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Old 05-02-2015, 09:46 AM   #1
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Cab A/C seems to get warmer and not blow as hard when getting up to freeway speeds

Wanted to see if anyone else seems to feel any difference with there dash ac system when traveling freeway speeds? In our 45 foot Tuscany the wife and I both notice this when traveling. Seems like the coach pressurizes or something . Then pulling off and going down to low speeds the air feels way colder and seems to blow harder.
I had a Monaco dynasty I felt the same thing in so think it just might be common issue in these big pushers. Anyone else notice this and is there any remedy?
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Old 05-02-2015, 10:00 AM   #2
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Sounds like the water flow control valve to the heater core isn't shutting the flow down properly.
At higher engine speeds the coolant from the engine is hotter and circulating faster, so the A/C has to over come more heat to cool.
Does it act the same on MAX A/C ?
Max A/C should shut off the flow completely.
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Old 05-02-2015, 08:02 PM   #3
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Quote:
Originally Posted by Skip426 View Post
Sounds like the water flow control valve to the heater core isn't shutting the flow down properly.
At higher engine speeds the coolant from the engine is hotter and circulating faster, so the A/C has to over come more heat to cool.
Does it act the same on MAX A/C ?
Max A/C should shut off the flow completely.
Using Max A/C is what I must do. The Evans control valve need replacing, but it's easier and cheaper to always use Max setting.
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Old 05-03-2015, 08:35 AM   #4
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I think Skip may be on to something. The heat/AC controls are fairly primitive compared to a car's controls, at least on my rig. A car typically has a heater core, and an A/C evaporator, and the temperature control is a door that mixes air from both sources: when set to full cold, the door closes off the heater core and all the air goes through the A/C evaporator core. This gives you fine control over the temperature, and fast response to changes.

The dash air on the motorhome is much simpler (at least on mine): all of the air always goes through the heater core and the A/C evaporator core. Temperature control is a matter of controlling the amount of hot water going through the heater core. This means that temperature changes take a lot longer to take effect (the heater core has to actually warm up or cool down before the change is felt) and it also means that a control valve that doesn't completely close can make things warmer than desired.

On my rig, setting the dash air to Max A/C does nothing to stop the flow of hot water to the heater core, all it does is close off the outside air intake and recirculate the inside cabin air. Since it's cooling the same air over and over, rather than always having to cool down outside air, that can help overcome the effects of a leaking hot water control valve. But on mine, at least, the control valve is controlled only by the temperature dial with a strictly mechanical linkage.

Of course, different coaches have different arrangements, but I think Skip is onto something with the potential of a control valve that doesn't close all the way.

A quick check may be to find the control valve (on mine it's easy to see once the generator is out, it's right on the passenger side firewall.) Turn the temperature knob and see how the valve moves. Now, turn the knob to full cold, and try to manually push the valve further closed. There might just be some slop in the mechanism that is preventing the temperature control from completely closing the valve?
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Old 05-03-2015, 09:15 AM   #5
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If you have a Evans AC/Heater unit you may have this problem which many of us have had.
Does not matter what MFG coach you have if it has a Evans.
There is a simple fix than also you can buy a new control valve just make sure you install correctly.
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Old 05-03-2015, 09:42 AM   #6
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Makes perfect sense as is to why this is happening. Leaving from so California this afternoon for a 300 mile trek across the desert back to Utah , so a great place to test this theory as it will be 90 degrees in the desert. Will leave on max ac to see if that helps this issue then take it to Freightliner for the new valve. My coach is a 2013 model too. Thanks so much everybody for the info!
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Old 05-03-2015, 11:51 AM   #7
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Freightliner didn't install the valve , so you may have to source it yourself.
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