I believe it is the same 'effect' as when you switch on A/C ... i.e. there is more load on the alternator, and the extra 'drag' on the fan belt requires more power than the idle setting can sustain, so the idle speed drops. I would suggest cleaning both sets of battery terminals, and the ground strap connections to the body as well. Wire brush them all until bright and shiny, then apply Silicone grease before tightening all connections!
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Warren Hall
1973 911S Targa
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