here is a good write up from my buddy David, he's *my* source of info for the SOHC swap
"The reason I did things the way I did, was because there was no way a service technician was going to be able to correlate the 180SX, and two versions of the 240SX "if" things didn't work. I can hear it now, "you have a bad condensor fan," or something along those lines. Heck, I worked aviation electroncis for over 20 years, then electronics for another five years.
And how in the heck does one get custom lines made? One of the lines that "should" be built is the high side coming from compressor to condenser. On the SOHC, the condenser is in front of the radiator, but behind the front mount intercooler. (FMIC) The front bumper and FMIC have to be removed to even get to that line. So does one take the car to a shop, then have the technican remove all that stuff? Not on our car. No way. Can't drive it with that stuff removed, so the only option would have been to tow it to a shop to have the line made, or buy the tools and make it myself. I chose a third option, which was modifying the line. The low side line goes from the compressor, behind the engine, to the firewall connection. That line can more easily be made with the car in an operational condition.
To modify both of these lines, I cut off the extra mounting hole and flange from the lines. SOHC compressor has two mounting bolts per line, DOHC has one. But the lines WILL fit without having the flanges cut, but there is little room to deal with that extra metal down there. I spent a long time getting the lines to seal, but with a large box of o rings, and using my lips (no jokes please)
I was able to get it to seal finally.
The SOHC compressor plug does not fit the DOHC compressor. In my case, I solder spliced the SOHC receptical to the DOHC compressor. I used heat shrink tubing on it. That allowed the stock SOHC plug to fit. I could have done a million different things to make that work, it's only one wire after all.
I purchased a new OEM SOHC receiver/dryer. SOHC because I was using SOHC lines and didn't want fitment issues there too. One of the few parts I could find. I went through two condensor fans before getting one to work. However an inexpensive electric 12" fan would have worked just fine. On the original SOHC configuration, the condensor fan is controlled by a thermoswitch located in the lower radiator hose. (KA) Obviously, the SR20 doesn't have this, so there needs to be a way in which to have the fan turn off and on based on temperature. In my case, I already installed a temperature switch at the top of the head in a special housing for the radiator electric fan system we installed. I "could" use that, but I worry about running too much current through it. So the answer for that would be to install a second relay to operate the condensor fan, just like the radiator fans are configured.
Or, I could get an additional temperature switch and special hose and use that without a relay. What I have so far, is the condensor fan jumped, (shorted) so that any time the AC is turned on, the fan turns on. It's very hot here, (97 today) so for around town, that works. But on the highway, I would worry about the airstream loading the condensor fan and burning it out. So eventually this needs to be controlled via temperature.
Everything else I did is pretty much out there...ECU pinouts were spot on, except that everywhere mentions pin 10 as being for AC. It isn't listed as doing anything on the 180 schematic, and it isn't doing anything on our car. Just pin 106 from the AC relay which supplies a ground for the relay, and 41 to the control box from plug F3 located above the passenger kick panel.
That's about it. As long as the swap was done correctly, the idle-up and compressor cutout (accelleration) should all work. In our case, we found that the ECU swap consisted of wires twisted together and taped. We solder-spliced and covered with heat shrink. But the wiring was ran correctly despite the crappy splice job. I went ahead and got my own little R134 fill kit, and four cans of 134. I didn't vacuum it, didn't do anything "proper" but wanted to make sure it all worked before taking to a shop and getting the vacuum done, and exact pressure and oil added. That's next."