<VV> Idle curiosity - Air Conditioning project

[Craig Nicol]

In a culmination of a 12-year quest, I have installed (mostly aftermarket)
A/C on my '66 EFI car.  Pretty straightforward job except for the need to
custom fabricate a compressor bracket that would work around the EFI fuel
rails and injectors.  

 

The "cool" thing I wanted to share was the computer-controlled idle-up
function. GM ECMs control the idle speed through a stepper motor in the
throttle assembly.  There's an ECM input called "AC request" and the wire
from the AC thermostat is routed there instead of directly to the AC
compressor. 

 

When the ECM senses the compressor ON signal from the AC unit, it commands
the idle stepper motor open a couple of clicks and then after a very slight
delay, the ECM closes a relay to turn ON the AC compressor clutch.  With
this system active, I no longer get a 200 rpm drop in idle speed, in fact
the idle is rock-solid and remains at the same rpm with the clutch ON or
OFF.  Additional benefits: The ECM turns the AC clutch OFF at
wide-open-throttle and when the engine is overheating.   I wondered if this
feature would be present in my ECM since the donor car didn't have AC and
the ECM plugs didn't include the wiring.  I was delighted to find that all I
had to do was to install the terminals in the ECM plug; the AC functions
were included in the ECM programming and internal hardware.

 

A small victory!
Craig (cooler now) Nicol