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From the new FAQ:
Evaporator Replacement.
Procedure. The most difficult thing about this job is finding all the screws that need to be removed. Plan for a full day. Patience is vital, because if parts of the evaporator housing are damaged, you will have a very difficult time remounting the blower and obtaining a satisfactory seal. I paid $175 from fcpgroton for a new evaporator. The dealer will charge you about $1000-1200 to replace the evap and recharge the system.
Take appropriate safety precautions. Consider disconnecting battery for extra safety.
Properly drain refrigerant
Disconnect evaporator to drier/receiver connections in engine compartment. Be careful to use 2 wrenches to apply the proper amount of counter-torque to prevent damage. Be sure to have new o-rings on hand. Tightly plug the drier open connections immediately because moisture may damage the drier. Since I was replacing the compressor too, my parts supplier suggested a new drier. Might be something to consider with a new evap anyway.
Remove right side sill trim, side panel, panel under glove box and the glove box. Removing the dash is not really necessary. Doing this job is much easier if you remove the front passenger seat, then lay some padding down on the floor.
Disconnect control module cable (clip), remove control module (2 screws), and its mounting bracket (3 screws)
Disconnect electrical connections to blower and resistor (3 plugs). Remove 3 screws to remove the blower.
Remove approximately 9 screws around front, side and back of the evaporator housing. Note that 740's/760's/940's/960's may have different evaporator housings so the number of screws and procedure may vary. What you are trying to remove is the lower half of the duct which extends from the cylindrical portion containing the blower going to the left to a point where you may see a label for the evaporator. Coming down vertically from this point, underneath, you will see 4 circular grommet looking things. That is the other end of the lower housing you will need to remove. Being extremely patient and flexible is a vital skill. Make sure you find all the screws because they are hard to find, particularly near the firewall. I used a combination of sockets (7mm), ratchets, flexible socket driver, and bought a cheap box wrench and bent it into shape to be able to get all the screws out. When you have all the screws out, gently pry the lower half off. This may require a little bit of force, because there is very sticky mastic sealing the lower half to the upper. It may be a little difficult to tell the difference between mastic and a screw, but if you leave a screw in and break the duct, you’ll have to install some more screws to get a good seal when reinstalling the housing. I found that I had to install some new screws anyway, because various parts of the housing broke anyway, and plastic doesn’t seem to care for having screws removed and reinserted. I had to oversize some of the screws to get a good bite. [See Tips below.]
Pull out the old evaporator and install the new one. Add compressor oil to the new unit in the correct quantity. Transfer the filter and rubber seal from the old evaporator to the new. This will require cleaning the old filter and gluing on the seal. I used duct tape to keep the nuts from sliding down the pipes while installing the evaporator. Also, this will take some trial and error to get the evaporator up into the housing high enough to get the threaded connections at their proper location in the engine compartment.
Remove all of the old mastic with a flat-blade screwdriver and replace with new. It's sold as windshield sealant, and it comes in 20' rolls at AutoZone. I bought 3/8" diameter sealant and stretched it until the diameter was appropriate for the A/C cover. I also put a bead of silicone around the drip tube foam where it contacts the firewall. Replace the lower duct housing, blower, electrical connections and trim (See Tips below).
[Notes from Adam/Randy:] It is hard to get the lower housing back in place [see Tips below]. The problem is that the back of that lower cover (nearest the firewall) needs to slip over the evap, which has a pipe loop hanging at the lowest point. That pipe loop hangs right where the firewall takes a bend towards the cabin, and the proximity of that pipe and the incline of the firewall makes it nearly impossible to sneak the cover between there without breaking the plastic. One thing that helps is to reach back with one hand and press the evaporator away from the firewall a bit, since the housing catches on the tube on the bottom of the evaporator. I also cut the drain foam plug in two on the bottom and resealed it with RTV, which gives you a little more room. See the Tips below.
Reinstall evaporator cover screws. The screws are easy to strip from too much torque. It's best to watch the box while you're turning the screw. Stop turning when the cover contacts the box (as opposed to turning until the screw "feels" tight.)
Reconnect evaporator-drier/receiver connections in engine compartment. Again, be careful to use 2 wrenches to apply the proper amount of counter-torque to prevent damage. I recommend using new o-rings.
Evacuate and refill with refrigerant.
Reconnect the battery.
Enjoy cool air.
Tips For Removing and Installing the Evaporator and Plenum Cover. [Jan Kinner] Taking an evaporator out of a Volvo 940 is not an easy job!! There is ZERO clearance to get to some of the screws that need to be removed (see illustration of MCC plenum unit in right side footwell). And even when all the bolts were taken off I still broke the evaporator drain spout pulling the thing out of the car. [Editor's Note: Broken drain spouts are quite common, so be VERY careful. ] The mating surfaces are filled with sticky mastic which must be pried apart. The evaporator, once disconnected from the a/c tubes on the engine side of the firewall, came out easily enough, but the new one was as stubborn as a mule. Note that the evaporator has two tubes that extend through seals in the firewall. We finally got it through the firewall and hooked back up, but that was the easy part. The tough part was putting the evaporator cover back on -- this took almost 6 hours to do. Tip: take out all the insulating material around the drain spout that goes through the fire wall. That will at least give you enough clearance to put the cover over the evaporator. Another suggestion... everytime you get a screw back in take a break. You are laying upside down in the car working over your head in the dark...very frustrating!!!
Tips for Removal of Lower Cover. [Adam Burke] It does not appear possible to remove that lower cover without breaking it unless you first remove the bolts that hold the HVAC box in place. The evap drain extension is in the way. When you have removed the screws and you are trying to figure out "what else is holding the cover", it's wedged between that extension and the protruding evap drain. To get around this, remove two bolts at the fresh air intake and one on the firewall behind the receiver/drier. Instead of removing the exterior cowling and rain guard to get at the fresh air intake bolts, reach up through the fan shroud from below with a 10mm open/box-end combo wrench. Looking through the hole where the fan motor goes, the bolts are on the top of the metal bar, facing upwards. It's easy enough to do with the right tools and saves a LOT of time. The bottom evaporator cover comes out much easier front-end (firewall end) first. It's counter-intuitive, but there is more room to pull the drip tube clear of the firewall drain hole this way. The wrong way: If you pull the back of the box down first, the front part pivots against the bottom of the evaporator and pushes the drip tube forward against the firewall -- not what you want. Worse, this puts a load on the drip-tube area of the box, and it's likely to break at this point. (You can't see what's going on while the lid is in place, so you can't actually see the cover wedging against the evaporator.)
Tips for Installation of Lower Cover. But - if you've loosened the bolts and air intake mount that anchor the HVAC box, it probably will have dropped some. If it has dropped even 1/4", the lower cover won't fit at all: It will run into the firewall before the screw holes line up. If I had to do this again, I would first see if it would help to have an assistant push up on the evaporator while I remove (and later replace) the lower cover, although this will be difficult because you both need to be in the same place at the same time. . If that wouldn't work, then I would remove the HVAC box mounts and pull the whole thing away from the firewall as much as it could go before I would work on the lower cover. A wedge - such as a rag stuffed behind the box - might help hold it out temporarily but frankly it didn't work for me. When reinstalling if you don't have a helper, use a ratcheting strap (those flat, nylon straps with ratcheting tighteners you can get almost anywhere) wrapped around the HVAC box just to the left of the evap and anchored around the pillar between the front and rear doors. It doesn't take a lot of force, but it helps to have something holding the box away from the firewall. You also need to have the box pushed up while being held out, so place a small hydraulic bottle jack and a large, flat piece of wood under the box (about in the same location as the strap). Again, it's not force that is needed; just something that will sit there and patiently hold the box for you. To re-install the evaporator cover, put the rear in place first, then swing the front of the cover upwards. You'll still have to wrestle with the whole thing to get the drip tube back up into the firewall, but it won't be nearly as difficult as trying to put the cover on front-end first.
Cracks in the Lower Cover. [Adam Burke] I don't know of any way to repair this kind of plastic. It doesn't respond to the plastic primers and glues that I normally use. If you sand it, JB Weld epoxy will adhere, but JB Weld won't give you the strength the plastic had before it cracked. I also tried to patch it from both sides with fiberglass, but the fiberglass wouldn't stick - even to the sanded surface.
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