FIXED!!!
This is a copy of the original writeup: http://www.explorerforum.com/forums/showthread.php?s=&postid=667436#post667436 I may update the main post, please check there for new thoughts.
I purchased the truck in the winter knowing the A/C wasn't working. Even on warm days, the compressor would cycle on for a few seconds and then shut off for ten. Pressures were low.
Diagram of A/C system, courtesy of www.ackits.com
Daignostics
On 1996-1998 V8 Explorers and Mountaineers, Ford wrapped the accumulator with a foam blanket. My guess is they figured since the accumulator is in the low side (hoses are cold) and it is right over the passenger exhaust manifold, they wanted to insulate it. However, the accumulator sweats, the foam trapped in the moisture and the steel rusted.
Comparison of New and Old Accumulators
After peeling away the foam, I found a mess of rust, PAG oil, and flourescent dye. (Ford adds dye in the factory) A new Motorcraft Accumulator was $65.
Repair
The system was discharged, and the accumulator removed. One of the fittings is a simple hex nut, the other is a spring disconnect. The spring disconnect tool is also used on the fuel system, and can be purchased at Autozone for less than $10.
While the system was open, I wanted to inspect and change the orifice tube. Located in the inlet to the evaporator (the one that get's cold, near the dash). It is a filter/metering valve.
Old Orifice Tube, 109,500mi
Often times the orifice tube is an excellent indicator of system health. If a compressor is going, little metal fragments will start to clog up the screen. If the compressor is gone, "black death" will load up the orifice tube with gunk.
http://www.ackits.com/index.cfm?fuseaction=Black Death
My orifice tube was fine, and did not need replacement. However, since the system was open, the tube was out, and a new Motorcraft OEM tube was $4.00, a new one was installed. A special puller is needed to remove/insert the o-tube, I "rented" one from Autozone for free.
When assembling the system, the o-ring seals were dipped in the air conditioning oil, PAG 46. 2.25 oz of PAG were also added to the accumulator before it was reassembled to replace lost oil. The Cycling switch was re-attached to the accumulator.
Evacuation
After the system was buttoned up, a vacuum was pulled. This is where the difficulty was. A/C systems
MUST be vaccumed to remove air. The vacuuming accomplishes two things: 1) It removes air from the system. When expanding, air does not get cold like 134a, yet has volume, reducing cooling capacity. 2) A vacuum will lower the boiling point of water(moisture). Moisture in the A/C system can form acid.
The ideal vacuum pump is a dedicated electric refrigeration pump, capable of pulling down to 50 microns, or 29.9 in Hg. No one in the Boston area rented them, they cost $200. I had used a ventrui effect air-driven vacuum pump on two previous R12-R134a conversions, which cost me $30.
Robinair Air-Vac
The the AirVac however did not pull past 26inHg. My medium sized compressor (5hp, 5.1scfm@90psi, 22gal) could not keep up with the 90psi the AirVac's needs. I held the 26in HG vacuum for about 1 hour, with the Air_Vac running. I cycled it on/off not to strain my compressor. I wasn't too worried about moisture since the system was open for less then 30min, on a very dry day. The excess air, would mean higher vent temps. Oh Well. I have since heard that the pull to 26inHG is almost instant, but the AirVac can pull to 29.7in HG if given enough time.
EDIT:Many people who use these make a manifold to run two compressors, to maintain constant high pressure and not damage the compressors. Also, I've heard that it is possible to make a vacuum pump from an old compressor (Refigerator, dehumidifier, window A/C). I may look into this, there a window unit from the 70s in our basement)
Charging
With the engine off, the first 12oz can of 134a was discharged into the system's high and low side upside down as a liquid. The can emptied 100%. A second can was attached, right side up. The high side valve was closed, and the engine started. The A/C control was on "Max A/C," the temp all the way cold, and the fan on max. All windows/sunroof were open.
Gauge Manifold
Charging was done using the temperature probe of my Mac Tools 710 Digital Multimeter. Very accurate, it's remote probe reads in tenths of a degree. I added gas untill the vent temps settled on 40.5 deg F. The Ambient temperature was 76 degrees, High side pressure was 190psi, Low side pressure was 30 psi. Adding gas took a VERY long time, almost 30 minutes. I've heard people say that you can dip the can of 134 into hot water to speed of the process, but others say not too. I chose to take it slow and easy. You can see when the clutch cycles by the increase/decrease in vent temp. Often the difference will be by 3 degrees. As the system nears full capacity, the cycling will stop, and vent temps will remain constant. The sticker on the dash says the system will hold 1 lb 14 oz of R134. This is 30 oz, exactly equal to 2.5 12 oz cans of R134. I estimate I was able to add about 26-27 oz of R134, leaving about 3oz of air in the system.
The clutch does not cycle often now, and the air is nice and chilled! Special thanks go out to Chris, Glacier991, who gave me advice along the way.