I just installed a new alternator and every time I turn on my headlights the idle starts to get messed up. It will jump to 1500 rpm when the headlights turn on and then it will start to idle bad, the rpms will go up and down.

any advice?

 

What was the reason for replacing the alternator?

Reman alternators are notoriously bad for FC's. I think the FC uses something like a cheesy 75A unit. Couple that with aging wiring and connectors, and you've got some voltage issues. The best solution would have been to upgrade to an FD alternator. There are numerous writeups for it. However, since you're already locked in to the reman you bought, you should get a clamp on ammeter and test the amount of current the alternator's pumping out. It also may be a cheap reman, so it could have a really crappy regulator causing flutuating voltage under a load. This could, in turn, load and unload the engine which explains your funky idle. The BAC valve will try to keep up with it, but along with the fluctuating voltage, it'll make it hard to do its job as well.

 

I replaced the alternator because it wasn't putting out enough volts. And it came directly off an s5 engine (same as mine) so i don't think its Reman alternator. ( i haven't checked)

also can it have any affect from the colder weather? because the past few days it been in the 40's and thats when its been acting weird, and when I went to test it to see if it still does it, and it didnt. It was 80 degrees when I tested it. so idk if i should take that as a factor as well.

but it would probably have been a better idea to upgrade to an FD alternator :/

 

ahh.. see, you said new alternator

Anyways, it's probably the cold weather then. I don't have any knowledge of s5's so I can't help you there. Hell, come to think of it, I can't remember anything about the s4's cold start now It's all been removed and replaced by a simple standalone.

 

For future posters' info, I don't mean the cold start assist system that injects antifreeze, I'm talking about the thermowax, fast idle cam, etc.

 

What does the idle jump from, and does it do this when the engine is cold, warmed up or both.

 

usually after the engine is warmed up, and before i take off in it (at the time) it was usually idles from 1k. But before i put the new alternator in, it would idle around 600rmp with headlights on.