Blower operation makes idle drop

2006 Toyota Corolla , 100k, manual transmission. Good maintenance, I know the car. The owner told me that the use of the highest setting of the blower motor will cause the engine idle to drop. I stopped-by his place and he started it up, engine was already warm due to recent use, he then turned on the heat, went through the blower settings. I watched the tach and saw the engine rpm drop by almost 300 when he turned it to high. Recent battery and cables (Summer 2014). Any comments? Rocketman

Are you sure that he didn’t have the HVAC system set for “defrost”?
Some people seem to use that as their default setting in the winter, without considering that it causes the A/C compressor to turn on, frequently dropping the revs slightly.

Start by having the charging system checked.

When cruising at 60 mph does changing the fan speed affect engine performance?

The electrical load from the blower increases alternator drag.
Normally the computer compensates to keep the idle speed constant.
This engine has “drive-by-wire” throttle control, thus no IACV, unless its an XRS (about 10% of Corollas).
Sounds like the computer is reaching the limit of idle adjustment when the blower is on high.
My first suspect is a dirty throttle body.

Sounds OK to me and you can’t trust those dashboard tachometers enough to use them for anything.