My Toyota e80 sedan 4 doors having problem when the idling speed was set at 900 rpm. The engine will vibrates when air conditioning turns on and the engine rpm drops to 700 rpm. I need to depress the throttle pedal until engine rpm above 1000 so to avoid engine stalled at this point. A faulty idle speed control actuator can't function properly to self adjust the idle speed above 1000 rpm when air conditioning turned to maximum level. Kindly advise if can replace the faulty actuator or it is common problem for this type of old sedan car.
I'll take a swing at this ...
The engine in your car, whether it's the Single Overhead Cam (SOHC) or Double Overhead Cam (DOHC) version, has a vacuum actuated (non-electronic) idle air control valve. It looks something like this:
Since it is vacuum operated, I'd suggest you check the vacuum lines to ensure they are in good order (check for cracks, tears, and ensure they are in place).
If these check out okay, the next thing I'd do is clean it. This might require you taking it off the engine to do so. There'll be a gasket on the back side where it connects to the engine, so it would need to be replaced to do this.
If that doesn't work, track the vacuum lines back to their source. There should be some type of vacuum switching unit attached to them. Ensure the proper operation of the switching unit. (Note: I couldn't find a switching unit when looking for parts for your vehicle ... that the engine has a switching unit is an assumption on my part.) I don't have a clue how you'd check the operation of the switching unit, but would assume it's electrically operated and should be energized by a 12v power source. Also check the vacuum lines from it to ensure they are in good working order.
If all of this stuff looks good, ensure the vehicle is tuned properly, with fresh O2 sensors, spark plugs, wires, and distributor cap/rotor. If the engine is getting bogged down under the strain of the A/C, it may just need a tune up for it to maintain its engine speed.