1999 Jetta (old body) P0506 Fault code

This code only comes up when the A/C is on. If I do not turn the A/C on the light stays out. It will never come on. As soon as I turn the A/C on and drive it a little the light comes on and this code comes up. (P0506)
Please someone let me know what can cause this.

lower idle speed than expected. does the engine speed increase when you turn on the A/C? If not there is where you problem lies

No it does not increase. What could cause it not to increase. When I turn my A/C on the rpms go down and sometimes it idles high and then low. Idle air Control motor maybe. Let me know. Thanks Travis

I would check the throttle position actuator, and the closed switch for proper gap and operation. This is assuming you have the mono motronic fuel injection (one injector)

If you have the mototronic system, which I belive you have due to saying the idle air control motor in a previous post. The first step on that system is to clean the IAC valve and the throttle plate area.
The IAC should buzz when energized and should not indicate an open circuit. The AC on signal goes to the ecu then it adjusts the amount of bypass air.

A good cleaning should fix it up. If you need pin outs for the signals or expected electrical values please let me know the engine code (AAA)?

Look up cleaning the throttle body & throttle valve.