I recently had to have my pcm replaced (twice, it's been in the shop for 3 MONTHS, two dealers refused to look at it, the symptoms were wacky) Anyhow, I've got it back and all seems well except the rear ac blows hot now. The front is fine. Is this coincidence or is there something related to the pcm?
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Speaking from experience with 1999...
I don't think it's related. As you may know, the rear shares the flow of refrigerant with the front, with one compressor for both. However, the rear has its own evaporator core and thermal expansion valve.
Also, the radiator coolant flows through the same AC evaporator in the back, I think vacuum controls the flow -- on my '99 2500 van it's either full on or full off-- you can't mix them to control temp in the back like you can the front.
Maybe something got messed up and your getting hot coolant flowing through it? Or maybe the expansion valve. I am not sure how to diagnose the rear expansion valve being bad, but maybe you could check the temperature of the lines from below the floor -- feel the AC liquid line (the small one)-it should be hot vs. the AC suction line (the big one) - it should be cold. And the two coolant lines should both be cool because they are not supposed to be flowing unless the heater is on.
Also -- take the cover off the blower and be sure there's no air blockage and be sure it all cleaned out. There's a filter that picks up dust and dirt.
Do you know where the vac cutoff is for the rear? I have located a cut off for the front and can see it working. There is a vac line to a ? in the rear heater hose but it doesn't look like a cut off, maybe it is? The vacuum to that line is constant, heat on or off. I'm going to check the A/C lines but I'm rather sure the coolant lines are not getting cut off. Thanks for the heads up Lee.
I can't remember where that valve is, but it does seem like I remember taking the cover off mine in the back and checking its operation. If yours is like mine -- I have two removable bench seats and I take the bench seats out. Then there's three screws holding the cover. Lift it off and the evaporator is revealed. The hot-water valve may be there.
I think I was curious one time to see how it worked in conjuntion with the dash controls. But there should also be a foam piece there that can be cleaned to improve air flow.
It's a conversion van but the system looks as though it is the same just covered with plywood and cloth. I've got the cover off and there are definitely no vacuum or cutoffs in the back, just several relays. Based on your first post I went back and checked the system over and believe that the 2000 is on or off nothing in between. I have confirmed that the a/c is working and that the coolant flow is not being cut off. I believe that the switch must be the one that doesn't look like a switch as it is the only thing in line with the rear heater hose and it has a vacuum line. Now I just need to find the time to figure out if the switch is bad or if it is not being told to cut off. Thanks for your input!
If it's a coolant cutoff issue, then, does the rear AC get cold at first and then get warm as the engine temp rises?
I had a subscription to AllData ($20 / year) and that helped a lot when I replaced the front evaporator. However I let the subscription lapse and have not re-upped it because this forum is so great. Also, the other guy here, Stev, I think speaks highly of the Haynes manual for vans. He sent me the manual info -- check with a search in this Van forum. I have not purchased it yet, because I have no van-specific problems -- knock on wood.
I got to pull out the two bench seats to access the rear AC/HTR unit on my 12-passenger 2001 Ram Van. The Hayes Repair Manual is somewhat lame on the rear HVAC unit. [>:] I have a rear refrigerant leak on the AC side for the unit. It's in the fitting connection under the floor body. It could be a $2 dollar o-ring that has gone bad.
In some of the other forums I've read for this rear HVAC unit, the AC is either on or off with the front system. However, I'm not all sure about that since your front is cold and your rear is hot. The rear heat is controled by an inline water-valve to the system. Maybe the water-valve is stuck to the open position?
Twice a year I have to pull the seats out, take the trim off and clean the unit. All kinds of stuff gets sucked into the unit and out the vents.
Chump and others who frequent this and other Dodge forum sites have Dodge complete service manuals. For the price, it's worth getting one. And if you sell the Ram Van down the road, you could always resell it again for a better price as things become harder to get years later.
I had some oil leaking in the same place (under the floor coming into the rear AC evaporator), too. I was going to make a $2 o-ring repair, but the corrosion between the dissimilar metals and my heavy-handedness tore the tubing (on the larger one the the "suction" side). It's a $144 part 
If I had it do over, I would squirt the PB Blaster on it good daily for a week before trying to turn the bolts.