The LED-FCD is a variation of the "two diodes and a trim pot" design as published elsewhere on the web. I seem to have misplaced the link to it and can no longer find it in search engines. Please send the link to me if you have it so I can link it here and give credit where credit is due. As with all sub-€1,- electronic FCD's (the most common one being the zener FCD) it bleeds some current off the MAP sensor signal above a certain cutoff voltage to hide extra boost from the ECU. This prevents fuel cut ; so beyond that point !!you're on your own!! in judging whether your pistons and the stock fuel system are capable of dealing with the extra boost or not.
There are a few advantages to doing it _this_ way.
Here's the schematic. I think it's pretty much self explanatory. Putting it all on a small piece of circuit board makes it neater and more reliable, but you could also tape it all up into the wiring loom if you really wanted to, as with the Zener FCD. I can build one for you too, if you want.
If you really want to know how it works and why I made my changes to the original design, look here. It's a long and boring read though.
The first one of these was put into Edwin's Turbo ST162. He's using a '91 MR2 Turbo 3SGTE (Twin entry CT26) in an FWD ST162 Celica chassis. He ran into fuel cut soon after rebuilding his turbo and removing the cork from his tailpipe. My version of the FCD came about when I thought I still had a bag full of diodes, but when I wanted to grab two, it turned out I only had very few left. That and curiosity made me look into how the diodes-and-pot style FCD worked anyway, and why it needed two diodes. Because I had plenty of LEDs, I thought it would be fun to see if running the bleed current thru a LED instead would be enough for the LED to light up whenever the signal was actually modified. Turns out it was!
In the meantime, this FCD has been installed on an ST165 too, along with a ball and spring boost controller. Another local ST165 will get the same "kit" soon. If you are local to me (netherlands) and want this upgrade for your 3SGTE too, contact me for details. It's the cheapest 30+ extra hp you will ever get from your 3SGTE.
Note that this one has two 100 ohm resistors. I've updated the resistor values in the schematic after my experience dialing the first one in. I've also built one in "stick" shape instead of square so it's easier to work into the wiring loom.
The LED-FCD has three wires you have to hook up. As with the zener FCD, you can install it either near the sensor in the engine bay, or in theinterior near the ECU. I recommend the latter, otherwise you will have to "weather-proof" it. Especially the trim pot is probably sensitive to moisture. Other than that, an engine bay install is pretty straightforward. All three wires can be hooked up to the MAP sensor wiring. On a 91 3SGTE the brown wire is ground (E2), the center pin is map sensor signal (PIM) and the other one is 5 volts (VC).
Near the ECU, the same three wires are available. You can tee into a ground wire, but grounding it to one of the ECU bolts is probably easier. On a 91 3SGTE, the other two are pin 1 (VC) and pin 13 (PIM) of the 16-pin ECU plug. Don't know which side of the connector is pin #1? Hint : pin #7 is missing, top row is 1 thru 8, bottom row 9 thru 16.
The trick is to raise fuel cut just enough for your needs. Baseline is with the pot turned all the way to the 5V side. Find a long stretch of quiet road to do test runs on and turn it to the E2 side a little at a time. The car I tested on needed the pot about halfway to raise fuel cut to 13 psi (from stock 12 psi). 14 psi was 1/10th turn more, and another 1/10th turn saw boost stabilize at 15 psi without fuel cut occuring. We left it that way. The trick is not to turn it too far, so fuel cut will still work. If you turn it too far, the fuel cut voltage is never reached, because the MAP sensor maxes out. That happens around 17-18 psi. If disabling fuel cut completely is what you want, you're probably better off with a Zener FCD because its sharp cutoff lets the whole signal thru up to just under cutoff point.
When you're testing and it hits fuel cut, we all know it won't let you boost again until you shut off and restart. You can do that while driving, for quick back-to-back tests. Turn the key to ACC and back to ON while still in gear. DON'T let it go into steering lock or engage the starter on the way back to ON. (You have to press a button to get the key to OFF in a Toyota, but most of us probably automatically press that button whenever they turn the key back!).
If I have to tell you what to watch out for when running (too?) much boost, you shouldn't be doing this mod. Also, never EVER do this without an accurate aftermarket boost gauge. Not only is your stock boost gauge grossly inaccurate, it is fed from the same PIM signal the FCD modifies, so the FCD will affect the boost gauge too, causing it to read low!
Disclaimer: Anything you do to your car is YOUR responsibility. I am not a Toyota engineer so I can never be 100% sure this is going to work on your car too. If it blows up in your face, I'm not responsible. I'm just reporting my OWN experience, YMMV.