Chevrolet Repair: 1991 Chevy p/u, AC temp indicators flashing, chevy pickup, gmc truck

Question
QUESTION: I have a 1991 Chevy pickup that has the digital display for heating and AC controls.  When I try to start the heater or AC, the temperature scale cursors all begin to flash.  This lasts for 2 minutes and then they go out till I press any of the control buttons for fan speed or temperature change.  Any idea what this indicates?
Thanks for your time.
Robin

ANSWER:  There is a code stored in the temp controller that is indicating a self diagnosed problem.  You need to access the controller and it will tell you what it thinks is the problem.

wayne


---------- FOLLOW-UP ----------

QUESTION: How do I access the controller?  I was of the impression that this model does not generate codes.
Thanks.

Answer
Here is the bulletin.  This is COPIED from service manual.  Sorry for the delay, it is easier for me to cut and paste this then go from memory (which is like swiss cheese at the moment) My son's team is in playoffs right now and I am the head coach and have been trying to scope out our next rounds all-stars, have special and extra practices and manage a busy shop.  
 The Auto HVAC of those years have a fairly archiac form of self diagnosis but it is incredibly effective.  This bulletin needs to be checked first.  I have run into this at least 3 times a year usually in the spring.  We go from an average temperature of -18'c in the winter to and average of 32'c in the summer around here so I see lots of heating and cooling problems.  If you check this, the low side pressure switch and the controller I will help you further from there.

w  

Bulletin No.: 461202

Date: May, 1994

SUBJECT:
A/C CONTROL HEAD FLASHES OR ERRATIC OPERATION OF A/C CONTROLS (POLISH
TERMINALS)

MODELS:
1988-94 CHEVROLET AND GMC TRUCK C/K MODELS
1990-94 CHEVROLET AND GMC TRUCK TOPKICK/KODIAK MODELS WITH C60 OR C69 AIR
CONDITIONING

CONDITION:

A/C Control Head flashes, performs erratically or malfunctions.

CAUSE:

Light surface corrosion on the Control Head terminals.

CORRECTION:

Remove the A/C Control Head and while holding the unit with the terminal
cavity down (toward the floor) so no particles will fall inside on the
circuit boards, polish the terminals on both sides of the cavity with a lead
pencil pink eraser. Do not use an ink eraser or other abrasive such as a
small wire brush or sandpaper, etc. Doing so will remove the terminal
coating and degrade the contact surface.

Performing this procedure, in many cases, will make replacing the Control
Head unnecessary. Using this procedure in conjunction with the A/C circuit
diagnosis found in the Electrical Diagnosis Manual and especially before
replacing a Control Head for any reason is recommended.

After polishing the Control Head terminals, but before reconnecting, check
for proper terminal contact as described in the applicable electrical
diagnosis manual. If contact is good, apply dielectric grease (P/N 12345579)
to the harness connector terminals. Please do not apply grease to the
Control Head.