Hi Lowrider.
This is from my notes and should help.
HVAC Temp door actuator testing
With everything connected, and the key on, the dark blue wire (which shows the position) with door at full cold should read about 4 volts. At full hot the Dark blue wire should have about .5 volt on it.
The light green and light blue wires are the power and ground to move the motor. It should show a voltage between 6 and 7.5 volts on these wires when the motor is trying to move the door position. The polarity depends on which direction the controller is trying to move the door.
Here is a link to a thread with a schematic in it for a 1994.
the 1988 system is different then an 1994. A 1988 thas a position sensor circuit for the recirc door and a 1994 does not. I would think there are other differences too.
http://www.truckforum.org/threads/ac...-silver.20021/