I'll skip over how I diagnosed the problem, but suffice to say I figured out the problem was the expansion valve. When I went to order a new one for $25, I found that a new evaporator core was only $50, so I ordered one of those, too, just for good measure. Finally, because everyone says you should, I ordered a new receiver/dryer for about $15 and a bag full of o-rings for $3.
This post isn't meant to be a detailed how-to, but it is a "you can". This job was dead easy, except for crawling up under the dash to yank the old evaporator core out. I used this tutorial to guide my actions and it was very applicable to my Miata. The evaporator core is housed in a black plastic box, bolted to the firewall right behind the glove box, with two small 10mm nuts. On the right, it connects to the blower housing. On the left, the center air control unit. Two aluminum pipes stick out through two holes in the firewall, where they connect to some more pipes that go to the compressor and the condensor. I didn't need to mess with either of those things. I disconnected those pipes, unbolted the box, released the clamps that connect the box to the left and the right, and yanked. With a little persuasion, being careful not to damage any aluminum bits, it came out. The evaporator box is held shut with some metal clips that pop right off with a screwdriver, and two screws. The whole thing clams open and you can pull the core right out.
|The old and new evap assemblies. After I assembled the expansion valve to the core.|
|Here's the whole evaporator assembly ready to go back in. I just had to be careful sticking those aluminum tubes through the firewall, not to jack up the threads.|
|Hard to get it all in the picture but this is how the vacuum pump is hooked up. The blue line is the low pressure side, the red is high, and the yellow line is the line through which you apply vacuum, or feed refrigerant.|
So to sum up, this was an easy project that should make my '94 a lot more comfortable in the Florida summer, which is fast approaching. I commute almost two hours every day now, so having this A/C working is going to be pretty important.