There is a feature on some GM fuel injection computers that will not allow the ECM to turn on fuel delivery until it recieves a certain signal from the car's security system. This signal is sent when the system detects the correct PASSkey-encoded ignition key in the ignition. Since I no longer have the rest of the Camaro security system, and need the engine to run in a 1972 Nova, I had to defeat this system, or at least reconfigure it to do what _I_ want it to do!
Tis PASSkey II signal is a square-wave pulsed-ground. The diagram in my Haynes manual for the 1991 Camaro shows the PASSkey wire going into the ECM, this is the wire you connect the bypass circuit to. If you wish to interface an alarm to it, it's easy to let the alarm's "grounded when armed" wire ground this wire constantly, therefore preventing the ECM from seeing the signal, and keeping the engine from running, even if they have the key, the engine won't start until the alarm is disarmed. Even if they hotwire it it will not start until tha PASSkey signal is applied to the ECM. So anything that grounds out or breaks the path of the signal, or interrupts power to the oscillator will prevent the engine from starting.
On my Nova, the oscillator is in a housing about the size of a pack of smokes, with a 3 pin connector. If I am goint out of town, or feel the need for extra security, I simply unplug it and take it inside. Without this part, the car is just as useful as if it had no ECM. It is visible in the collage on the previous page, right at the bottom of the ECM, sitting at a slight angle. (middle right pict.)
*Note. This circuit DOES work. I didn't select the resistance and capacitance values for this circuit. The creator of the circuit claims it produces a 50HZ signal but one individual has argued that it may not. Whatever frequency it produces will make a 1228746 ECM start fuel delivery. I have never measured the frequency but I would darn near bet the farm that my Nova will start when I turn the key. You can adjust the resistors to adjust the frequency. Several other people have built the circuit from the diagram from this page and their cars start too.
This circuit will produce the signal to allow
General Motors ECM's with the VATS PASSkey II systems to start
fuel delivery. It produces a 50 HZ square wave 50% duty cycle
pulsed ground signal. Just get a 555 timer IC and connect it up
as follows. A small IC circuit board from RadioShack can hold the
555 and most of the other components.
16 pin DIP IC PC board
two 0.1 µF ceramic capacitors
1 µF electrolytic capacitor
3.9 K resistor
22 K resistor
100 Ohm resistor
14 Volt Zener diode
Pin 1 is Ground. Pin 8 is + input. Connect a 0.1 µF ceramic cap between these
pins, with short leads, for filtering. Otherwise it may misbehave. Connect pin
8 to 100 Ohm resistor. Connect a 14V Zener diode between pins 1 and 8, banded (+)
end to pin 8, to absorb voltage spikes from starter solenoid or system overvoltage.
Other end of 100 Ohm resistor connects to ECM's "ignition on" signal wire.
Tie pin 4 to pin 8.
Tie pin 6 to pin 2.
Add a 3.9K resistor from pin 7 to pin 8. (A 3.3K will be close enough)
Add a 22K resistor from pin 6 to pin 7. (You could adjust this one for
precision setting, but it's not needed for this application)
Add a 1 µf capacitor (electrolytic OK) + side to pin 6, - side to pin 1.
Add a 0.1µf cap from pin 5 to pin 1.
Output is on pin 3. Connect to VATS wire of ECM. Also connect
alarm to ground this pin to disable fuel delivery when armed.