due to ambient temp but is likely around 40 on the low side and maybe 170 on the high side) the problem is the pressure switch for the light. If it is well over 200 on the high side it is probably the expansion valve. I have seen a failed expansion valve cause high side pressures over 350 PSI.
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Truk, I discovered just yesterday I now have the exact same problem you are experiancing. My compressor kicks in and the dash panel show compressor engauged. After about 20-30 seconds the hi pressure light comes on and the light on the dash panel goes off.The air starts getting hotter, and then cycle starts all over again. But mine works just fine when engine is at idle or fast ilde(which I think describes your same condition because this is when your gauges are hooked up , at idle). But when I get to highway speed (RPM), thats when mine cuts out. I can turn off and back on and it again works for about 20-30 seconds. Let me know if you find the problem and I will do the same.
Bill S.
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OK Bill, exact problem I have, we’ll get it sorted out.
I have your e-mail address as well and anything I do, I’ll let you know.
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Hi
I worked on one with a similiar issue last summer. When it was all said and done, the unit was over charged. You really need to have a set of gauges on the unit and watch the pressures when it cuts off. If it is not slightly overcharged it could be a plugged drier or expansion valve.
Mikee
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Tom,
Its my understanding this does not happen on a coach with slides!
I wish I had an answer for you but right now I’m dealing with a “Check Trans” and “DO NOT SHIFT” indications. I’m looking at a small rendition of a wiring diagram for Engine Control. It has a relay and a solenoid in the Emergency Stop Motor wire but my red lights don’t stop the motor. It keeps on running.
Hate to “pile on” but Nancy clearly wants a coach with slides. That would solve your problem, after all you have been a TBC member in the past, why not again!
Good luck in finding your gremlin and I’m not referring to an early 1970’s restored tow vehicle.
Merry Christmas and Happy New Year to all and to all a good night.
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Will, that was harsh. My guess is Tom is right now revising the invoice for your dues doubling and maybe tripling the cost.
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Mikee,
The charge is dead on, Jeff’s gauges and the damn thing won’t do it setting for hours on high idle, only on the road at higher RPM’s.
I tried get Jeff to ride on the hitch for a road test, but I can’t tell you what he told me here on the forum. I do remeber it sounded like an impossible act.
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I might be overlooking a potential source for the problem, but three things that I mentioned earlier stick in my mind, but one has to be first ruled out.
When the AC is engaged I believe the fan does not cycle but is locked “on”. It is important that its operation is verified. The subtle clue is if the fan is not locked on you will see slightly higher temps whereas if it is locked on the temps will be cycling on the engine thermostat. If that can be verified you probably have an issue with the expansion valve. If it is not opening fully or as required the compressor may not generate enough system pressure at high idle, but at highway RPMs it is likely building excessive system pressures due to a restriction or failure to properly operate. I am assuming when the system was opened to replace the expansion valve a new dryer was put in place and the system was vacuumed.
Get a cheap set of gauges, hook them up and put a brick on the gas pedal to get the revs up to 1500, Make sure the inside of the coach is hot by running the Webasto or Aquahot, turn the AC on full and if the high side pressures are over 200 and suction side is low, like 25 or 30 it is likely the expansion valve. I don’t know what the high side pressure switch is set for, but it could also be faulty something your gauges will tell you if pressures are OK and the light comes on.
The reason the fan is my first place to look is because everything can be perfect, the charge, expansion valve etc, and if the condenser isn’t getting adequate air flow you are going to get high pressures. I just cannot imagine the fan cycling instead of locked up as revs increase.
2005 XL Thompson Coach
Rarely tow anything behind us.
Sold: 1993 Liberty that we loved for 12 years. -
My fan is working easily verified by the drop in coolant temp and with compressor on it always maintains that lower operating temp. When on high idle, the fan is locked on high.
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Jeff or you will just have to watch the gauges and it is likely with the fan ruled out as a potential issue that the expansion valve may be the bad boy. Is the right one in place if it is a new one?
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I’m pretty sure it was the right one and maybe that’s the problem. I think we’ll change out the valve again and dryer and go from there.
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A switch is cheaper so it is well worth sticking gauges on the thing with it running at 1500 RPM.
It will not hurt it to run at that speed. In fact you can just floor it. The engine won’t hurt itself. When the light comes on with the gauges hooked up you will know if the light is showing high pressure or the switch is wrong.
Just my opinion.
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