What does the vertical switch on the rear of the AutoProm do?
The vertical switch on the backplane of the AutoProm changes the resistance between pins A and B on the ALDL interface cable. The topmost position places 10k ohms between the pins. Some ECMs require this 10k resistance in order to connect to the ALDL datastream (1227165 is an example). The middle position opens the connection between A and B (infinite resistence). This is the “default” position. Most cars can be datalogged in this position. Cars that require 10k to connect can be switched to this position after connection. The bottom most position shorts the connection (0 ohms). Most GM ECMs dump trouble codes when pins A and B are shorted. This switch position makes doing so simple and convenient when a PC is not available.
