I posted this on TMF a few years ago when I had my boosted auto P5.. it's been a while but I may still abe able to answer some of your questions. Also, see the attached file from Flyin' Miata for details.
Note, this was a MAF and primary O2 sensor voltage clamp.
Tools needed:
10mm socket
Wire strippers
Mityvac
Multimeter
Regular screwdriver
TINY phillips screwdriver
Attached is a file from Flyin' Miata that contains the installation instructions. It's pretty straightforward but I'll go over most of it anyway.
1. The first thing I suggest is calibrating the pressure switch to click over at the right vacuum reading. This varies depending on altitude, so everyone will have different adjustments based on where they live.
If you have a boost/vacuum gauge then you can probably leave this part for later once you have the wiring hooked up and you can start the car again, but I suggest you do it now before the clamp is installed because it's easier. You'll need to get a Mityvac (O'reilly auto parts is where I got mine from) and hook it up to the pressure switch. Slowly get the Mityvac to read between 2-3 in.Hg and adjust the brass screw on the switch until you hear it click. Test it a couple more times with the Mityvac and make sure that you hear the click around 2-3 in.Hg
2. Now it's time to install the clamp. Disconnect the battery, and unplug the ECU. The ECU is located under the glove box, by the passenger's foot area. Pull back the carpet, and remove the 4 10 nuts that hold the plate. Watch your fingers, the plate is SHARP on the sides!!
Now you'll need to go follow the instructions on the last page of the attached Flyin' Miata document. There is a total of 4 wires that need to be spliced off the clamp onto the ECU. I used butt connectors and wire taps, but soldering would be the preferred method (if you know hot solder.. which I don't). Once all the wires are hooked up, you'll need to run a small vacuum line from the intake manifold to the pressure switch. Using a vacuum block will make this a lot easier (you should get one of these, because you'll probably need vacuum lines for other things as well).
3. Reconnect the battery and ECU, and start the car. Let it idle for a few minutes and get out your multimeter. Measure the voltage at the blue wire that you spliced, and confirm that it's between .1-.9 volts. Now remove the vacuum line from the switch, and observe the change in voltage. It should now be a solid number, and we want it to be .3v. If it's not, adjust the small screw inside the voltage clamp until it's at .3v, and repeat the measuring procedure several times. Attach the vacuum line and observe .1-.9v then remove the line and observe .3v.
And that's it for the voltage clamp. The hardest part of adjusting this gizmo is the pressure switch. Using a vacuum/boost gauge makes it a lot harder to precisely tune because you can't hear the click while driving, and it's hard to keep the vacuum at 2-3 in.Hg without breaking the speed limit and possibly crashing cuz you're too focused on the gauge. Get a Mityvac and use it instead!!
