[Gmecm] IAC control??

[Cowen]

Hello! I'm trying to trouble shoot a 1988 Pontiac 2.8l
with a '727.  Problem is, the IAC retracts the pintle
for starting, and then (intermittently) doesn't extend
again.  Then the car idles at 2000 RPM. This car is
not pulling any codes - I checked a few times, and I
just get a reassuring "12". 
  If I put the ECM into diagnostic mode, the pintle
extends just fine, every time (so the IAC seems to be
OK).  If I extend the pintle, leave the IAC
disconnected, then start the car, and finally
reconnect the IAC with the car running, it runs
perfectly.  It even compensates for power steering
load.  
Question is, what inputs does the ECM need to set
idle?  Coolant temperature, power steering pressure,
and what else?  
Other notes:  the battery is crap, does the '727
increase idle speed when Bat Voltage is low? Also, the
temperature GUAGE works fine, but the temp indicator
light is always on.  If the ECM uses the same
(faulty?) sender as the indicator light, shouldn't it
pull a code?
Thanks for any suggestions!



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