GM vats

[john spears]

All of the TPI systems that used VATS, used the 30 hz signal as was stated.
On the newer LT1 engines, a 50 hz signal is used, except on Corvettes. They
used a special data stream signal on the serial data line. These eproms are
much more difficult to use on projects, because of this. On these, you must
go into the calibration and toggle the vats/no vats option. Even the new
flash memory PCMs are using the 50 hz setup.  BTW, the early system was
officially called PASS-KEY and the 50 hz is called PASS-KEY II. I always
grab these newer ones in the junkyard for my projects that use the flash
PCMs. Sometimes the key is still in the ignition and I can just measure the
resistor pellet; otherwise I just take it back and try each of the possible
values using a decade box, (waiting 4 minutes between tries) until that 50
hz signal pops up on the oscope! Then I solder in the correct resistor. You
can build the same basic thing with a 555 chip, but I like the challenge of
figureing out the correct resistor! Plus, they are quite plentiful now that
more and more GM cars are using VATS.

        John Spears
Speartech Fuel Injection Systems