Yes, AC eat some HP outright and the Power needed also eats some more HP as everything loads the alternator. More so w/ SI Alt use on most Fiero but 88 use CS130 > 90 amps peak power as OE.
If have wiring problems anywhere then can have this problem and Volt meter on dash likely anything but stable.
AC compressor w/ problems can cause more load then normal.
Alternator iffy or replace w/ smaller unit can cause this.
"Power Pulleys" that alone can cause this and other problems.
Before pulling/replacing parts... Start w/ clean/fix all grounds in engine bay, battery cables and + "box" under C500.
Coat ends bolted/screwed to engine etc w/ silicone or brake grease.
This often will help many problems not just this.
See my Cave, Wire Service
and rest of section.
Also pull AC/Heat Blower Resistor and Motor and look at them and inside the box...⚠️ Warning:
These Resistors w/ crap buildup can burn a car down.https://www.fiero.nl/forum/Forum2/HTML/141959.html
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave