Does changing the orifice tube to a variable one necessitate an adjustment to the volume of R134a? Does the replacement of the AC compresssor in a dual system require flushing of both systems? And finally, if it does require flushing of both front and rear, how does one flush the rear system in an Excursion with the expansion valve? Does it require removing the valve?
I have very high pressures on the high side and am looking for possible causes.

 

To answer your questions:
1. No change in the amount of refrigerant with the VOV. However, in my opinion, VOVs are a gimmick in most applications. They also introduce another point of potential failure to the system.

2. Yes. After a compressor failure, both systems must be flushed.

3. Yes, the rear TXV must be removed for flushing.

Repairing dual AC systems after a compressor failure is quite labor intensive.

I need to ask, why are you asking these questions? You say that the system has high High Side pressure. That sounds like the compressor is functioning quite well.

The most common cause of excessive High Side pressure is restricted condenser airflow. Dirt /debris on the face of the condenser or between the cond. and radiator, or a bad fan clutch.
To check for poor airflow, hose down the condenser and watch the High Side pressure. If the pressure plummets quickly, there is an airflow issue.
At 8 years and 250K, I'd bet on the fan clutch.

 

Compressor failure would give you low, high side pressure. High and low pressures would be closer together.

 

Ok here's the situation. I took my Ex to my regular AC guy this summer for issues with not cooling at an idle and only half cooling on the highway. It was determined to be a faulty compressor as it wouldn't build pressure and the orifice valve was plugged with crap out of the compresor. So I ordered a new Ford compressor along with a NAPA dryer and VOV. Mechanic installed it all and now says that High side pressures are through the roof. Over 400 psi before the safety switch shuts it down. Mechanic says the fan starts but doesn't bring the pressures down. I don't think he flushed the rear system so am wondering if there is too much oil in the system which is causing the high pressure.

 

Def possible, the only way to be sure would be a complete drain and flush, and start over.

 

I would be leaning toward a plugged condenser. Parallel Flow condensers can't be reliably flushed and should be reolaced after a compressor failure.
Check for debris in the orifice tube again. If there's. Debris, replac,e the cond.

 

Are there any more possibilities? What else could cause high, high side pressures?

#1 Too much oil
#2 Defective cooling fan
#3 Plugged condensor

Anything else. I would just like to have a list of possibilities to present to my mechanic....