After much forum searching and discovery of several helpful how-to guides, I wanted to share a summarized write-up of the AC compressor replacement process on my 2001 C320. The guides I read made it sound fairly complicated and said you need to remove a bunch of parts like the radiator fan, drain the radiator, etc but I discovered it's easier than that.
My guide here is intended for those who have some basic knowledge of AC systems and who have the interest in looking up the proper specs so I didn't list all the specs here because they vary a bit depending on your model and year. Also this isn't intended to be authoritative on how a shop would do it...this is just the basics of getting cold air again.
Yes, you should replace your receiver/drier desiccant, and you should replace your condenser and possibly your evaporator and expansion valve, and possibly even do a complete system flush, but I didn't do those things on my car because it is 18 years old and has 300K+ miles, so I just wanted to get cold air again before the summer gets too hot.
Estimated Time: 4 Hours
Estimated Cost: $75
Difficulty Level: 2/5
1. Rent a vacuum pump (will use later) and a manifold gauge set from your local parts store and check the high & low port pressure with the engine running. If you have readings of 0 on both, then your refrigerant leaked out and you're ready to get to work. If you have greater than 0, take it to an AC shop and have them remove the refrigerant first since it's illegal to vent that to the atmosphere.
2. Remove both cold air intake tubes, and remove the serpentine belt (17mm or torx socket on your belt tensioner, depending on whether it is the newer or older version). In my case, the compressor pulley separated from the center of the compressor when it failed and was spinning freely but loosely, so I put an AC bypass serpentine belt on as a temporary fix until I could replace the compressor. The serpentine belt you need for that is "Duralast 910K6" or "6PK2310" - Kudos to Benzworld member
for that tip.
3. Disconnect the low pressure line from the top of the engine bay near the ESP module
4. Jack up the driver's side and remove the bottom engine cover (6 8mm bolts)
5. Disconnect the high pressure line on the Thermal Expansion Valve, gently pull it out while keeping the adjacent hose in place (10mm nut)
6. Place an absorbent pad/catch pan underneath the power steering pump, you're going to lose about a quart of power steering fluid
7. Disconnect the power steering pump hose (22mm) - be careful to catch the 2 copper washers that seal it
8. Remove the 3 torx head bolts holding the compressor to the engine - 2 on the front and 1 on the rear of the compressor. You'll need to slide the inner tie rod boot out of the way to be able to fit a long socket extension behind there.
9. Disconnect the electrical connector on the top of the compressor.
10. Rotate the compressor 90 degrees so the pulley is facing the passenger side, and gently maneuver the AC hoses around the power steering pump and other stuff in the vicinity. You can actually get away with bending those hoses quite a bit without damaging them.
11. Pull the AC compressor towards the passenger side and diagonally up in between the radiator fan and the engine - voila, it's out!
12. Replace O-rings on hose connections, and move old hoses to new compressor, lubricate O-rings with PAG-46 (might be able to dab some with your finger from the old compressor).
13. Fill the compressor with the right amount of PAG-46 oil and install it with reverse of steps 2 to 11.
14. Hook manifold gauge set up to the high and low ports, and connect the vacuum pump. Open the valves and pull vacuum to as low as it will go: usually between -20 to -30. Close valves, turn vacuum pump off, take photo of gauges, let sit for an hour, then compare current gauge readings to your photo to see if gauges have moved at all. If the vacuum held, turn vacuum pump back on, open valves, and pull vacuum for 45min-1hr to remove moisture from the system, then close valves and turn pump off.
15. Start engine and fill the system with the correct amount of PAG-46 oil and R-134a refrigerant. You can buy and use a can of compressed PAG-46 and R-134a blended mix. Before you open the valves, you should use the bleed valve on the manifold gauge first to get any air out of the fill line so you're only adding refrigerant to the system. Fill system to spec and use thermometer to determine coldness of air coming out of vents. Also refill your power steering fluid, probably need to turn the steering wheel left and right a bit while you top it off.
16. Glorious, you have cold air again. I did the whole job for $75 including a $20 used compressor from a pull-a-part yard, O-rings, a quart of power steering fluid from the dealership, and the cans of refrigerant and compressor oil at my local big box store. The tool rental was free at my local auto parts store after I had my deposits refunded.