After searching and reading, I'm curious about anyone's opinion about my idle.

First - car idles fine, smooth as glass. When it's dark out and I turn on the headlights (or, to a lesser extent, anytime i press the brakes) my idles drops. now, it sits at 800 solid, and when i press the breaks i will drop maybe 50 rpms, but at night with headlights on and such, my idle sits at 700 and will dip to 600 for a second before bouncing back up to 700 or so. put the headlights down, and up my idle goes.

my volt gauge sits at 13 when im driving along without headlights but drops to just above 12 if my headlights are on.

My thoughts were either a sticking BAC, since i have so many miles or my alternator not putting out voltage enough.

 

I'd guess alternator first.
Voltmeter should be reading just over 14v, no matter what.

 

wow - it's never over 13 on my s4.

 

If the car was brand new it wouldn't do that but that was some years ago. If you disconnected the brake switch the idle wouldn't behave as such so this tells you how much of a drag the brake lights are in concert with how meager the alternator is with respect to output.

 

for an 80s car it's fine. It's not surging or freaking out. Lower your expectations.

 

I would check all grounds, dubble check the alt, and think about swaping out to a higher powerd alt, IMO 70-80 amps is just silly

 

The S4 70A alternator sucks, but you still shouldn't have voltage drop to ~12v with lights and accessories on. If all connections are decent, the alternator is dying. But you really have to upgrade the alternator to have any chance of stable voltage at idle.

 

upgrade it to what?

 

FD or Taurus alt would be a start.

 

are those simply a bolt in? no surprises?

 

the taurus alt needs a minor bracket fabbed, and the pully drilled out, and some minor electrical work. its a 140 amp alt, and you can score it cheap at any local junk yard/parts store. I dont remeber the details on the FD alt install, but non of them where to hard.

 

If you're set on no modifications at all then having the original alternator upgraded to produce the amps necessary would be the avenue to take.

 

While this alternator issue will improve your charging system, it won't alter your idle problem. Definitely sounds like your BAC isn't working. Easiest thing to do is swap it for a known good one. You could also check the voltage at the connector. What happens when you demate the BAC connector?

 

I was experiencing a similar issue with my 86, there is a ground wire from the engine to the body with a slip on connector for the body side i found that in wet weather mine would corrode and need to be cleaned with a bit of sandpaper. i eventually replaced it altogether with a slightly heavier gauge of wire. no idea why this affected me only when i applied the brakes tho. my alt sits at 13+1/2 during daylight driving and just above 12 with fans and lights and wipers and brakes all on.

 

Which means it's not putting out enough amps.
A higher output alternator and it'd read 14v (or so) all the time, no matter what.

 

thanks for the replies guys! I am swapping the BAC tomorrow and if that doesnt solve it, I will get the alternator swapped out too.

EDIT - I will post back my findings so I can contribute to the community.

 

useless alternator

 

Yeah, looks like it.

Oddly enough, my alternator took a crap yesterday.
It was a FD unit I bought used 3 years ago, one minute it was fine, the next, not.

Naturally, no one in town has one and even a reman from NAPA is @$160, which is way more than I wanted to spend on a motor that's probably getting replaced soon, so I spent a bunch of time on the interwebs researching options.

Turns out, several cars use alternators very much like ours...929's, 626's and even MX-6's.
The MX-6 even uses a v-belt, so the pulley would probably work without swapping to the RX-7 piece.
I pulled one from a mid-nineties 929.
It runs a ribbed, flat belt but my already modified pulley (bored to fit the FD alt) was a perfect fit.
The wiring was plug-n-play for the S5 wiring, absolutely no modification required.
Nominally rated at 90A, my junkyard unit tested at 110A at Autozone (for whatever that's worth).
Cost me $25.

You might consider visiting a junkyard...