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Main engine AC 2000 Windsor


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Looking for information on refrigerant capacity on the main AC unit.

R134 capacity is critical to get a good performance. To much refrigerant and the higher pressure will shut the system or blow the relief valve. Too low and will not cool properly.


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Don't know if anyone can tell you that exactly,  other than what is says in the manual, I think mine says around 4 pounds. I did not care about the amount but went by the pressures. Kept high pressure around 200 psi in Texas summer,  low pressure port reads a bit low but it's a long distance from the evaporator.  If I had access to the evaporator, I would measure temp there but will not take the dash off for that. It blows cold, we have to turn the vents away from us up front.

Its been a while and I had no need to change anything.

Edited by Ivan K
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I  have the low pressure 38 to 40  on a 90 degree days and be sure to have a shop fan on the condenser or the high pressure  will get to high on the coaches with the condenser in the radiator stack.  Your total amount  of 134 will depend on the length of the coach.

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My sig owners manual indicates the refrigerant quantity (4.0 lbs). I believe to determine the correct quantity, you really need to be able to measure the amount of liquid in the receiver, which is impractical in a service setting. The pressures will be in range (with low suction as Ivan mentioned) as long as there's some liquid in the system and there's enough surface area available (not flooded) in the condenser to condense gas. Mine was cooling just fine before I recovered 2.5 lb to fix a leaky hose.

4.0 lb appears to be a pretty common value with Roadmaster chassis / SCS Frigette-based systems, so it's probably a safe bet. If you go too far, the high side pressure begins to rise as the effective surface area of the condenser is reduced by being stacked up with liquid. You're at least 1 lb overfilled at that point.

Edit: the 2009 Dynasty manual in the downloads section also indicates 4 lb 134a. 👍

Edited by trailmug
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When I repaired my Dash A/C on my 2000 Windsor 6+ years ago  It required around 4lbs on a 40' coach.      

My unit has an expansion valve, so low side will change from 25-40 PSI depending on temp.   High side max is 250 PSI, however with the length of hose, I kept way under, in the 200 PSI range for max on a 90ish degree day. 

What I did do to verify was to use calibrated gauges and do a static pressure test after the system has been off for some time vs Outside air temperature at the condenser.   



Edited by b_faster
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I'm getting potentially wordy and obnoxious here, but I spent a good deal of time with the SCS system after multiple shops had failed to fix it and I found a significant amount of contamination/sludge (photos attached). I got really suspicious about the 4.0lb recommendation after reading the Sanden service manual. Factory oil charge is sufficient for up to 4lbs R134a, with more oil recommended for systems with longer lines which require more refrigerant.. I can't imagine longer lines than on a bus, so did Monaco recommend 4.0lb just so they could bolt the compressor on, evacuate, charge, and be done with it? It'll last the warranty period, at least! I flowed liquid into the evacuated system and it easily took 5 lb with the system off. Aha! I knew it needed more.

Finally got around to measuring subcooling at the condenser a year later and it was 18F, which is a reasonable upper bound for a critically-charged, receiverless system to have a liquid refrigerant reserve in the condenser (a receiver system doesn't need this). At low system loads it was probably costing efficiency / compressor life due to refrigerant stacking in the condenser and raising compression ratio. The receiver looks close to a pint in size (I realize a dry ounce of R134a isn't 1fl oz, but we'll say it's close enough), so I recovered a pound to get back down to 4.

For academic purposes: at 4lb the condenser measured 11F subcooling with a thermocouple at the liquid output of the condenser reading less than the saturation temperature on the high side gauge (with a front-mounted condenser). I would use that to validate the charge in the future if I don't evacuate and re-weigh. Charging beyond this went to 14F at 4.5 lb and 18F at 5 lb. The gauge pressures don't move substantially within this range.

In summary of this overabundance of typing, I feel like I successfully "shade tree" validated the 4.0lb charge.
If you replace any components, the Sanden manual has approximate amounts of PAG oil to add for each part replaced.
If anyone ends up doing a full system flush, I settled on 4 oz additional oil due to pooling in the long, hilly suction line.

Thanks for reading. 🙂



Edited by trailmug
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