...the ECU uses the ABS speed sensor to determine vehicle speed, then correlates this with engine RPM to figure out the selected gear. Your best bet is to mess with the vehicle speed sensor, as the engine speed sensor input is necessary for proper operation of the engine.

The vehicle speed signal is a square wave that varies between 0V and B+ 29 times per wheel revolution. The speed sensor is connected to the ECU at pin 39 (a pink/black wire) and grounded at pin 25. You can't simply disconnect this wire as it will cause the ECU to default to low boost in all gears.

Instead, you need to trick the ECU. A small circuit would very easily do this. All you need to do is create a battery voltage square wave at the right frequency. By my rough calculations (check these numbers, no guarantee of accuracy), for a car with 205/50/16 tires, your fake square wave at a frequency of 464Hz would make the ECU think it's always going ~70mph.

There is a possibility that this modification would cause boost adaptation problems, but I can't be sure. Other than that, it should have no other bad effects. From what I can tell, the speedometer/odometer are not connected through the ECU. Don't take everything I've said here as truth, do make sure you check and recheck the information before trying anything. I won't be held responsible for a burned out ECU!

For a much simpler fix that requires no soldering or hacking of the ECU wiring harness, try a manual boost controller. It will give similar results, although it cannot defeat the shift up light.

posted by 207.225.42.208

• 1+2nd boost < 3,4,5th. Rewire ECU to always think to be in 5th?, Eric, Sat, 4 Aug 2001 14:47:17
  • You will get 90% of this effect with a series MBC n/m, SteveW, Fri, 30 Jul 2004 17:22:43
  • Good idea... but is it safe ?, Newbe 900, Fri, 30 Jul 2004 15:11:08
  • Well..., LDH, Sat, 4 Aug 2001 15:56:08 <-- Viewing This Message
    • How do I build a circuit to fool computer? LDH Help!!!, Eric, Wed, 29 Aug 2001 13:15:02
    • Re: Well..., eric, Sun, 5 Aug 2001 15:54:33
      • Cruise control, etc..., LDH, Mon, 6 Aug 2001 04:26:00
    • Unfortunately, no..., LDH, Sat, 4 Aug 2001 18:15:53
      • Oops...this post RE: steveC's, LDH, Sat, 4 Aug 2001 18:17:53
    • isn't there a fuse for the shift up light? There was in my '88 nm, steveCi nBC, Sat, 4 Aug 2001 17:11:18
  • Re: 1+2nd boost < 3,4,5th. Rewire ECU to always think to be in 5th?, Brad, Sat, 4 Aug 2001 15:31:16

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