When I turn either my AC on or my defrost.....my car idles up and down....when neither are turned on..it's fine. Is that a normal thing....or is something messed up?

 

Part normal, part not.

The A/C compressor comes on in Defrost. That part is normal.

The idle should go up slightly when the A/C turns on. That part is also normal.

When the pressure in the low side of the A/C system drops below a point at which the temp would be low enough to freeze up the evaporator, the low pressure cutoff switch nouted on the accumulator is supposed to open the circuit to the compressor. When the system is calling for the compressor to be on, the idle kicker solenoid is always energized, whether the compressor is running or not. So it's normal that the idle should go up when the cutoff switch cuts off the compressor.

If there's enough Freon in the system for the A/C to come on, but not enough to be completely full, the low-pressure switch will cycle the compressor off after it's been on for a few seconds and the suction-side pressure has dropped as a result. So that's normal.

The behavior you describe is normal for the circumstances. The only thing that's wrong is that the system needs more Freon.

 

Even when it's on the defrost though? The air is ice cold and the defrost works fine....it's just that that the idle goes up and down. Let's say a system had no freon in it.....what would happen then? My brothers Z28 has no freon and it works fine. My other friends 91 RS has little freon, his air isn't that cold, but his idle dosen't kick down and up when he has his air or defrost on.

 

Watch the compressor. Is the clutch kicking on and off--my guess is that it is. Defrost or A/C, the compressor and low switch don't care. With no freon the compressor should never kick-on. With low freon it can kick on and off IF the freon is low enough. The air can be plenty cold even if it's low enough to kick in and out. Bottom line is that if the compressor is kicking on and off, it's probably just low on freon.

 

That's all good info, except that you don't have a high idle solenoid. In your car, the ECM will raise the target idle RPM by about 50 RPM when the compressor request input is active. The ECM will try to maintain an idle 50 RPM above the normal curb idle by regulating the IAC pintle position. There is a little lag in the system, so if the compressor cycles off from a low refrigerant charge, there will be a tempprary increase in idle RPM, then a temporary drop as the compressor cycles back on and the ECM tries to compensate via the IAC.