O2 ?

[lionelhutz]

Yes the Tan wire circuit 413 is a separate wire that is GROUNDED at G160 along with Circuit 450 Sensor return ECM ground
In the ECM the pin that 413 is attached too is almost always grounded at the ECM buss as well.

Why is a separate wire needed if it's just grounded to the other ground pins?

As you can see the manual for the engineering and design tool used for the creation of the ECM...

Where can I see the original design documents used for the '7747 ECM?

Here is the Chip or similar used most commonly from 1995 on (1991 on some models)
LM9044

On page one under Typical Application you see the diagram of the chip internals it is a differential amplifier followed by ground referenced OP amp.
This is the stye used from 1995 on when the chips where first made available.

That chip still has the same pin connections as the chip for the O2 in the '7747. The '7747 chip has different pin numbers on the package, because it is DIP, not PLCC.

What are you meaning with the internal description? A differential amplifier allows both input pins to float from the ground and it will still output the difference.The stage after is internal in the IC so it doesn't need to be differential. Right from the datasheet the IC will work with up to -3V on the O2 input pins.

Did that style of chip become available in 91 or 95? You're contradicting yourself.