I saw that some others had issues with using ATX power supplies.  I wanted to share how I approached using one for a bench rig I made today, adjusting the 12V output to exactly +12.5V and wiring in some helpful switches.

WARNING: Like just about everything else on this forum, doing this involves modding things in ways that could be dangerous, stupid, or both.  Be careful with electricity and attempt this entirely at your own risk!

When I first cracked the PS open and started testing the various voltages, I found that the 10 amp +12V output was really only putting out +11.4V.  The lower amp -12V line was also reading low and since my PS only had the -12V line rated at .8 amps, I didn't want to use it either.

Google to the rescue!  I rejected a few mods that combined the +12V and -12V lines for 24V and then allowed the voltage to be lowered from there because the .8 amp issue could still be a problem.

I knew that one of the chips on the PS board would control the output.  As it turns out, my PS uses a TL494CN controller chip.  I googled this and found the following link:

http://older.4hv.org/index.php?board=3;action=display;threadid=22578

Using the "Mod #2" technique at the bottom of page one of this thread, I was able to solder in a resistor and potentiometer that allowed me to precisely raise the voltage, watching the output with a digital multimeter, until it hit 12.5V.  (If I tried to turn it up much higher, the built-in safety features of the PS caused it to shut down.)  My higher amp lines are now running very stable at 12.5V.  Your controller chip may be different but if you search on what ever yours uses, you will likely find some similar hacks.

I then wired three switches into the rig:

Switch one:  "master power" that brings +12.5V to the OBD2 connection, to ECU pins 62 and 21, and to switch two.   
Switch two:  "Ignition Switch Emulator" puts +12.5V to ECU pin 3, booting the ECU separately from the OBD2
Switch three:  "Boot Mode Select" grounds out pin 24 to enter boot mode.  (Currently through a 4.7Kohm resistor as per another user's suggestion. Still waiting for my G-1260 so I haven't tested this yet.)

I ended up finding a use for the -12V line anyway.  I had some 12V "automotive" LED's from Radio Shack.  The switches I used were DPDT so I rigged up the LED's to be switched on via the other switch poles using the low amp, isolated -12V. 

Now I have a visual indicator of what's powered up and whether or not the boot mode pin is shorted!  I'll be mounting the switches and the LED's in the cover of the PS.  I had a spare engine wiring harness which provided an ECU connector and my female OBD2 connector is on it's way. 

This was a nice little indoor project for a day when it was just too damn hot to do anything in my garage!  As a complete tuning noob, I'm looking forward to using my new rig and all the helpful info on this site to learn how to tune my car myself.   

Bit of an FYI here.

I've just built myself a bench flashing harness, and have been chasing down "boot mode inactive" error messages for a couple of days.

I knew the ECU was going into boot mode, as the ME7_EEPROM tool worked in bootmode, and VCDS stopped connecting when boot mode was enabled. I assumed the galletto lead was faulty and ordered another, but same issue.

After much puzzling, i noticed a thread where another member was having flashing issues and fixed it by dropping the voltage.

I'd built my harness using a 14v 4A switched mode PSU, as people seemed to suggest connecting a battery charger while flashing, and most battery chargers output between 13.8 and 14.2v. After a rummage i found a 12v 2A PSU and quickly made some temporary connections to it and tried it out, and it worked perfectly.

So it seems that 14v is too much for a Galletto lead, and yet 12v, which most people say isnt enough, worked perfectly.

I just stumbled over this. I do have a MPPSv12 which works fine in a S3 8L APY (incar).
Now i did some tests on the bench and build a wire without pin21 because most users say its not necessary. Vagcom worked for me, ME7_95040 tool did his job too.
So i am able to go into boot mode but MPPS and Galletto wont work with or without boot mode.

Now i added a cable for pin21 and put it to +12 (always on). Nothing changed
Maybe its necessary to feed it with ignition +12 via switch?

I do not really understand why this pin 21 is necessary. MPPS told me to pull out the dash fuse. So no Immo here, like on the bench.

Will try with only 12v tomorrow

Worked very well with 10-11volt at the moment!

Those cables are complete ripoff.
Go to a breaker, cut off the plug from a million looms there (I got my plug for 2 EUR).
It will probably take you an hour or so to make the cable... ordering it online for stupid money on some obscure website will also take a while, and then you have to wait for it.



Cost about 5-10 EUR in materials to make...

Looks nice and would save me a trip to the scrapyard
Where can i get one??
I'm UK based

Nothing to write, it's only K-Line and switch on power.