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36' motorhome factory AC terrible Pages: 12

sc3283 on Tue May 21, 2013 5:14 PM User is offline

Engine Size: 300
Refrigerant Type: r134a
Ambient Temp: 80
Pressure Low: 22
Pressure High: 225
Country of Origin: United States

hi, factory installed dash AC was ineffective at time of my used purchase

high pressure 400 and up as purchased
Unicla 150 compressor, dual V belt clutch 5.5" diameter
condenser is in the radiator stack
No ability to speed the hydraulic cooling fan when AC is on

I added 400 cu in of parallel row condenser up frt with pressure switch activated electric fan. It is plumbed in series. Now pressures are acceptable having over 800 sq in of condenser total. I ran out of hot weather last summer to get a good evaluation of the modifications.

Now this year....we are comfortable to 90 degree ambient in bright sun whereas prior 85 on a humid night we were uncomfortable.

I feel it can still be better....as I see NO expansion valve cycling on the pressure gauges. This tells me a couple of things.

Compressor too small and or compressor speed is not fast enough

Compressor speed is 2682rpm when cruising 65mph. From what I have seen on your site...that is not fast enough

When coming to a stop the cooling becomes nothing at idle during 95 degree weather

When cruising down highway (95 degrees bright sun) dash vent temp is 55, if I downshift the trans to raise engine speed to 2000 rpm from 1650.....dash vent begins to drop in temp

What do you recommend?

thank you


ice-n-tropics on Wed May 22, 2013 9:27 AM User is offline

Prior trouble shooting motor home AC results in following general conclusions:
1) Idle cooling poor because of hot engine compartment air recycling back into condenser especially w/ tailwind. Add air recir. dam along sides of rad. and build belly pan to direct air toward rear of engine. Heck, some nere-do-well MH install cond downstream of rad.
2) Driving cooling needs 200cc+ comp swept volume (if your engine is 300 HP)
3) Comp drive ratio (dia. crank pulley/comp clutch) approx. 1.8 to 2 for gas and 2.1 to 2.4 for lower rpm diesel
4) Suction hose ID greater than 5/8". Bigger is better.
5) Too much oil decreases cooling.
6) 100% recirculated air thru evap
hotrodac

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

sc3283 on Wed May 22, 2013 12:04 PM User is offline

ice...TY for the reply


200cu in compressor, ear mounting, 2 V belt pulley..Unicla is the only 1 I have found with this size and configuration(which makes it a bolt on swap)....any clues where to buy?

thanx

bohica2xo on Thu May 23, 2013 2:25 AM User is offline

Clearly a badly matched compressor drive.

A 200cc / rev compressor is a decent sized unit.

What make & model of engine on this coach?

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Thu May 23, 2013 5:40 AM User is offline

Equally lost, 36 footer could either be a front gas powered unit or a rear powered diesel unit and how old is it? Is R-134a stock or is this a retro? Tried looking at my crystal ball, still has a crack in it.

Anyone know how to repair a cracked crystal ball? Then I won't have to ask these questions.

94RX-7 on Thu May 23, 2013 9:48 AM User is offline

Quote
Originally posted by: NickD

Anyone know how to repair a cracked crystal ball?

Throw in a can of stop leak. LOL

sc3283 on Thu May 23, 2013 11:20 AM User is offline

96 Monaco REAR diesel, Cummins 8.3c, 300hp

R134a is stock

OE copper tube compressor in rad/cac/atf cooler located in left side rear

New parallel condenser in RF with high side press switch activated fan

over 800 cu in of condenser now

drier in stock location...rear of chassis after OE condenser

Edited: Fri May 24, 2013 at 9:18 AM by sc3283

NickD on Thu May 23, 2013 1:26 PM User is offline



Nice, this one is on the market for only $18K, prices of these sure came down. Now I am wondering why your low side pressure is so low, should be up in the 27-28 psi range. You could add if this is a TXV system where a super cold evaporator would switch off the compressor. High side seems a bit low too.

Just going back to basics, do have have means to switch from vacuum to refrigerant without changing hoses? Just a wee bit of air in there increases pressures and greatly reduces cooling. Also that long suction line more than likely running underneath has to be well insulated.

Sure bohica and ice will add more.

Do you average around 10 mpg with this home? Remember when diesel fuel use to be cheaper than gas?

sc3283 on Thu May 23, 2013 1:46 PM User is offline

thats the vehicle

pressures stated in opening were @ idle...550 rpm

pressures at 1000 engine rpm are:

35 psi low, 250psi high @ 80 degrees ambient temp


it is an expansion valve system

ALL incoming air to evap is recirculated. NO outside air enters the evap box

the hot water valve for heat is DEFINITELY stopping ALL water flow to heater core

with the super dooper ULS fuels now.....9.2 is the best I can muster

Yes it is NO prob to go from vac pump to freon charge for me.

I will admit 35 yrs of slinging wrenches BUT all my R134a experience was in a car dealer where we had a stated qty of charge....this RV I charged by adding R134a slowly, watching the gauges and checking dash vent temp. I may be undercharged...but was making sure I didnt overcharge due to the negative results with overages

Suction line insulated? Why I must ask? It is just a return for the compressor....and once compressor pumps the refrigerant, it will become hot..I do realize cool incoming the compressor may make the compressor run a bit cooler

thanx!!!

Edited: Thu May 23, 2013 at 2:03 PM by sc3283

mk378 on Thu May 23, 2013 2:00 PM User is offline

Why insulate the suction line? Heat gain to the suction line is still heat gain that has to be dealt with. Cold gas entering the compressor is more dense, meaning you have more mass flow for a given suction pressure, rpm and displacement.

A large TXV system should have a suitable receiver, which means you can overcharge slightly past the point of full performance. The excess refrigerant will simply fill up the receiver as liquid.

sc3283 on Thu May 23, 2013 2:09 PM User is offline

mk, makes sense on density

the reciever/drier is the old standard issue (about 6" tall, 2" diameter) like what was used commonly

http://www.man-a-fre.com/pa/pictures/88470-14010.jpg

Ex valve is standard issue 90 degree with bulb sensing type

https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTHm0Tc73QbNNXK_iFpbcgcZe7gYLuDFNSVRyCY2gjcuoFgsz9AZQ


thanx

NickD on Fri May 24, 2013 7:18 AM User is offline

Yeah, that long suction tube can be considered a part of the evaporator, if it heats up, pressures go up as well creating more back pressure for reduced refrigerant flow. Doesn't make to much difference with a 2-4 foot long suction tube, but yours has to be over 30 feet long. Again, all basics.

Motorhomes can present special problems, with my old 82 Pace Arrow, GM claims Pace Arrow installed the engine air, Pace Arrow claims that GM installed it. Had to trace any model numbers I could find on the components. Then the controller had all spade lug connections on it where some idiot had all the clip on wires in the wrong place. Had to take that apart and make my own circuit diagram.

A major problem with these things is that huge windshield, my girls would complain about that, so when starting off, switch on the generator and also add cabin air. After it cools down rapidly, the engine air does fine, but then its too cold, LOL.

You could try contacting Monaco and hopefully find somebody there that knows the capacity of your system and has some specs on it. Could be lucky.

I run into this all the time with the FAA and some very old equipment trying to get information, no circuit diagrams, lowest bidder that goes out of business, hostile corporate takeovers, and sometimes get lucky. Called one company, an old engineer came back out of retirement about my age. Not only got the information I needed from him, but we spent another 30 minutes on the phone BSing. Got that old unit working again. These new kids know nothing.

NickD on Fri May 24, 2013 7:25 AM User is offline

Oh, I looked around for a Monaco around central Wisconsin, while their stock is huge, really huge, want over twice the price for these things here. When I purchased this Pace Arrow, was talking about will we ever see two bucks for a gallon of gas? Guess what? Would cost me over 500 bucks to fill both tanks.

Went back to that http://www.mhsrv.com/ site, that 96 was already sold, next cheapest Monaco is $70,000.00 ranging up to $200K for used ones. Local dealer is loaded with RV's in the $350-400K range, soon as you buy one, be lucky to sell it for a $100K the next day.

Didn't know the EPA was also playing with diesel fuel like car gas, over the winter was lucky to average 30 mpg with my new Cruze with this stupid $4.20 a gallon gas. Least now, back to averaging 42 mpg. Was hitting 15 mpg with my Pace Arrow if my wife would let me drive at 55 mph, but drops to 12 at 65. But that was with summer gas.

Gas use to be 120,000 BTU's per gallon, now can be as low as 90,000. Wouldn't be that bad if they dropped the price in proportion. But they don't do that.

Edited: Fri May 24, 2013 at 8:11 AM by NickD

sc3283 on Fri May 24, 2013 9:43 AM User is offline

Nick, TY for the reply

factory charge spec is useless with the additional of the new parallel 2nd condenser. Monaco built this thing with "after-thought", "after-birth" AC...and they have been sold at least 2 times since this RV was built....assistance from them that is valid is doubtful.

I realize these vehicles have large heat gain from sun due to the large windows. I still feel comfortable dash vent temps can be achieved even though the whole cabin can not be cooled via the dash AC....

I did NOT change the rec/drier location upon adding the up front new condenser. It is in the rear of the coach.

SO far it has been suggested....replace comp with 200cc(12cu in)...which was along my original thinking

Should I move the rec/drier up front and put it between new condenser and exp valve at the same time of comp replacement?


thanx again!!

mk378 on Fri May 24, 2013 10:08 AM User is offline

I would think you want to flow compressor -> rear condenser -> front condenser -> receiver drier -> TXV.

Just to be clear, the receiver must the last part before the TXV since its purpose is to separate liquid from gas and deliver a steady flow of liquid refrigerant to the TXV. It is a cylinder with a dip tube to pick up liquid off the bottom. That also means it must be mounted vertically and connected respecting the "in" and "out" ports.

sc3283 on Fri May 24, 2013 10:20 AM User is offline

mk, gotcha....when I added the condenser last summer....I never gave drier location a thought or I would have brought it up front after the new condenser

bohica2xo on Fri May 24, 2013 12:00 PM User is offline

Oh, it is one of those...

Motorhomes in general are thrown together from low bid parts. Because of the relatively low production volumes, the companies are at the bottom of the food chain as well. Diesel pushers are an even smaller subset of the coach market.

My experience with diesel pushers tells me that they can be made to cool well with enough work. You have several problems to solve:

1) Horrible compressor drive mismatch. Since that 8.3 will never go past 2750 rpm, you need to find a drive that will get you to 2.4:1 overdrive on the compressor. Adding a larger compressor will help with the idle cooling, but you still need to fix the drive error.

2) Oiling issues. Every pusher I have worked on had them. By the time you get enough oil in the system to make the compressor happy, it cools poorly. An oil separator is the best cure, but requires some work.

3) Added heat from the long suction line. Already covered here some. Use good insulation, then wrap that with a good IR barrier. Road heat under a vehicle can be intense.

4) Condensor air flow issues. You describe a hydraulic drive fan. What controls the fan in this vehicle?


You don't mention a complete flush in your posts. Do you have any idea how much oil is in the system?

Adding a large condensor did not fix it, simply covered up a problem. It could be that the system simple has way too much oil. Or the OEM condensor is partially blocked with the remains of several compressors that failed from poor oiling. Compressor failure from poor oiling is common on pushers. So the lot boy at the RV place swaps the compressor, adds some oil & sends it along. Next kaboom, more oil & another compressor...

I would flush it completely dry, and replace the OEM condensor. Even if you bypass the OEM condensor you can get a quick idea of what it is not doing. If the OEM unit is restricted it does more harm than good.

Relative to the system components, where are the gauge port fittings?

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Fri May 24, 2013 1:46 PM User is offline

What about installing a York? Even one with a sight glass on it so you can be sure the sump is full. Didn't think about that, my MH has a York.

Is there any kind of axially compressor that has a sump? Another question I am too proud to ask, is it really necessary to change the oil in a York if you want to convert from R-12 to R-134a?

What I do know with PAG, practically have to flush everything and replace the dryer if the system is opened for any reason unless you live in Death Valley. PAG is tantamount to sludge.

Another question with a rear pusher, how much oil do you add?

sc3283 on Fri May 24, 2013 2:48 PM User is offline

bohica..TY for the reply

1. Yes, the comp drive speed is now 1.6 to 1 (stock from manufacturer). I have a drive pulley system drawn on paper..yielding a 2.09 to 1 ratio which was giving a comp rpm of 3456 when cruising 65mph..I guess it is time to refigure. Your 2.4 ratio will yield a 3960 comp rpm @ 65 mph cruise

2. understood and comprehend

3. understood and comprehend

4a. the stock hydraulic fan is pump driven off a pump which is crankshaft driven and thermo wax valve controlled based on radiator temp. It has NO ability to ramp up to high speed when AC is on (unlike later model year comp controlled DP systems).

4b. The added condenser is cooled with 16"electric fan (shrouded and sealed to the cond)which is high pressure switch activated(200 psi "on")

4c. with added condenser....800+ sq in of cooling area combined

System worked at time of purchase BUT VERY poorly. That is when I discovered the extreme high pressures. I know the coach has had 2 compressor replacements prior to my purchase...as to what procedure was used for replacing???...I'm sure the simplest

While I had system open...I did flush with drier removed, lines disconnected at ex. valve and compressor...I added the correct qty of oil per Unicla (cant recall their spec at this time), then began charging until I got to the above pressures.

Prior to any repairs but seeing the pressures...I felt every inch of tubing in the OE condenser and felt NO col/cold spots. (Was taught that back in the 70s as a blockage will act like an expansion valve and be cold/cool just after the blockage). Based on that, my opinion was no blockage.

I have gauge ports on each line @ compressor and have high side port at new condenser up front(for the cond fan press switch)

and THANK you for your time and expertise



Edited: Fri May 24, 2013 at 2:56 PM by sc3283

bohica2xo on Sat May 25, 2013 9:27 PM User is offline

Most diesels are setup to cruise above 75% of the RPM band. That engine does not wind up very high, where is the WOT shift point? Running the compressor faster really pays off at idle. You spend very little time at max rpm,and most compressors are just fine with short excursions over 6,000 rpm.

The wax valve is easily bypassed. You can either add an electronic variable speed valve, or a solenoid that takes the fan to full speed with the clutch signal.

Back in the 1970's all we had were tube & fin condensors. I can block 75% of the passages in a serpentine condensor, and all it will do is get hotter. It all depends on the type of condensor you have.

With the gauge ports you have, an inlet vs outlet measurement can be made on the OEM condensor. Also remember your suction pressures will be higher at the compressor than the actual pressure at the evaporator.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

sc3283 on Sat May 25, 2013 11:17 PM User is offline

WOT shift point? honestly no clue....but guessing about 2200

My high pressures are not excessive any longer with the elec fan on 2nd cond. I don't see a benefit of getting into hydro fan controls to ramp that fan speed when AC is energized

thanx for your time!

sc3283 on Tue May 28, 2013 2:14 AM User is offline

I verified WOT shift point this weekend...2200rpm

plan of attack is looking to be

Larger compressor
insulated suction hose
new ex valve(due to not wanting to open system again)
flush system again to be SURE old/excess oil removed
remove OE located rec/drier and inst new drier up from between cond and exp valve

any benefit of a larger diameter or longer length drier? Drier on vehicle now is pretty much the std issue aftermarket #6 O ring type 2 1/2" dia by 7" or so tall

Am I forgetting anything?

thanx

sc3283 on Tue May 28, 2013 2:14 AM User is offline

I verified WOT shift point this weekend...2200rpm

plan of attack is looking to be

Larger compressor
insulated suction hose
new ex valve(due to not wanting to open system again)
flush system again to be SURE old/excess oil removed
remove OE located rec/drier and inst new drier up from between cond and exp valve

any benefit of a larger diameter or longer length drier? Drier on vehicle now is pretty much the std issue aftermarket #6 O ring type 2 1/2" dia by 7" or so tall

Am I forgetting anything?

thanx

bohica2xo on Fri May 31, 2013 9:13 AM User is offline

With a shift point of 2200 rpm, your drive ratio could be as high as 2.7:1 for the compressor. That gives a compressor speed of 1485 at idle.

If you look at a large compressor like the V7


You can see where the capacity is on the curve. The V7 can run to 7500 rpm during upshifts without damage, and 6000 for longer periods.

Keep in mind that the 6kw line on that capacity curve represents 20,400 btu/H That is a fair bit of cooling at idle.

I used the V7 as an example, simply because I have the data handy. If you are changing to another type of compressor, check the manufacturers data.

If you are still considering making a new drive pulley I would do it as a serpentine design. I know there are some made for that purpose already We used one on the bigger C15 Cat engines to drive a screw compressor. Might have to ask around the trucking folks. A pulley that will give you 2.4 to 1 at a minimum, should work for any compressor you choose.

You still need to determine if the OEM condensor is blocked. First, what type of unit is it? A condensor that is not getting much airflow should still have a measurable temperature drop from inlet to outlet. Spraying some water over it should drop high side pressure quickly if it is an airflow issue.

Dryer size is not a big deal with that long high side line. Do check the sight glass on the dryer, an undercharge will still show up there.

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Fri May 31, 2013 at 9:22 AM by bohica2xo

sc3283 on Sat June 01, 2013 1:21 AM User is offline

factory installed AC from Monaco (factory R134a system)

dash portion is SCS(Frigette)

original condenser brand? NO idea...but it is tube and fin design

current compressor....Unicla 9 cu in

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