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Old 06-23-2021, 01:21 PM   #1
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Dash A/C - pls explain manifold readings

Calling all experienced A/C helpers...

My dash air is not cold enough - system seems to 'work', but it just seems like it should provide more cooling.

First thing I did was check the vent temps - never colder than 49 degrees. So I decided to add r134a. I used my a/c manifold gauges (for the first time). The gauges certainly seemed to read 'low'.

I admit, my skills are quite lacking in the a/c area, so I was struggling to get any improvement. I added two 12 ounce cans, and still couldn't get the gauges to read what I thought was correct, still too low (?) (based on training from YouTube University). So fearing that I had over-charged the system, the next step was take the coach to a shop.

Mechanic added "a little under one pound" of freon and took my money. I could tell from his explanation that he did expect much improvement.

So, I brought it home and checked the pressures again, and here is what I see:

* motorhome is parked - not moving
* engine is running 2100 rpms
* a/c is set to 'max' and blower is set on 3rd position
* ambient air temp outside is 90 degrees (with very low humidity)
* air temp inside coach is 78 degrees (I ran the roof air for an hour) - this is the air that is recirculating at the return vent below the passenger's feet. It's the air that is being cooled by the a/c.

Given these 'settings' above, 47 degrees is as cold as I get from the vents (I have a quality digital thermometer stuck in the vent).

Shouldn't I expect a greater than 31 degree temp drop?

I have a video showing the manifold gauges and the reading as the compressor is cycling. I'll try to add that video here. In any case, the readings are:

* Low side: 38, then drifts down to 16
* High side: 170 then drifts up to 235

... then, rinse, repeat as compressor cycles.

So, the end result at this point is an additional 2-3 pounds of freon with little to no improvement (over-charged ?).

All ideas are much appreciated.

-Doug
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Old 06-23-2021, 01:56 PM   #2
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On a 90 degree day a 47 degree vent temp is better than normal. The vent temps on the majority of class A coaches would be 50 to 55 degrees. Your dash A/C is doing better than most.


And I agree, you have overcharged the system and have no idea what you are doing. You better hope the compressor doesn't lock up or a hose burst. Your expectations are wildly beyond reality and will wind up costing you a great deal of money if you don't quickly take your coach to someone that knows what they are doing and properly evacuates and recharges the system.
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Old 06-24-2021, 01:26 PM   #3
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Your 49 degrees at the vent on a 90 degree day is good. IMHO

Given the competing elements of windshield, ambient temps, and a big cockpit, salon, and galley area, if you get a 20 degree reduction in inside temps you have done well.

What temp is your rooftop AC showing at the vent?
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Old 06-24-2021, 08:29 PM   #4
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First of all are both gauge valves closed when checking. Those are only open for vacuuming or low side for charging, or you make a slip stream around compressor. Are you sure no hot coolant going through heater core.
Check AC performance guide down a bit on page for psi on gauges Check more at about 1500 rpms.
https://www.proairllc.com/design-eng...empcon-guides/
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Old 06-24-2021, 10:30 PM   #5
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Quote:
Originally Posted by 153stars View Post
First of all are both gauge valves closed when checking.
Yes, I was careful about those valves being closed.

Quote:
Are you sure no hot coolant going through heater core.
That's a good reminder. I will check that.

Quote:
Check AC performance guide down a bit on page for psi on gauges Check more at about 1500 rpms.
Thanks for that page - I'll study a bit there.

-Doug
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Old 06-25-2021, 12:06 AM   #6
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This should provide some guidance: Heavy Duty air conditioning service manual
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Old 06-25-2021, 05:20 AM   #7
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Quote:
Originally Posted by deandec View Post
Your 49 degrees at the vent on a 90 degree day is good. IMHO

Given the competing elements of windshield, ambient temps, and a big cockpit, salon, and galley area, if you get a 20 degree reduction in inside temps you have done well.

What temp is your rooftop AC showing at the vent?
My thoughts also.
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