When the key is in the on position, car not started, my warning lights dim whenever I either step on the brake or turn on the headlights.

The alternator has less than 1000 miles on it, was recently tested and the battery is strong.

It hasn't been started for awhile, and last time it was running, I had installed one of those small air horns. While I was driving, I used the horn and it seemed to drain the battery.

Limped it home and haven't gotten it running since.

Any ideas?

 

If the car is not running then the alternator is not being used at all. When you press on the brake pedal it is using enough voltage to dim the lights. It's probably normal.

 

Thats good news. Thanks. Itll be great news once I figure out how to get this thing started.

 

Yeah, brake lights are a big current draw, especially noticeable on just the battery.
Same with headlights.

So, a theoretically good battery and alternator, yet one use of the horn kills the car?
Obviously, the first thing I'd do is remove this vampire device but that's probably not the issue...I just dislike air horns.
If the batt/alt are good, then the wiring between them must not be.
Is this new alternator a stock replacement or an "upgrade" that might have slightly different wiring?

 

It's a replacement 3rd gen alternator.

 

I know the FD alt is a direct plug-n-play for a S5 but am pretty sure the wiring needs changing to work with your S4 harness...was that done?

 

Look at the wiring diagram provided in the FAQ section and see if that's how you wired the new alternator. If not, and you wired it up just like the previous S4 alternator then it will drain the battery over time.

 

Y, that was done when I first changed it out back when it was running last year. I've since rebuilt the engine and will have to double-check that wiring.

Of course, the main heavy gauge wire is setup, but need to check the 2 signal wires.
If i remember correctly, one goes to the dash and one goes to the battery.

If those wires are reversed, will it keep the car from starting?

 

No, it won't affect starting the car. You didn't respond to my earlier question of how you checked for fuel flow. Was it done w/key to start or w/key to on and the fuel check connector jumpered.

 

Something something battery drain from the two signal wires not being hooked up properly after the alt upgrade

 

I didn't jumper the ?fuel check connector?.
I pulled the uim and the primary fuel rail and had someone turn the key to start.