AC dryer and expansion valve

[ekgflashnet]

I'm setup to get the A/C serviced next week.  I'm pretty sure the symptoms are pointing to a bad evaporator/drier.  I'm going to have that replaced.  After that, I'll have to go on a drive during hot weather and see if the compressor stays running or once again stops while running the AC after some period of time being on.

steveg 2005 DDIV

[vanwill52]

Perhaps I'm misreading something, but if the low-side pressure is over 40 PSI and the compressor will not engage, it would seem the low-pressure switch is not closing.  When evacuating and recharging, I usually remove the connector from the low-pressure switch and put a jumper wire in the plug.  This forces the compressor clutch to stay engaged as the system first starts filling.  Otherwise, the compressor short-cycles and it takes much longer to take the charge.

Although those with more experience might be able to tell when the system is properly charged just from the gages, I use a weightscale to measure the Freon tank weight and add what the system spec says.

If the low-pressure switch is faulty (stuck open) put a jumper in the low-pressure switch plug.  If the compressor engages, the low pressure switch is faulty.  Not all systems have a high-pressure cutout switch.

[David Pratt]

The Pressure Switch on the old R12 Systems were just a single low pressure switch to cut the power to the compressor if the low side of the system got to low to protect the compressor.

With the introduction and use of R134 in 1994 the manufacturers used to different pressure switches depending on the make up of the system.  One pressure switch is called a Binary Switch and has a Low and High pressure cut out and can be identified with only two wires coming out of the switch. The other switch is called a Trinary Switch and has a Low and High pressure cut out and a third feature to turn the Condenser Fan on/off when the compressor is operating and can be identified with three wires exiting the switch. The Binary Switch is the most commonly used switch in our motor homes when the condenser is mounted adjacent to the Radiator. If the condenser has an electric fan it may have a Trinary switch instead.

Using a scale is the correct method to weigh the proper amount of Freon that is added to a completely Empty, Evacuated System. If all the components of the system are working properly this will give you the correct Low and High Pressure readings with the gauges and the outlet temperatures exiting from the Dash Vents.. Those pressure readings and temperatures are then the Bench Mark for properly maintaining the correct levels of Freon in the system. When the system starts to get low on Freon, the Temperature exiting the Dash vents will start rising, the low pressure side of the system will get lower and the high side of the system will rise.

The only correct way to properly add Freon to the system then is to use the gauges to add only the amount of Freon to bring the pressures and temps  back into the proper range.

The Dash A/C System is probably the most neglected system in our coaches. The pressures should be checked at least annually and the Dash Vent Outlet temperature checked more often, especially if the coach sits idle for any length of time. A rising Temperature at the Dash Vents is the earliest sign that the system is getting low on Freon. The Dash A/C system does not like to sit as seals and hoses have a tendency to start leaking with age.

[vanwill52]

A few years ago, I decided I must be the last mechanically inclined fellow on the planet that knew nothing about servicing A/C.  So, I started reading, then had a lengthy discussion with my Christian heavy-vehicle mechanic and he gave me some good info.  One of the things he told me was that the "average" consumer could frequently get good results simply by using the "Refrigerant in a can" devices available at auto parts stores.  They frequently have a gage that reads the low pressure side.  If your compressor won't engage because the low-pressure switch is preventing it...and if as you add refrigerant, it engages, chances are you are low on refrigerant.  But reading the low side doesn't tell you all that you need to know.  My heavy-truck guy convinced me to buy a good quality set of gages (although he admitted that amateurs could get the same result from $50 gages as from $250 gages, a good quality vacuum pump, R134 in bulk containers, and a digital scale.  He advised that if you don't know what is wrong, check the static low pressure (compressor not running).  If it's below 80 PSI (min), you likely are low on refrigerant.

I took his advice, bought the gages, the bulk refrigerant, a high-capacity (Yellow Jacket?) vacuum pump, and the digital scale.  A bit "over the top" for an A/C amateur, but I'm a "tool guy"

I've spent the last few years repairing my own, and friends, A/C problems.  I've replaced three compressors, two condensers, a half-dozen low-pressure switches, plus "topped off" refrigerant in a dozen vehicles.  I tend to follow my mechanic's advice and simply evacuate every system to start with.  Once evacuated, let it sit for a few hours.  If the vacuum diminishes, you've got a leak you've got to find.

Luckily, I have never had to replace an evaporator.  It seems that no matter the vehicle, replacing an evaporator is an ordeal, if it is an in-dash one.

Just as a FYI, I had an Internet friend in AZ who seemed exceptionally knowledgeable about A/C.  He said he had determined from the Sanyo (Sanden?) specs for his compressor, that it was being run at too low an RPM with his pulley setup.  He changed pulleys (engine pulley, I think) to give him the proper speed.  He said his other option would have been to choose a higher capacity compressor that would provide adequate flow for the coach.  He did so, and said his A/C improved immensely.

One thing I would suggest EVERY RV owner have is an A/C duct thermometer.  About $10 on Amazon.  If you KNOW what the air temperature is coming out of your vents, it removes a lot of guesswork.  As Dave Pratt said, a good functioning dash A/C will bring vent temps to the 40*F - 50*F range under most conditions.  In keeping with my "A/C education"  I have a dash vent thermometer on every vehicle I own.

I'm sure Dave Pratt's (and many other's) experience far exceeds my own, but it has been satisfying to take care of my own A/C problems.  It is no longer the mystery it once was.

YMMV

[Happycarz]

Two duct thermometers would be better than one.  One for the output duct register and the other one at the input grille. Measure the delta T.

As I live in AZ, the interior of a parked auto or motorhome can easily reach 125°.  Even on recirculating mode and 125° input air, the output air is certainly not going to get down to 40°. Your delta T at the outlet duct will vary depending on both the evaporator inlet temperature and the delta T across the condenser. You can throw in high humidity to maybe cut the delta T another 5°  (7° humidity here, as I write this.)

Good airflow across the condenser goes a long way in helping the system keep you cool. Annual condenser cleaning and making sure the fan, electric or mechanical, is coming on and moving air.

BTW, I’m not the one in AZ that changed my pulley. Ought to look into it though.

[vanwill52]

Harry,

You are a gem in this community.  No BS.  Just facts;

Van

[David Pratt]

The discussion on AC Drier and Expansion Valve is about the Dash AC in the coach not the Roof AC units. But you are correct on your info on the temp inside and outside.

There almost is no way to put a duct thermometer in the input side of the evaporator on a dash AC. Just measuring outflow temp at the duct will help monitor the system. Also, while traveling and running the roof AC's will help tremendously to lower the temps of the dash outflow temps.

I will run at least two of the four roof AC units while traveling, normally the front and rear unit set at 75 Degrees F and it keeps the coach very comfortable and the Genny gets its proper exercise.

[Happycarz]

Dave, not sure if you are referring to me, but I am talking about the dash air.

When in recirculation mode, air enters the evaporator through this grille.