i would be surprised if it were setup any differently than how it is on every other GM ECM:



since O2LO is tied to ground, the cap running from O2HI to O2LO after a 100 ohm resistor is a RC low-pass circuit. without knowing the value of the cap, hard to say what it allows though.

A28 should be a pulldown, not a pullup? it's possible I stuck my probe onto a 5V reference pad and saw no resistance, but I'm fairly certain I just used the gigantic ground planes to be certain. as-is, a 1K pulldown with a 0-5V signal will draw up to 5mA, I would think that would already be well within the wideband controller's capabilities? a 10K would obviously bring it down to .5mA peak.

with the pulldown being where it is in relation to the isolation resistor, it isn't going to form a voltage divider, if that's what you're thinking.

I do remember A28 being a weird setup.... as if the factory intended to use it for something but dropped it for production vehicles. the code never references using that channel on either PROM, so what exactly was going to be done with it, I don't know. considering that there was barely enough space left on the T-side to perform an A/D conversion, I get the feeling that there was either a 64KB PROM on at least the T-side at one point(I never check to see if the PROM's pin 3 trace was broken out or not), or it was being used as an engineering-specific extra a/d channel... no idea what they would have used it for though.

something I'm remembering at the moment is that for GM MAP sensors, there is supposed to be a 50 or 51K pulldown for the A/D channel, though I couldn't say why(a guess would be due to the MAP sensor itself being fairly high impedance), it's just something I'm remembering seeing on the datasheet for one of them. something to keep in mind should a baro sensor route be taken.