While it may look the same, there are several differences between the R12 and R134a evap assy.
Because it looks identical and the fitting is the same, both type will fit on any NSX apart from the difference between the LHD and RHD so in order to distinguish the difference, Honda placed small sticker on the R134a spec parts.
As Shawn mentioned, it's the purple one.
You can find the same sticker on some of the pipes, expansion valve, receiver, hose, etc on R134a system.
The o-ring material, operating pressure, etc are different between the two system and thus, the expansion valve in the above photo and the two o-rings (hidden under the couplings) are different between the R12 and R134a evap assy.
Hence, the different part no..
Even with these differences, when you retro fit the R134a conversion kit, the A/C will work even using the old used R12 spec o-ring, expansion valve, evap, compressor, drier, pressure switch, etc after the conversion.
Although the o-ring is not compatible between the two, when the old spec o-ring gets in contact with the R134a, it will first expand (swell) and then slowly deteriorate over years so seemed to allow some time before start leaking.
The chemical used inside the R134a drier is more efficient than the R12 one but the diameter of the OEM R134a drier is smaller than the R12 one so if you want to install it properly like from the factory, you will need to change the bracket although you can do quick DIY modification.
Again, even after the conversion, you can still use the R12 drier although strictly speaking, probably better to switch to the correct spec next time when disconnecting any of the fittings.
Having said this, some of the owners are on-purposely using the R12 drier even after the conversion or even on the R134a system for the extra capacity of R12 spec drier.
If the system is sealed and operating properly, the A/C gas should not leak for many years.
However, eventually it will leak at slow rate from the o-ring and corrosion at the fitting thread.
When someone re-charges the gas, sometimes people could over charge the system. This will shorten the life of the system especially the evaporator on our NSX and it will cause puncture.
When R12 drier is in use, it seems that this extra capacity can act as a buffer in preventing the puncture at the evapo or other issues related to overcharging of the gas.
As a side note, after converting the system to R134a, it seems that the original R12 compressor tends to fail prematurely and starts making some noise. You better act at that point otherwise the metal dust from the broken compressor will travel inside the pipe and it could cause partial blockage at the R & L condensors as they are connected prior to the gas filtered at the drier.
For this reason and considering the age of any R12 compressor still in use on early model NSX, it may save your time and money by replacing the compressor to the later spec R134a one (P9K) at the time of conversion.
From efficiency point of view, R12 is better than the R134a and thus, it won't loose too much eng power when the a/c is in use on R12 system. One of the reason why some owners stays with R12 even when the gas is so expensive these days.
Even for the R134a system, the later spec R134a compressor (P9K) is much better than the early spec one (PR7) that you won't notice too much drop in eng power when the compressor has engaged.
Kaz
