89 suburban R2500 454 TBI idle does not go up with A.C. on

[GhettoBurb]

My 89 suburban R2500 with 454 TBI seems to have a problem. (It really has a 502 in it now but everything else is stock)  I have front and rear air. When I turn on my A.C. the idle does not go up. The A.C. works, compressor comes on and it blows cold air. The only problem is the idle doe snot come up. Now that I think about it, it does not come up when cold either any more. Any idea where to look first?

[GhettoBurb]

well it apears to only drop about 200 RPMs when it drops. I think it actually drops lower fo ra second then stabilizes there. I did replace the IAC about a year ago. I am wondering if it is causing me problems. I will have to test it to find out. I did unhook it then start up teh trck and it riddles way higher with it disconnected. I only let it run long enough for the idle to stabilize, but not very long, only 10-15 seconds tops. It did stabilize before that. I think i will try testing the IAC and see what happens.

[Jims86tpisub]

Get a diagram , and find out where the ac power wire(Green) splits and goes to the computer. Its a thin green wire on my TPI set up. If that wire is broken, it is not sending the signal that the A/C has been activated.

[GhettoBurb]

I'll check that out. I reset the IAC and it seems to be better. I will check that lien also, maybe I am getting a short through it. The idle does not seem to go up during warm up so I am thinking maybe the I am having problems with IAC that I need to check closer. I will check both out and see what I come up with. Thank you for your response.

[VileZambonie]

Is the engine stalling? If not then what's the problem? It's tbi so you shouldn't feel a sudden change in rpm. The iac is a stepper motor and will display it's counts on the scan tool.

[Jims86tpisub]

If you are having problems on warm up, you may need to set the(minimum air) idle and tps voltage all over again. somebody musthave messed with it.

[Stowburb]

I havn't fired my big block up lately but as best i remember it will idle up for a few mins. when its cold, then drops back to normal when it gets warmed up, but i dont think the idle changes when i turn on the AC.
 Is the SES light on? i do know that if the  coolant temp sensor( the 2 wire sensor w/ the yellow and black wires) next to the thermostat housing is bad, the ECM will not know what temp the coolant is,therefore not adjusting idle when cold.
 If you have a scan tool, you can watch the temp in the  live data. i'd check that out first.

[GhettoBurb]

The Coolant temp sensor has been recently replaced. I don't currently have access to scan tool to do any testing that way. When I unplug the IAC the RPMs raise quit a bit. When I plug it back in they are normal again.  I read some directions to reset the IAC by unhooking the battery for 15 seconds  then hooking it back up and going through a series of turning the ignition key on and off and then starting the truck. I did that but nothing changed. How would I set the minimum air idle (or is that what I already did?) and the TPS voltage? WHat should the TPS voltage be at idle? The check engine light is not on.