Sorry if this is too long but maybe I can help just a bit......
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Jaysford: If your A/C compressor is original it was designed for use with R-12---it might not work as well with R134a. Consider replacing it---new I’ve found suitable compressors WITH clutch for $160 all over eBay. The filter/dryer and expansion valve or capillary tube should also be replaced at the same time. While somewhat expensive I’d carefully check the hoses and fittings too---they do go bad and contribute to lowered A/C efficiencty. Proper oil level is quite important as well as the vacuum operation. Most recharging systems I’ve seen work mostly on a timer and not by the actual negative pressure of the vehicle’s system. Any moisture not removed also affects efficiency. Vacuum needs to be as close to 29 inches of mercury or as close as possible----this can take longer than most mechanics are willing to wait. While I don’t know the specifics of converting an R12 vehicle to R134a someone here will certainly have the info you need.
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On the E vans with A/C there is a vacuum line controlled by the function selection switch that exits from under the footwell insert just behind the B pillar---it might be in a bundle, blue in color with a shrink-fit cap and quite possibly labeled. Most vans later than ’97 with A/C seem to have a wiring harness that allows for the heater control unit (fan speed, temp control and heat/vent/A/C/defrost) to be swapped out with the control unit intended for auxiliary heat & A/C.
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The heater core shut off value is a great idea and was once common place on many vehicles until the concept of an electronic blend door temp control replaced them. On both my ’97 and ’00 E250 cargo vans I added a rear area auxiliary heater with Tee fittings placed in the front units send and return water lines. For different reasons I also added a hand operated three-way valve which allows heated water to flow out from the engine thru the rear coil then returning to the front coil finally returning to the engine block. When rear heat is no longer needed I divert the water flow away from that coil. FWIW I’ve noticed the lines running to the rear coil is not even warm to touch when the valve is closed to the coil. Also of note there is a heavy vinyl curtain between the front seating area and cargo bay which makes each area heater that much more efficient.
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A company called The MTG Group (Limousine parts and accessories - Modern Technologies Group, Inc) have various vacuum and electric solenoids in addition to compact vacuum actuated 2-& 3- control valves. One each electric/vacuum solenoid and a control valve shipped cost me about $51---they were the only outfit I found with these two items making it easier to purchase. Worth looking into perhaps.
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Bill11012: At full speed the blower motors use the 13.2 VDC of the alternator---you can’t increase the voltage beyond the alternator output and even if you did you’d severely shorten or completely burn up anything it feeds. When the heating and A/C system is working properly the blowers work well---keep in mind things like insulation and the length of time they’re running affects the sensible or “comfort temperature” of the van’s interior. On a 100 mile trip once the interior reaches its comfort level typically blower speed or temperatures are lowered to maintain those temperatures. Instead of “cycling” heat or A/C on and off like a home thermostat find a suitable level and adjust to keep it steady.
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Sorry to yammer on------just have a bit of experience with this--hope this was hepful.
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It does work well, I just wish it could have a little more.

As hot as it gets here the A/C stays on max the whole trime you drive it. You can drive 300 miles at 70 with out a stop and never turn the A/C down.

The air is very could and gets it down to about 80 or so, I just want to make it blow harder. If the system could move more air it would be perfect.

 

Hate to disagree about moving more air----actually lowering the air velocity would give more cooling efficiency. If a higher speed blower could be used you have nothing but a fan--the A/C effect not really working to design.

My E250 with 235K miles and original compressor works just fine except on the hottest most humid days. If your's isn't up to snuff I'll look elsewhere for any "problems".

 

My 15 passenger turns into a refrigerator on trips with the front AC fan off and rear on medium, that was in my trip to Florida in July, the faster the fan blows, the less efficient, face it, you're condenser works best with slower moving air to pick up the chill.
I discussed with a guy on a valve to close off the heater core, he told me the factory may have not done this due to the way it receives the coolant, closing off the flow could possibly hamper the cooling of the engine by restricting flow through the passages. The older engines were tapped into the block where they just received coolant aside of the actual blocks flow, I'm not quite sure on the plumbing in the newer engines, and why the factory would overlook closing off the heater core from the coolant flow. I use my rear air to cool my van, the front just helps bring it down when I get in, but lately I've noticed how ineffective it is in traffic, I fear the clutch fan isn't working as it should under the hood, or my radiator I replaced the busted factory one with is not as it was advertised. I plan to relocate my transmission cooler to the spot behind my bumper and install an electric fan up front to increase cooling when at idle, but on the move, it's cold as ice.

 

What kind of outside temps are you talking about the air being so great in?

Your right about the fan being faster just making the air warmer, thats something that I never though of. At what speed does it become worse rather than better?

My A/C is much better out on the road than in traffic too.

The heater shut off is just a matter of cost. The EX and crew cab both got it and it had no negative effects.

 

I never use max, it is no better than normal high, I put a fan in my van to help circulate air too, move it around to get cool faster. As for improvement, I believe an electric fan outside, behind the grill, blowing in the fins will improve the AC far better than anything else.
I wonder if it was left out for a reason, and it'll not surface till years down the road.

 

I would like to do that fan Idea.

Have you though about how you would wire it?

 

Relay tripped by the AC clutch engagement wire, much like Electric fans are wired, usually they are switched via a heat probe in the radiator or engine block, then an always on over ride on the clutch wire, due to added need.

 

Sounds great. I think the one out of a CVPI would be the perfect size and they are easy to find.
I will measure it in the morning and see.

 

In front of the radiator you need an open fan, no shrouded housing, like those sold at the parts stores, I don't know of a factory fan like that, the police have them on some of the cars.

 

Why no shroud?

 

You're putting it in front of the radiator as a pusher, a shroud would restrict air from the factory fan pulling it in.

 

I looked and I thing the spare CVPI fan could be made to work and would work very well.

Bad news is my CVPI's fan went out when I was driving it today so the spare will be going to it rather than to this.

 

That would be the norm man.

 

I was thinking about doing the same thing. I have a 1997 with front and rear A/C. This is what I don't understand. Why does the rear A/C have a heater control valve for the heater hose and the front does not. Anyone know which hose is the right one to install a Vacuum control valve for the front heater core? Oh yes, It is a 4.6L engine