Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

Custom-built system -- how much refrigerant? Pages: 12

SpinRite on Tue June 16, 2009 2:51 PM User is offline

Year: 1994
Make: Holiday Rambler
Model: Navigator
Engine Size: 8.3L Dsl
Refrigerant Type: R-134a
Ambient Temp: 77
Pressure Low: 14
Pressure High: 125
Country of Origin: United States

Hello to all -- great forum you have here!

I'm a pretty handy DIY-re, but this adventure is my first real venture into the world of air conditioning.

I've had this '94 Diesel-powered motorhome for a couple of years. When I got it, the A/C wouldn't cool and I finally found, and replaced, a bad clutch coil. Compressor pumps OK now, but not really cooling well.

Since I have no idea what others before me might have done to the system while trying to trouble-shoot the clutch problem (which was intermittent and difficult to find) I'm ready to pump it down and re-fill.

How do I know how much refrigerant is "enough"? I can't find any stickers with filling info.

Its a custom-built system of the type found on a bus. Compressor (Zyxel) is in the rear, driven by the diesel. Large condenser is side-mounted, with two 18-inch electric fans that blow whenever A/C is running. The evaporator is up front, about 35 feet away. All plumbing looks to be the usual 1-inch O.D. hoses common on automobile systems.

Can you "guesstimate" how much R-134a a system like this might need to fill it up?

And, how do I know when to stop adding refrigerant? I'll need every bit of cooling I can get out of this system when I drive through AZ on I-10!

HVargas on Tue June 16, 2009 3:12 PM User is offlineView users profile

On something like that your best bet is going to go by pressure and go by vent temperature. Unless you can get a hold of a Holiday dealer and see if you can get it out of one of there service people but we all know how well that usually goes.


When considering your next auto A/C purchase, please consider the site that supports you:
Contact: Arizona Mobile Air

mk378 on Tue June 16, 2009 3:43 PM User is offline

It's going to take quite a bit just to fill up that long line with liquid. I figure 5.6 pounds for 35 feet of 5/8 inch ID hose (liquid R-134a at 100F is 72 pounds per cubic foot). Then add 3 or 4 pounds for the rest of the system, especially if the condenser is a large tube and fin type. Better buy a 30 pound cylinder unless you can get a heck of a deal on cans.

Pressures show it's rather undercharged, maybe a leak or maybe the last guy just didn't believe how much it would really take. Also while it is empty it would be good to replace the receiver-drier just as a routine measure.

SpinRite on Tue June 16, 2009 5:14 PM User is offline

Thanks for the replies.

But, remember that I am a rank amateur, when you say "go by pressure and vent temperature" ...?

I know there are not exact numbers, but should I look for the readings to level off or something?

How do I know when I have added too much?

Thanks for your help.

chris142 on Tue June 16, 2009 5:23 PM User is offline

2.0 to 2.2 times the ambient temp is a start.

If it's 80F outside aim for 160 on the high side and see how it does.

SpinRite on Wed June 17, 2009 4:11 PM User is offline

I've been reading up a lot on all this and I guess I'm ready to open the system. Have a few more questions before I start...

Since I've never really seen this stuff before, let me make sure I'm clear on what everything is:

1. Is this the Receiver-dryer?
Can I just buy a replacement, or does it come apart somehow to put a new bag of dessicant in it?

2. I've read a lot about the orifice tube and seen several pictures of them, both new and suffering the "Black Death". But, I'm not sure where it is actually located. These hoses are just entering my evaporator. Is the OT inside that aluminum tubing where the couplings are -- the one marked with orange tape?

I'm pretty sure that I need to pull it out an check it for "crud" -- any special tricks needed to get it out? Unscrew it, or just yank?

3. Is this block just a flow-splitter for the evaporator -- or is there some kind of valves or oriffice in it? Do I need to open and inspect it?

4. Is there any signifigance to the blue stripe painted on the block, or is it just some mark made during manufacture.

Thanks again for all the help.

mk378 on Wed June 17, 2009 4:27 PM User is offline

1.Yes. Replace the whole unit; it is not serviceable, kind of like an oil filter. The switch unscrews also.
2 & 3. You have a TXV system, not OT. The yellow block is the TXV (called a "block type", imagine that). As the system seems to be basically working, no need to mess with it.

chris142 on Wed June 17, 2009 5:21 PM User is offline

The reciever dryer is a universal. You will have to have it matched up as there are several different sizes, length doesn't matter really. Should be #6 fittings on each end. Make sure to transfer the pressure switch from the old one to the new one.

You have an expansion block instead of an orifice tube. Leave it alone if it's working.

SpinRite on Wed June 17, 2009 5:47 PM User is offline

Thanks, mk378.

When I was trying to find the clutch for my compressor, I learned that parts stores are unable to identify anything, except by using the "vehicle's year, make, model." Since this is a custom-built system, I can't give them that info, so finding parts is very difficult.

I already looked for the R-D on ACKits and several other on-line sites, but no luck. In fact, ACkits was the only one I found where you can at least browse the pictures of their stock without having to first enter "vehicle's year, make, model." Maybe I just don't know where to look for A/C parts.

I'm hoping that one of you experienced guys on here can just eyeball the picture of the R-D above, and recognize it as the one commonly used on a certain "year, make, model"

bohica2xo on Wed June 17, 2009 7:37 PM User is offline

I generally use THIS ONE when I need the switch port. You get a real mount instead of pop rivets. A quick bracket may be required.

Pay attention when you open it up, there should be some oil on the inside surfaces of all of the plumbing. That is a long way for the oil to migrate, and balancing the oil charge can be tricky with some of those motorhomes. Drill a hole in the bottom of the R/D, and drain the oil overnight. Measure it, and replace it + some for what is hanging around in the desicant bag.

Good luck with your project.


"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.

HVargas on Wed June 17, 2009 7:40 PM User is offlineView users profile

If you go into our search feature, and type in part number 37-10849 that should be the correct drier. We keep them in stock if you need one.


When considering your next auto A/C purchase, please consider the site that supports you:
Contact: Arizona Mobile Air

JACK ADAMS on Wed June 17, 2009 7:49 PM User is offline

Here is all the data for the drier that Adam listed for you. Part# 37-10849

Part Description2.5 X 8 3/8 MIO W/LP PORT & FP
Diameter 2.5in
Length 8.0in
Fitting In 3/8 MIO
Fitting Out 3/8 MIO
Desiccant 4.0ci
Show ChartY
Part Type DRIERS
Aftermarket Num04-4082

SpinRite on Wed June 17, 2009 9:04 PM User is offline

Wow! Is this a great forum, or what?

Thanks a lot for all the great come-backs. everybody.

TXAB on Thu June 18, 2009 10:38 AM User is offline

Originally posted by: SpinRite
Wow! Is this a great forum, or what?

Thanks a lot for all the great come-backs. everybody.

This is one of the best forums.

"Don't get stuck on stupid!"
---- Lt. Gen. Russel Honore

mk378 on Thu June 18, 2009 10:47 AM User is offline

Also on the receiver drier pay attention to flow direction-- ports should be marked with "in" and/or "out." In from the condenser and out to the TXV. Plumbed backwards, you will have a bad time. They must be mounted vertical or nearly so, like your old one, because there is a tube inside to pick up liquid refrigerant from the bottom.

SpinRite on Thu June 18, 2009 5:07 PM User is offline

I have a small compressor from an old refrigerator -- it was working fine when removed a few years ago.

Should that pull a good enough vacuum for this purpose, or will I need a "real" vacuum pump?

JACK ADAMS on Thu June 18, 2009 8:01 PM User is offline

They will work as long as you get a good vacuum on the system. I heard of people take a vacuum lead off the engine and run the car till they get a good vacuum. Not to sure about that one but people do what they have too. You would like to see at least 28inch of vacuum for at least a good hour on this big of a system.

Hope this helps and good luck!!

SpinRite on Thu June 25, 2009 1:33 PM User is offline

While awaiting arrival of my new R/D, I attached the vacuum and pumped system down -- let it sit overnight -- no vacuum lose.

On advice I got from a friend, I let in a little bit of refrigerant, just to bring pressure up to 0 psi. Let sit overnight "to absorb moisture," then evacuated system back down to hard vacuum. Repeated this twice.

New R/D arrived from ACKits, so I removed the old one and drilled a hole in the bottom to see how much oil was in it -- there was none. So, I cut it open with a hacksaw and the inside is bone dry -- no oil on sides and the felt pads are not oily at all -- in fact they are full of a gray dust which shakes out! Aluminum, maybe?

Hmmm, now what?

bohica2xo on Thu June 25, 2009 2:06 PM User is offline

Bone dry R/D. Ruh-Roh. Any oil residue at all in the low side lines?

How does the compressor feel when you turn it over by hand?


"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.

SpinRite on Thu June 25, 2009 2:18 PM User is offline

Compressor turns easily and smoothly. I didn't open any other lines, only the R/D which is next to condenser.

SpinRite on Thu June 25, 2009 3:03 PM User is offline

Although the original subject of this thread was "Custom-built system -- how much refrigerant?", I guess the question I have now is "How much oil?" And, what viscosity?

BTW, here is the old R.D. As seen here, the desiccant beads and the felt pads trapped a lot of powdery stuff seen below the pad on the right.

SpinRite on Thu June 25, 2009 5:16 PM User is offline

Pulled off the hoses going to the evaporator and tested inside with a cotten swab. One hose was clean inside, the other coated with that fine powdery dust.

I checked the dust and it jumps onto a magnet, so my guess was wrong about it being aluminum.

Anyway, it is the upper hose going to the TXV that is dirty, the lower hose is clean. I suppose the evaporator must have a lot inside of it.

Any special cautions, or guidance, on flushing the TXV & evaporator. I read one thread on here where the guy took his TXV apart to clean and that was a mistake. Do I back-flush the TXV, or go in the direction of normal refrigerant flow?

Thanks for any advice...

bohica2xo on Thu June 25, 2009 10:10 PM User is offline

Good thing you looked. it would not have gone well with "a couple ounces" of oil for a dryer changeout...

How does the compressor feel? The iron came from someplace. Turn the clutch plate by hand, and see if it grinds. You can do the same inspection at the compressor, by pulling the lines & swabbing like you did at the evaporator.

You have a lot of it to do. The long lines, the evaporator & condensor. There may be a HECAT in your future.

The TXV is removed for flushing. You can clean the TXV with compressed air. A little light oil / solvent (PB blaster, LPS#1, etc) sprayed into it & blown out should get it clean. Inspect it for signs of corrosion on the internal parts.

The evaporator pic is a perfect example of why volume is so important in flushing. See the 4 spaghetti tubes? Imagine one of them totally blocked - you could still blow plenty of air through the rest of them. Even if not blocked, one section of coils could still have oil & flushing solvent, but air would blow out the rest of the tubes. An MVAC compressor can produce as much as 21 CFM of flow @ 100+ PSI. To blow all of the flushing agent dry, you will need to find a piece of hose that will clamp over each tube that is currently attached to the TXV. The discharge side should lead to a catch can (a 5 gallon pail with lid works good, be sure to leave an air vent). The air inlet (spaghetti tube side) should connect to an air plug or ball valve with the same bore as your shop air hose. Volume is important here. If you have a small compressor, let it build pressure before opening the valve. This will generate higher velocities, and clear all of the tubes.

Your condensor is ahead of the R/D, so the big pieces that did not make it to the R/D are in the condensor. Is the condensor a simple Tube & Fin type? If so, it will flush without too much trouble.

If the compressor feels OK, it should be removed & flushed with clean oil of the type you plan to use.

Oil charge.
Hard to guess what it will take to coat everything inside, plus what it will take for the R/D, compressor etc. Usually 15% of the refrigerant charge is close, but you do not even know that yet. A big system like a dual A/C E250 holds 72 ounces of refrigerant & 10 ounces of oil. You could probably start there without problems. I would add oil proportional to refrigerant once you go past 72 ounces. Starting with 12 to 14 ounces of oil might be better.

I would use BVA 100, it should be thin enough to migrate well.

Where are the gauge ports in this system?


"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.

SpinRite on Thu June 25, 2009 11:11 PM User is offline

Thanks for the detailed info.

Both gauge ports are right at compressor hose connections.

Compressor turns easily and smoothly, no scraping or grinding -- but I have never felt one before, so I have no comparison. In fact, perhaps it turns TOO easily -- one finger on the clutch plate will move it -- it's not loose, but there is only slight resistance. Of course, the system is evacuated right now, and apparently doesn't even have an oil film to make it sticky.

Is there any kind of manual compression test I can do, that will give an indication of condition? If I pour oil into the suction port and try to turn it fast by hand, should it lock up until the oil seeps through?

Edited: Fri June 26, 2009 at 1:58 AM by SpinRite

bohica2xo on Fri June 26, 2009 2:42 AM User is offline

You can bench test the compressor, here is a detailed post:

Compressor flushing & testing.

With the test ports on the compressor, you may see some low side pressures that are not in line with what is actually happening at the evaporator. The high side will be actual discharge pressure.

When you get to the point of charging the system, you should put the oil charge in at the compressor. Put most of it into the discharge line. Add a couple of ounces to the compressor inlet, and turn the pump by hand several revolutions to be sure there is no oil slugging. You should charge the bulk of the refrigerant charge as a liquid, with the compressor stopped - Through the high side fitting. This is important. The compressor discharge valve keeps the liquid refrigerant moving toward the evaporator, dispersing the oil on the way. Refrigerant is a great degreaser. Charging several pounds through a compressor with a very long return path is a death sentence for that compressor. Once you get 60 or 70 ounces in there, close the high side handwheel on your gauge set before you start the compressor. An evacuated system pulls in liquid refrigerant quite well.

Because the control panel is 35 feet away, I like to rig up a local compressor switch for those whales. That way I can stop the compressor if I need to while charging or testing.


"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.

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.