Download the appropriate electrical troubleshooting manual (ETM) from this site:
http://www.wedophones.com/BMWManualsLead.htm
Since I haven't driven an M10 equipped car in almost 20 years, I can't offer more specific advice. On the later E30s with M20 and M42 motors, the car's computer boosts the idle when it senses the AC compressor is on.
Good luck sorting it out,
RM