A blog by Bryan Pryor. Random thoughts on Life, Liberty, and the Pursuit of Happiness. Also contains good technical bits to help you along the way.

Tuesday, April 22, 2008

50Hz square wave generator for VATs/Passkey


My weekend project: create a circuit that would create a 50Hz square wave signal to feed into the ECM/PCM on a 1990's caprice to circumvent the vehicle anti-theft system. Solution? A couple of capacitors, resistors, and a 555 timer. Result: the car runs like a dream. More to come.
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Anonymous Steve said...

I need to build a similar square wave oscillator - do you have the values for the R1/R2/C1 components?

July 9, 2008 9:17 AM

Blogger Bryan said...

I should have done a better job of documenting this.

Here you go, a site for reading resistor codes:

R1 = 10,000 Ohms
R2 = 100,000 + 33,000 = 133,000 Ohms

Capacitors: Both are 0.1 microfarad

You are wiring the 555 timer to be astable. Here's a good tutorial on that:

With those values plugged into this calculator, this gives a 52.283 Hz wave with a 51.812% duty cycle. Hope it helps!

July 9, 2008 11:40 AM

Blogger Geoff said...

I am interested in seeing your circuit, but can't find it. I'm building a car using the 3800 supercharged engin and a 92 Buick pcm, so I'm looking at ways to bypass the vats. Sound like you may have the answer


October 5, 2009 8:13 AM

Blogger Bryan said...

Hi Geoff, I'll do some digging and find the circuit again. Just give me a day or 2...

October 6, 2009 11:22 PM

Blogger Bryan said...

Here's another good site: http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/index.htm

October 7, 2009 7:53 AM

Anonymous http://vatspasskey.com/ said...

Not sure why, but everybody around seem to keep calling "VATS Bypass" a simple resistor to install under your steering column or on the TDM's connector. But the problem with that is it will only bypass the key and the lock cylinder, while the entire core of the security system (the TDM or Theft Deterrent Module) still remains in the loop! That trick was born just to add a remote starter and made sense when those cars where all new - no more the case today. ...And guess what? It's exactly the TDM that is well known to die with the age of the car - even before her 10th birthday. That's what happened to my trusty Buick and is happening to other tens of thousands of unfortunate people in North America. Lots of people are even junking their cars since the time GM discontinued the production of the TDM back in Jan 2009.

April 19, 2010 5:09 AM


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