I wanted to share my experience on refreshing my a/c system and converting from R12 to R134a on my 220k mile LS400. My decision to DIY was driven by the nearly $700 the local Lexus dealer charged to "repair" my a/c last year, only to give up when they could not get it to work for longer than a day.
I began by replacing the evaporator and expansion valve. It is a labor intensive job, but definitely not out of reach for an average DIY mechanic. From there, I removed the Evaporator Pressure Regulator (sometimes referred to as a suction throttling valve) and disassembled it (six screws). After inspecting the rubber diaphragm for any tears or holes, I reassembled with PAG 46 oil and a new o-ring on the spring-loaded "piston". This o-ring is like a compression ring, so to speak. From there, I removed and sent out any rubber hoses (discharge and suction at compressor and the suction hose sections between the evaporator and the compressor) to be replaced at a local machine shop that repairs a/c hoses. I was very pleased with the quality of the work; the assemblies looked very much OEM, at a savings of over 50% versus the best online prices for new hoses. With the system this far apart, it was very easy to flush the remaining parts (ie the condenser and small miscellaneous lines) with a "flush gun", that can be purchased / rented at most auto parts houses. I then replaced the compressor with a rebuilt unit from a gentleman in Texas, who was a wealth of knowledge and provided a great product. The last new part was a new receiver-dryer. When all of the parts were ready, I reassembled the system with new HNBR (essential for R134a) o-rings, including several seals where two pieces of tubing thread together (the service manual refers to them as Brock joints) and use o-rings to eliminate leakage. After reassembling, I pulled a vacuum on the system with a small vacuum pump for about an hour and a half, to ensure that all condensables were removed from the system. The system held this vacuum for 15 minutes, meaning no leaks. I then added about 28oz-30oz of R134a to the system with a set of manifold guages, to ensure that the high and low side pressures were within spec. The center registers now deliver air at 38 degrees when the ambient temperatures are around 95 degrees.
For those interested, here is a breakdown of the expenses involved:
Parts
Compressor: $200 (1yr warranty)
Rebuilt Hoses: $150
Evaporator: $130
Expansion valve: $36
Receiver-dryer: $15
O-ring kit: $6
Parts Total: $537
Supplies
R134a: $21 (3 12oz cans)
PAG 46 oil: $9 (8oz)
Supplies Total: $30
Tools/Equipment
Manifold Gauge Set: $50 (may be rented/borrowed)
Vacuum pump: $90 (again, rented/borrowed)
Leak checker: $75 (optional, but good for piece of mind)
I happen to own these tools, because I love to work on my own projects.
Let me know if you have any questions.
Jeff