Rough idle 2003 when defrost/AC on

[jim43bob]

Have 2003 Saturn 2.2 Vue.
Whenever heater is set to defrost or AC button is pushed on, engine idles erraticly.
Have removed AC clutch relay and condensor fan relay from under the hood, but does not make a diiference.
Engine idles smoothly when just set to other positions on heater control with or without the relays in(AC does not work at this time for reasons I can't determine).
Does the head unit affect idle in any way?
Something is telling the engine to adjust the idle to allow for the load of the compressor but what could cause it to do this with the relays removed?
Any help appreciated
Jim

[fdryer]

The head unit only talks to the BCM (body control module) that commands the a/c relay and notifies the ECM (engine control module) that knows to bump up the rpm anticipating the compressor load. The ECM commands the idle air control valve (IACV) to open a little more in order to allow more air - this raises the rpm. Normally the load drops rpm's and this is detected by the cps output that the ECM needs for engine rpm information. A drop automatically forces the ECM to raise the rpm's back. The ECM actually plays with timing as well as the iacv to raise or lower rpms's as necessary for whatever parameters are programmed to adjust rpm's in whatever mode is running.

Removing the relays had no effect as you found out so there's something causing the erratic idle, not the act of raising the idle to compensate for the compressor load. Perhaps a tune up is needed?

[jim43bob]

Thanks for quick reply!
Will check out cam sensor and wiring on weekend.
For some reason I thought you were refering to crank sensor (CKS) .

[fdryer]

Ignore the two lines below the three dots - they're signature/references for reminders to those having EFI or a/c problems, nothing pertaining to your particular issues. :cool:

[jim43bob]

Ok I'm a bit lost.
I now realize you weren't refering to a cam sensor(didn't realize there isn't one on the 2.2), but I don't know what the letters "cps" mean when you say "the cps output that the ECM needs for engine rpm information".
Also when this started to happen I changed just the plugs and air filter , but still idles irraticaly only when AC or defrost on.
Thanks Jimbob

[fdryer]

Sorry, the crank position sensor (cps) is the only electronic device that tells the engine computer that the engine is rotating. There are machined 'teeth' on the crankshaft that are detected by the cps and tells the ECM precise timing information as well as rpm's.