More 5K Climate Control Woes

[Steve Sherman]

I have two 5KTQW, an 87 and 86, and both had similar but opposite
climate control issues.  One had the air too hot.  That turned out to be
a stripped gear in the flap control gearbox, so that the mixer flap was
stuck on the hot side.  Easy (at least it will be once I get the part).

The other one has a problem that the air started out warm, but quickly
(too quickly) turns to cold.  As though the mix was off.  Turn the temp
control to MAX works for a while.   At first I thought it was a similar
mixing flap issue, but the flap seems to be moving smoothly, and goes to
each end (hot and cold) as expected when the temp control is switched
from MAX to LO.

The only other things I can think of are:

1. Failure in the fan/temp sensor in the dash.  Hopefully not as this is
a PITA to get to.

2. Failure in the motor/gearbox/potentiameter that moves the mixing
flap.  Perhaps the flap is being moved further than then controller is
signaling.

My question for the forum is how does one go about checking out these
possibilities?  Is there a magic button sequence that will cause the
controller to display what percentage of mixer flap travel it is calling
for?  How about for a reading of what it thinks the inside temp is?

Any suggestions welcomed.

TIA