Have you looked into your Idle Control Valve?

I'm having the same issue with my '93 109E 2.3. The cars starts and runs perfectly, except the idle drops to 500 to 550 rpms in drive with the A/C on.
This much I do know, the Idle Control Valve is not operating. When I turn the ignition on it doesn't click, when it is running and I unplug the connector, it doesn't affect the idle. If I apply voltage to the ICV, it clicks and the idle rapidly increases.
With the ignition on, I have 11.15 volts across the ICV connector. Pin number 2 (plus side) reads 11.91 volts, which according to my schematic it comes from the OVP relay. The ICV has 8.8 ohms of resistance.
I'm stumped up to this point. The ICV will operate with a 9 volt battery touching the contacts, or 12v applied from the car battery.
I have replaced all hoses going to and from the ICV and took it out and cleaned it, although it looked very clean inside.

Let me know if you find the problem with your low idle.

Thanks,

Dave

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible