This is a popular mod in the VW forums and I should have done a search first, but what the heck! The purpose is to limit the amount of boost the ecu sees by limiting the voltage using a Zener diode to the equivalent of 5, 11, or 17 psi in order to avoid soft limp.

I should have taken pictures of my install, but here is what I did.

1. I bought two sets of male and female power connectors from Radio Shack (part nos. 274-234 and 274-224). These are PC power connectors that are used for disk drives.

2. I took three wires cut to about 6" and stripped .25" off each end (to attach the connector terminals).

3. I stripped (very carefully with a razor blade) about 1/2" of isulation from TWO of the wires about 2" from one end.

4. I then wrapped the bare wire around the wire from the diode and soldered it. I did that to the two wires that I removed the insulation from. However, I flipped one of the wires around so that its 1/2" strip was opposite the other. This is so that I didn't have to bend the diode wires and let me keep things kinda straight.

5. I noted which wire attached to the end of the diode with the stripe, which wire attached to the other end, and the third wire with no diode attached. I then put heat shrink over everything.

6. I then terminated one end of my rig with the male terminals and the other with the female. NOTE: DO NOT ATTACH THE CONNECTOR HOUSINGS AT THIS TIME.

7. I then cut my MAP wires about 6" from the connector, stripped 1/4" off the cut ends of the harness and terminated them with the male and female terminals. NOTE: DO NOT ATTACH THE CONNECTOR HOUSINGS AT THIS TIME.

8. On a diode (which looks like a little barrel with a wire throught the middle), there is a stripe around one end. There are three wires going to the MAP; GREY/BLUE, BLACK, and WHITE/BLACK. You want the end of the diode with the stripe to go to the GREY/BLUE wire and the other end of the diode to go to the BLACK wire.

9. This is where you test your setup. Temporarily attach the wire from your diode rig with the striped end of the diode to the purple/grey wire of the MAP; attach the wire connected to the other end of the diode to the brown/black wire of the MAP; and attach the third wire to the purple/green wire of the MAP.

10. Start the car and let her run a little while; even rev the engine. I got a CEL almost immediately because I had the diode hooked up wrong. If you get a CEL, switch which diode wire goes to which MAP wire.

11. Once you've checked everything is working correctly, attach the connector housings by pushing the male and female pins in from the rear. Make sure, however that you use the right housing so that you have MALE (MAP wire from ECU) to FEMALE (diode rig) to MALE (diode rig) to FEMALE (MAP wire to MAP sensor).

8. Now I have a "Diode Rig" that I can install and remove at will.

Below are some pictures of the final. I need to take some of the assembly process next time.

Now, for a little electronics. A diode is like an electronic check valve, letting current flow in only one direction. Anytime you have two or more electronic devices hooked up in parallel (the share the same voltage source and ground), the voltage across them is the same. This mod artificially sets the voltage the ECU sees from the MAP to be the voltage of the Zener diode. What I've determined is that a 3.9v diode equals 5psi, 4.3v diode equals 11psi, and 4.7v diode equals 17psi.

Lastly, the ususal disclaimer is that you do this at your own risk.

Good luck.


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and it sounds like you made the $5 ebay "chip"

 

what about adding fuel?

 

no it doesnt add power it just keeps the car from going in to limp mode when you over boost with a chip and n75

 

Well, here's the other part of the story:

I'm not suggesting this is better than chipping. It's just that a certain brand of software that I acquired when it was still in beta doesn't work on my A4. I've never been able to boost over 12 psi. Of course, the software manufacturer says it's anything but their software.
I've had my car in the shop where I got the software three or four times now. They have tried everything to prove it's a mechanical problem. They've smoke tested the car for boost leaks. They've reloaded the software. They've tested my wastegate. Still, the software people think it can't be their software.

So, to do some diagnostics of my own, I've run vag-com logs about a dozen times under different situations. Never has the specified (requested) boost gone above 1750 millibar (that's about stock boost). I've used a MBC to show that I can hit 17 psi and hold it under load until going limp. I just had my car back in the shop on Friday and they flashed the ECU with the stock program to run more tests.

Well, I did this diode mod to show that mechanically I can hit and hold at 14 to 17 psi. Because this is on the stock program, I don't want to run her too hard, even though I do have a 5 bar fpr in the car right now. This mod allows me to hit 17 psi without going limp. I'm very tempted to ask for my money back and go with GIAC, APR or even Neuspeed.

I'm going to give this certain software company about one more try and then quit wasting my time.

 

yea you should have went with giac in the first place, lol

 

what about running a mbc and not using the n75 valve to do boost

 

Right now I have a 5 bar fpr in the car. That adds about 12% increase in fuel flow. I am driving cautiously and am considering a rising rate fpr. The thing is this car hits 17 to 18 psi and is a smooth as stock. I'm just afraid to run her hard.

 

The N75 seems a little smoother. I did have an MBC in the car and the boost was a little spikey.