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So, I hesitate to spend an extra $50 on a missing link for my turbo 2gnt since I plan to megasquirt it in the next few months... that $50 will cover both IAT and CLT sensors I need for MS!
The other Fuel cut defenders people sell are nice, but also run ~$50 shipped.
Since I just needed to clamp the voltage at around 4.7v so the ECU does not see boost, I wanted to try out the zener diode method I had seen online.
The problem I saw is that most people just bias a diode between the MAP output signal, and ground. (Flat part of the Diode symbol pointing toward signal, not grond)
MAP Signal --- |< --- GND
The problem with that method is that it assumes the MAP output is high impedance and is not capable of putting out enough current to blow the diode. I think in practice this is okay, but using an unknown current supply (my variable power supply), the amount of current I allowed at 5v made massive swings in actual voltage at the diode output. Over a certain current (near the rated current of the diode), the diode started getting hot.
To keep that from happening, I calculated the current handling of the diode I had, and it looked to work best around 50-120mA of current. So, I did the little bit of math (V=IR) to find out I needed about 10 ohms of resistance. I used a 15 turn Pot to dial it in, because I planned on testing other resistances. The higher the current through the diode, the less the output voltage 'tracked' the input voltage.
I wish I had some scope shots of a voltage sweep, but my scope broke 
So, essentially what I eneded up with is a circuit that intercepts the MAP output, runs it through a 10ohm resistor, and then to the diode:
(grn/yl wire) (resistor) (diode) (blk wire)
MAP Signal -------10 OHM------ |< ------ GND
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Map Output -> |
This limits the voltage to 4.7v, and track the input pretty close through the rest of the range.
I've verified that it tracks, and clamps, but I have not really run the motor with it yet. Of course, the active methods using an op amp and analog comparator are much better, but this should do in a pinch, and keep from over currenting the MAP with a direct short to ground like other diode based DIY FCDs do.
Anyway, feedback is welcome on this, I just saw no mention of a load limiting resistor in other designs, and not much mention of any DIY setups at all.
The diode I used is a 1N4732A 4.7v 1W @ $0.31 ea.
Any 10 Ohm resistor should work okay if it is big enough.
For a 4.3v clamp, a 1N4732A works.
If you want the FCD selectable for any reason (enable/disable), simply put a switch between the diode and ground. Open switch = stock operation.
Also, I ran out of heat shrink, so don't judge
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