Miata A/C Evaporator Replacement

It's been at least a year since the A/C in the '94 Miata blew cold. It just gradually faded out so I didn't really notice for a long time, until I realized it was really damn hot in the car all the time. I thought for a while that I was going to have to take it to an A/C repair shop, which really goes against my grain. I also knew it would be expensive. At some point, though, I had a revelation. That revelation told me that fixing the A/C was no different from fixing anything else. All I needed was the right tools. A quick check online showed me that spare parts for my '94's A/C system were pretty cheap. I wanted to do it right so I needed the proper tools - a manifold set and a vacuum pump. I already had the manifold set from a previous effort at diagnosing an A/C problem in my old truck, and an appropriate vacuum pump can be purchased at Harbor Freight for around a hundred bucks, which I did.

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 old core.

Here's the new core with the expansion valve all connected up, set into the lower half of the box. There's a temperature probe that is wrapped to the big tube.  Ok, actually, I don't know what the hell that does. I just connected it up like the old one.

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.

Here are the connection points just ahead of the firewall in the engine compartment.  They were misaligned quite badly and took some persuasion to get threaded.  Then I initially forgot to tighten the big one down all the way, which became apparent later when I tried to put a vacuum on the system.

Ahh sweet vacuum.  This is what you want to see when you run the vacuum pump. The inside numbers read vacuum in units of inHg (inches of mercury). You run it for about 5 minutes at first. Then you want to stop the pump and wait 10 minutes and hopefully your vacuum doesn't go away. If it does, there's a leak somewhere. I had a big one, but it was easy to find and fix. Then I let the pump run for a good half-hour.

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.

Here's where my running pressures ended up after adding 22 ounces of refrigerant and two ounces of oil to the system (I'm banking on there being some old oil left in there). I wanted to see more like 200 psi on the high side, so I may need to add some more R134a, or possibly my compressor isn't strong. We'll see how it cools over the next few days. It felt pretty cold just running it in the garage.

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.