Re: HVAC saga continues

["Scott Mo." <scottmo@teleport.com>]

>Nick wrote:
>The new programmer seems to do its thing just fine, but only
>after I've  driven the car for about 20 miles/minutes.  When I start the
>car cold in the morning, the system maintains whatever temperature
>setting (IE, cable/flap position) it was in when I shut it off the night
>before, regardless what temperature I punch in on the dash <SNIP>

I had this happen on my previous 86 5kT years ago, I found that if I opened
the glove box and reached in and gently pushed on the programmer servo
motor "lever" when the temp wasn't being regulated, the motor would wake up
and begin to spin and move the cable to adjust the temperature. You might
try this shortly after you start the car and see if the motor lever begins
to move.

In my case, I found the position sensing potentiometer mounted on the servo
motor was intermittent and the potentiometer (pot) internal wiper arm had
too much contact resistance and was open circuit. Vibration or some gentle
persuasion with your hand can move the pot wiper arm enough to get it
working again.

In nerdy terms, this meant the potentiometer (voltage divider) output
voltage was floating at the same voltage as the input to the op amp
(comparator) that is used to drive the motor one direction or the other. In
other words, the open circuit potentiometer confused the control head into
thinking the temp regulation lever was in the correct position even though
it was not.

My web page has some details on this problem at

http://www.teleport.com/~scottmo/AC.html#servo

The fix was detailed by the folks at General Motors and involves cleaning
the wiper contact inside the potentiometer or by adding a 680k ohm resistor
across the middle and outer terminal on the pot to make sure the control
head gets the right voltage from this pot. This resistor ensures that there
will always be a voltage read by the control head if the pot wiper gets
open circuit.
HTH
Scott Mockry

http://www.teleport.com/~scottmo