GM ECM: Vats

[peter paul fenske]

>Hi Gang:
>I am currently working on GM code. What I am going to do is
>set up my engine run stand to check my alterations to the 
>lookup tables. Also there is a lot of room in the proms 
>to alter some of the code for custom routines.
>There are two engines available to me. A 87 L98 with MAF and
>a 91 L98 with Map. I am inclined to go with the 91 because of
>the Map being cheaper to replace and taking less room in air
>ducting. Also a 27256 eprom is used and a lot of it is empty.
>However the 91 also has VATS(vehicle anti theft system.)
>A description of the system is in order.
>First off gm has a resistor embedded in their keys. This can
>have one of 15 resistance values. When five volts is 
>applied to that resistor one of fifteen voltage values results
>The ECM reads the voltage through the AD. On entry to start up
>this voltage is compared to a value in lookup. If it is correct
>a start enable flag bit is set. If not it is cleared. It then
>counts about two minutes before the start cycle can be initiated
>again. Thus it can take 30 minutes to start the car if someone
>taps in the steering column connector with a resistor bank. 
>Actually a bit longer since the time between efforts increases.
>I believe the newer ECMS do this directly. The older ones
>used a separate VATS module which makes life easier.
>I can still get this project going since I will just splice
>the appropriate resistor and use it to start the engine.
>What I am wondering has anyone cracked this routine.
>This is not going to help any thief since to change the
>calpak takes nearly as long and aftermarket chips that have
>defeated this are available.
>
>Anyways tnx for any help rendered:
>peter