When I turn my a/c on the truck cycles back and forth from about 500 to 700 rpm at idle. As soon as I turn it off it idles fine. Any ideas?

 

your compressor turning on and off

 

Welcome to normal operation. It freaks you out, though, when you forget to turn off the A/C before you replace a cam, then fire up the motor for the first time...

 

It has never done it before though...and it's not like it does it every few minutes or whatever...it cycles back and forth every 2 or 3 seconds.

 

Ok found a thread with a similar problem....but I just put a new IAC in....do they have to be set at a certain position when installed? Or can you put them in at any position and the computer will adjust to where it needs to be?
https://ls1tech.com/forums/showthrea...iac+idle+a%2Fc

 

I know that Bryan at PCMforLess set's the idle higher when the AC's on...mine does the same thing. The ac compressor puts a big load on the engine and the higher idle helps to keep it from stalling.

 

OK got a vid of what it's doing, there is no way that this is normal. This is with a/c on and one of the two e-fans running on high. Quicktime, 9 megs. I tried to record the voltage dropping from 14.2 to about 13.6 also on the Passport, but it got weird for some reason. Does this look like a problem with the IAC or something else?

http://www.public.asu.edu/~cwessel/DSCN1246.MOV

 

Link's no good.

 

its working for me...

That doesnt seem right at all... alternator problem maybe? Are the interior lights dimming when it drops in idle?

 

Its not the alt...as soon as I turn a/c off everything is fine. If I bring it up off of idle a little bit it's fine. The voltage is dropping because the rpm's are dropping below normal idle rpm...the pulley size on my alt keeps the voltage at 14.2 at normal idle, but any below that and it starts to drop off quickly.

I was reading Handzel's Chevy LS1/LS6 V-8s book and just found a deal on the 2nd to last page.

The engine idle goes up and down for no reason

This is usually a combination of the timing being too far advanced, the IAC correction speed being too fast or slow, and the throttle blade being too far open when the computer thinks it's at rest. You can fix this by:

A. Setting the timing
B. Setting the fuel pressure
C. Setting the throttle blade position
D. Resetting the IAC adjustment - This is a simple electronic adjustment in the IAC calibration table
E. Resetting the IAC Opening - At idle with the engine warmed up, the IAC needs to be somewhere between 15 to 25 percent open. That's so what the engine is cold, there's adjustment left for the IAC to open more to let more air in the enginer. Set the throttle blade when the engine is fully warmed up so the IAC is in the right place.

I'm pretty sure at this point it has to be a problem with the IAC....but I don't know what to do to fix it. Where should it be set to when the engine is off? That's the first thing I want to look at, and if it is set right I guess I'll just have to go buy HPT or something and mess around with the IAC values.