Idle Speed "surging" When A/c Is On

[gabbypig]

I have a 2002 Sierra Denali with a problem idle. Only when I have the A/C on, the engine will idle erratically with a rythmic surge. This happens about 25% of the time and usually when I have started the truck recently. Does not seem related to engine temp. and will do it in gear or in park.

 

I have already changed out the evaportor and compressor (for differnent reasons) but still get the surging idle.

 

Is there an idle stepper motor that has gone bad?

 

Any ideas?

 

Thanks in advance.