Depending upon what you have, you will need to run either one or two wires to the new PCM.
The following are for Suburbans without Auto Trac

Circuit 1694, gray/black wire, pin 23 of C-4 connector
Circuit 1695, black/white wire, pin 19 of C-4 connector

The following is for Auto Trac

Circuit 1694, gray/black wire, pin 23 of C-4 connector


Circuit 1695, Black/white wire, is the front axle switch wire and goes to pin 19 of the C-2 connnetor(red or green, depending on year) of the new PCM
Circuit 1694, gray/black wire, is the 4 low signal wire and goes to pin 16 of the C-2 connector.