Dash Air Conditioning Conversion

[Tranquility]

Hi everyone.

First post as I have followed these threads and now have a problem I can't resolve. Forgive the rather long post, but I am trying to provide all the details. Skip on if you have not interest in AC systems.

My situation is: I have a 1986 Beaver Marquis Garnet 40' motorhome with Cat 3208 diesel pusher. I am having a problem with the in dash air conditioner. The system had lost R12 freon charge before I purchased it and I have subsequently opened, flushed thoroughly, installed new R134a o-rings and dryer, pulled extended vacuum, drained compressor and introduced 8oz PEG oil with UV dye and attempted to recharge the system with R134a.

The gauges hook up on the rear of the Sanden compressor and have back seating valves which were opened midway. I reused the old R12 condensor. My initial problem was that I was getting low low pressure as the high pressure climbed.

I subsequently changed the thermal expansion valve when the warming of the bulb had no effect on low side pressure. Vent temperatures were almost ambient. Thinking the THX was bad, I had to find a replacement based on physical characteristics as there were no markings on the THX head or body. I don't remember the specifications for the replacement, but tend to recall 2 ton rated. Upon trying to charge the system with the new THX installed, I am getting high dash vent temperature (82 ambient with register of 65) and the low side pressure won't go above 15 psig. The high side pressures are in range with the condensor fan running. (82 ambient with high side pressures of 225-240)

There is frosting of the evaporator outlet with constant condensation. The THX is externally regulated and the sensor bulb is well attached and insulated. The system is currently overcharged as there are no bubbles in the sight glass with the system in operation. I realize there is some inefficiency of R134a, but this isn't even close. I should have register tempuratures in the 40's. The measured superheat across the condensor was over 10 degrees, but I returned the specialized sensors and don't have accurate numbers. Even so, the top coils of the condensor can't maintain contact with the skin while the lower coils can be held, so vapor to liquid condensing is taking place there. I question whether I installed the correct THX with a replacement of the correct rating. The low side hose didn't show any restrictions during flushing and I can't imagine it collapsing internally with 15 psi inside. I also checked the airbox doors to make sure they were sealing and positioned properly.

Short questions: Would the wrong THX cause the low suction side readings? Would the location of the line taps affect the readings this much? While the air coming out the vents is cooler, it isn't cold...any enlightenment appreciated.

Brian Campbell

[dickandlois]

Brian,

Welcome to the FMCA Forum!

The AC systems can be a changeling!! Does the Low And High Side pressure equalize when you turn off the system/ compressor? gauges should equal out !!

How long do you do let the system sIt when its under a vacuum? When you run the vacuum pump - do you get an equal reading on both gauges?

If all the above items are in spec, when you charge the system and no more 134 is being drawn into the system, Stop the compressor and let the system equalize. Then start the compressor again, it might take a few cycles to get the charge up to the proper levels.

Oh, is the evaporator fan running when you start the AC system? Bad fan motor-relay-fuse?

Rich.

[huffypuff]

Sounds to me you have some kind of restriction from high pressure to the expansion valve. I would start by inspecting the line that runs up the frame from the rear of the coach to the front. If that check out check the condenser. If that is OK, check line from condenser to the valve. If condenser is on the side don't forget to check the line from the compressor to the condenser.

[Golden_HVAC]

Hi,

I normally call the thermal expansion valve a TXV. They come in two versions. External equallized and internal equalized. External has a 1/4" line that goes towards the outlet of the evaporator, and ties into that line with a 1/4" flare fitting. Do you have that kind? You will be able to install a 1/4" flare tee, and check the pressures there if you do, and that will help in figuring this system out. You will be able to start the system, confirm that you have about 15 PSI near the compressor, then check to see what the pressure is at the outlet of the evaporator. If there is a pressure drop in this line, you will know what part to replace.

Also there might be a huge pressure drop in the line leaving the condenser (at 150 - 200 PSI in your case) to the TXV (should be very close to the pressure at the outlet of the compressor - might be 20 PSI less). If there is a huge pressure loss, such as a bad freon drier, this will cause the freon to stop flowing. The freon drier will absorb moisture, and needed to be replaced when it has been 'open' and not charged with freon for more than a few hours. The drier being plugged will cause the system to act like it is.

Also if you had a internal equalized TXV and installed a external equalized TXV (or the reverse) then the system will not work right. So if the old one had 3 lines and the new one does as well, you are in good shape. If the old one had inlet and only 1 outlet, the replacement is the same, you are in good shape. But Don't change from one to the other without dealing with the extra freon line.

You might have a plugged TXV. It is possible that some junk in the condenser has migrated toward the TXV. IF you are going to the trouble to open the system, install a tee in the TXV external line connection and have to recover the freon anyway, you can blow nitrogen from the compressor fitting high side to the TXV. Loosen the TXV input line, and make sure that the nitrogen flows freely from one end to the other, all the way through the condenser. IF there is back pressure, there might be a freon drier in the way, that is plugged, and that might solve your problem.

How about remove the TXV bulb clam, and warm it. This will open the TXV all the way, and should increase the pressure in the low side a lot. IS the freon line leaving the evaporator already cold? My guess is that at 15 PSI, if you are only getting minimal cooling on the coil, that the air is warming what little freon is in there to about 65F and it will come out very warm. The TXV should be opening fully and the pressure should increase to say 30 PSI. This should flood the evaporator with freon, and cool it, when the outlet is cold enough, then the TXV should respond and close the valve a little bit.

You might also need to put a pressure port at the inlet to the TXV, so that you can determine the freon pressure into the TXV. This should be close to what the compressor output pressure is. So you are getting 150 PSI at the compressor outlet, it should be within 10 PSI at the TXV input. My guess is there is a freon drier inline between the condenser someplace that is blocking your freon flow, and your TXV inlet pressure is much less than the compressor outlet pressure. So my guess is TXV inlet pressure is going to measure only about 40 PSI, while it should be 200 PSI if that is the compressor output pressure.

Or it is possible that if you had a internal equalized TXV, and installed a external equalized TXV, that is the problem.

Good luck!

Fred.

[Tranquility]

Thanks for the replies.

I had some time to work with the system yesterday and using your advice, here is what was done.

In researching the dash unit, I found the unit components have A-R-A labels on them and while I could not find information on this company, the components look identical to the Acme bubble AC units. (Anybody know what the proper R12 charge weight was in these units with 40' hose?) The TXV is externally balanced and I like the idea of installing a tee to check evaporator outlet pressure. (Thanks Fred) It will take some time to get that done however as I don't have all the hardware. The system pressures do slowly equalize when the compressor is de-energized. Double checking the condenser fan cooling, I found a short cycling circuit breaker for the condenser fan which was replaced. One terminal of the CB had become hot due to resistance and slightly melted the terminal. The condenser fan is controlled by a triad pressure switch on the HP line just after the drier which controls a fan relay. This relay can be easily bypassed to run the condenser fan continuously while charging, which is what I did. , I reclaimed the refrigerant and opened the system. High pressure, high volume air flowed freely from the compressor at the DP to the TXV inlet, so I do not suspect problems with the HP hose, front frame mounted condenser, drier or hoses to TXV. I also checked the inlet and outlet of the TXV for debris and none was noted. Sealed the system back up and pulled extended vacuum for 1.5 hrs. I recharged the system until the drier sightglass showed some bubbles and the evaporator outlet fluctuated between 30-38 F, with slight frost/condensation on the first part of the return hose.

The unit still performs poorly in my opinion, or is just severely undersized. Using a Fluke thermal gun, I was able to obtain the following:

there was 10F subcooling across the condenser, the evaporator outlet temperature hovered in the 30's, but a scan of the coils revealed they were only in the low 60's. This was close to the vent outlet temperatures. The compressor pressures on a 73F morning were 200 high/15 low, again taken at the compressor.

I am now researching the evaporator size and configuration. I have read that some evaporators which split the refrigerant into capillary tubes after the TXV have a 1 bar (14psi) pressure drop. Still reading more on this, so I might be wrong. Just pure speculation, but if there is some blockage of the capillary tubes in the evaporator, would that explain the lower outlet pressure without the coils cooling effectively??? My thinking is that if some blockage is present, the overall tonnage rating of the evaporator would be less. What cooling does take place would be dispersed thruout the fins which act as a heat exchanger, but the dimenished capacity would allow exit temperature to read in the 30's. Any thoughts on this idea?

If the evaporator does need replacing, it will have to wait due to the ease of access and travel plan dates approaching.

Brian Campbell

[dickandlois]

Brian, The dash duct output temperature in the recycle mode for our dash AC reads about 20 to 25 degrees lower then the ambient.

So at an ambient temp. of 70 to 75 deg. will read 50 to 55 deg. at the vents.

The dash AC units on most Coaches will not cool the entire coach. So many owners run there generator and one or two roof AC units.

Rich.

[Tranquility]

Well, based on Rich's numbers, I am going to resign myself to the underperformance of this system. In looking at all parts of the unit, I also noted that the foam insulation in the bubble box is degraded, which would also increase heat load. I hope this insulation isn't falling on the upper side of the evaporator as it is difficult to access. The airflow at the vents is good, but I can't be sure all of the air is flowing over the evaporator. Thank you all for your ideas and suggestions.

Brian

[dickandlois]

Brian,

One item you might want to consider. Look and see if there is any or how much insulation is installed around the front of your coach.

I have install insulation on the floor behind the kick panel in front of the passenger side, on the inside of the firewall from the drivers side all the way across to the passenger side, under the extended dash area outside the coach and the outside firewall.

Think you will fine that the only material between you and the outside is the wood used for the firewall.

I took the time to add insulation on the outside of the firewall also.

Outside where it is exposed to the elements, I used a mylar style insulation cut to fit around every surface above the generator on our coach and the under side of the extended dash. The real trick is to find a good method to attach the materials to each of the areas.

The dash support in many cases is made of rectangular steel tubing. I used some expandable foam injected into these.

Wrapped the duct work - this reduced the condensation that forms when the cold ducts contact warm / moist air and maximized cool air transfer from the heater / AC blower box.

Time consuming job, but the difference in our comfort level was well worth the time both if summer heat and winter cold.

Rich.