Automotive Air Conditioning Information Forum (Archives)

jlb27537 on Fri June 17, 2011 9:47 PM

Year: 1996
Make: Safari
Model: Sahara
Engine Size: 300 CAT
Refrigerant Type: 134a
Ambient Temp: 90
Pressure Low: 45
Pressure High: ??
Country of Origin: United States

Hello,

I have a 35' diesel pusher motorhome. Uses 134a. System was built by Acme, now Mobile Climate Control. They have no records of capacity of system. Have taken it to a very knowledgeable shop in Springfield, Mo. They service a lot of vehicles like airport shuttles, motorhomes for their a/c problems.

The dryer failed, causing the TXV to fail. The original TXV was a Parker 031134-106. Was replaced with a Omega 31-10931-AM Re-charged with 3.5lbs. That amount was a guess based on the length of the coach. Mobile Climate Control says the TXV part number is 4103005, which does not cross to the Parker that was on it.

System was evacuated, flushed, re-charged, dye added

While I have complete faith they are doing the best they can do, Questions:

The dryer has a sight glass in the top of it. Should the glass be solid with fluid or look like liquid is running thru it? There are no air bubbles.

With it on fast idle of 1200 rpm, dash fan on high, inside ambient of coach about 90 degrees, shop temp about 90 degrees, low side is 43-45 lbs with 58-60 degree air out of dash vents.

I am not working on it, it seems like it used to have colder air out of the vents. I have not had it on the highway yet.

Thanks

bromodragonfly on Fri June 17, 2011 10:59 PM

Keep in mind I work in commercial refrigeration/HVAC, and am not an expert in automotive applications. But here's my experience...

Sight glass should be solid, and will most likely look like nothing is flowing. Without dye, it can be difficult to tell if there is even anything in the sight glass. Bubbles would be a sign of an undercharge, or premature flashing (lack of subcooling in condenser).

With suction pressures of 43lbs, saturated temp in the evaporator will be around 48F. 58-60 F out of your vents could be normal depending on the condition the evaporator is in.

A TXV requires a full column of liquid to work properly, so make sure that there's no bubbles in that sight glass. But if there are, keep in mind that increasing airflow (by driving at speed) over the condenser may quickly solve that problem. Also, more heat rejected by the condenser, and therefore more refrigerant subcooling will lead to an increase in the refrigeration effect. The refrigerant will be able to utilize more of its latent heat absorption.

I looked up the omega TXV that you used as a replacement, and it looks like it's sized for a 1.5ton system. The tonnage of the replacement valve should match the tonnage of the original valve, and unfortunately, I couldn't find any info on the Parker or the one that MCC specified. If it is a smaller tonnage, the valve orifice will not be a sufficient size to feed enough liquid into the evaporator, even if bulb pressure is high and the valve is 'wide open'... which will starve the evaporator if the load increases (on a hot day).

The superheat setting of the valve is not critical, but the replacement should be similar, and this setting will be determined by the equipment manufacturer... basically it's just to ensure refrigerant going back to the compressor is 100% gas. You should ensure that the capillary bulb has tightly strapped to the outlet of the evaporator so that the valve will sense that temperature and function properly. Also, since that omega valve is externally equalized, make sure whoever has installed it hasn't been lazy, and has actually connected the side port to the evaporator outlet.

But before any of that, I would get the motorhome out of that shop and drive it around for a bit, before you decide if the system isn't performing as well as it used to (and have ambient temperatures in mind as well when you think about your comparison). If you wanted to be sure, you could phone up MCC again and get the info off the original TXV, to see if the shop used a suitable replacement.

Anyway that's my two cents.

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There is no knowledge that is not power

chris142 on Sat June 18, 2011 12:03 AM

The sight glass is useless with R134a. With a properly charged system you will still have foam and bubbles in the glass. If you charge untill the bubbles go away the system is now overcharged.

Since the dryer came apart I would expect the dessicant to be partially plugging the condensor and it may need to be replaced.

bromodragonfly on Sat June 18, 2011 2:32 AM

Does 'drier' refer to a liquid line drier, or a suction accumulator that contains a filter element? I assumed it was a liquid line drier since he mentioned the TXV failing, and they are situated between the outlet of the condenser and the TXV. In some systems, the liquid line drier is contained in the liquid receiver.

If the sight glass is before and close to the TXV and there isn't a significant pressure drop between it and the condenser, I would expect a solid sight glass. The TXV is designed to feed 100% liquid. I would be concerned if I saw constant bubbling through a sight glass on the liquid line. To me, it would be an indication of a dirty condenser, or not enough airflow across it. Especially since his system has equipment sized for 134A and is not a retrofit.

I'm just relating my HVAC experience to automotive, but I could be utterly wrong. Please let me know if I am, I'm trying to learn the different quirks of vehicle air conditioning.

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There is no knowledge that is not power

Dougflas on Sat June 18, 2011 5:35 AM

If the inside temp of the motor home is 90*, the the low side will be high. Get the motor home cooled down, then measure the pressures again. The sight glass is mounted at the drier; I don't rely on them for 134a as a charging indication.

jlb27537 on Sat June 18, 2011 9:30 AM

I got a reply about the OEM Parker valve. The original had 134-106 stamped on the valve, which is 031134-106, in 2006 the number changed to 031134-117 (6" longer cap tube and 2.5" longer external. This crosses with a Omega 31-10970. The 31-10970 and the valve used of 31-10931-AM have the same specs except the super heat. The original valve had a superheat of 8 and the one installed is 5. Probably not a issue. Right??? The shop also could not get a reference on the orig valve, but using their experience they used the valve they did, plus they stocked it. This shop does a LOT of work on these non factory systems as used in diesel pusher motorhomes and on add on systems as used in shuttle vans on F450/550 cab and chassis.

MCC says their part number is 4103005 which crosses to a Omega 31-10707. The FOR fitting shows to be a 12mm while the others show 1/4"

The shop flushed out the system and felt the system to be clean.

The dryer is on the high side. It is about 18" in front of the TXV. The sight glass is clear, no bubbles, like solid liquid. But you can see liquid running thru it.

I appreciate all the help. Going on a road trip next week. I'll just see how it goes. I realize how hard it is to work on a 15 year old system with no documentation.

Thank You, Jim

bromodragonfly on Sat June 18, 2011 7:51 PM

All of the valves look suitable, their tonnages are the same. Too much superheat can be hard on the compressor, or be too much of an addition the condenser to remove. Too little and you may get liquid slugging back to the compressor. But a difference of a few degrees, as in your case, isn't going to matter.

the equalizer line fitting size won't matter either. All the equalizer line does is sense pressure at the evaporator outlet, instead of at the TXV outlet, just to ensure the pressure drop at the distributor doesn't confuse the valve.

stay cool on your trip

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There is no knowledge that is not power

jlb27537 on Fri June 24, 2011 12:28 PM

Left on the trip. Was 85/90 degrees both inside and outside the motorhome. Air out of the ducts was 60 degrees entire trip. 60 miles @ 60 mph. Feel it is not cool enough. Planning on talking with the shop when I get back Monday.

We have right at 3.5 lbs in it now. There does not appear to be any specs on what the correct charge is. The motorhome is 35' long, so probably 80+' of hoses plus the capacity of the evap and condenser.

My first thought is it is overcharged? I'll get both low and high side readings if I can. The low side was 45lbs when I left the shop @ 90 degrees.

Thanks Jim

bohica2xo on Fri June 24, 2011 4:13 PM

At 56 ounces you are most likely undercharged. We really need to see the high side numbers to tell you which way to go.

In my experience with pushers, 72 ounces is a starting point for testing. Some take much more than that...

B.

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"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.