On my 83 GSL, when I turn on any power accessory (heat, lights, wipers, window motors) the voltage drops a good bit. If I have a couple electrics going, the RPM will drop. If I have all electrics on at once, I can stall the car. If I'm not mistaken, the voltage reg on the s2 is in the alternator, right? If it was one system causing the drop, I would think it was a short in that system, but it's anything. It has been like this for a long time; really sucks on rainy night when I'm afraid to run defog, wipers and lights at the same time. Any ideas of the cause of this?

 

1st gens like most small cars from the 80s have marginal electrical systems,when new everything worked ok...now with 35+ yrs of oxidation,corrosion resulting in added resistance in wiring it's not uncommon to have voltage drop in the electrical system.

Things to check(or have checked) in this order:
1) Have the battery tested,be sure it is right size battery for the car. Fully charged GOOD battery will show 12.6-12.8 volts without car running.

2) With a voltmeter,check voltage at battery with car running at idle(800 rpm) and no accessories on,should be 13.8-14-2 volts,if not alternator should be tested for output. Be certain alt belt is tensioned properly and not slipping.Car should have a 55 amp alternator. If you are not getting these kind of #s from testing(test and post results here) you need to...

3) Check (and clean)all connections both power and ground from /to battery including battery cables pos & neg. Feed wire from +battery cable to main fuse link and from there to alternator output terminal. Pay particular attention to battery ground cable where it attaches to the body at left strut tower and where it terminates at starter. Each of these junctions/connections is certain to be in need of cleaning/tightening after all these years and each connection if not clean can drop system voltage .2-.5 volt,enough of these can easily drop system voltage 1 or more volts.

4) It IS normal to see 100 rpm drop or so with load from accessories as it takes horsepower to turn alternator under load,however car should not stall out. With car running note system voltage at battery(should be about 14 volts),turn on headlights and note voltage(record) now turn on heater fan and record,turn wipers on and record voltage(at idle). Now with everything turned on raise rpm to@2500 and note and record system operating voltage. Post results here.

5)Once all circuits have been gone thru and tested for voltage drop(do you know how to test a circuit for voltage drop?) and all is as it should be,you can increase electrical system reserve by installing an FC 70 amp alternator,basically plug and play.

You need a multimeter(don't rely on dash voltmeter for testing purposes) for diagnosis/testing. They are relatively inexpensive,available everywhere and a valuable addition to your tool box.