Looking for a bit of electrical knowledge here. S4 N/A

To start I have tested the batery and charging system.

Battery voltage with engine off was around 13V
Charging at 14.3V (NO LOADS)
Charging at 12.8V (headlights, blower motor ON)

When I turn the headlights on the idle will drop to approx. 550-600 and takes a while to return to 750. If I am idling with headlights on and i press the brake pedal the idle will once again drop to approx. 550-600 but it will NOT return to 750until I release the brake pedal.

As well I can always hear a constant buzz in the interior. When I turn on the headlights it will get louder. If I press and depress the brake pedal the pitch will change.

I have checked all connections including body grounds as well as battery terminals and battery ground on the starter and drivers shock tower. No corrosion found anywhere and they were all nice and tight. Also checked alt belt tension.

I am stumped here guys. I want to take the easy route and blame the alt but I think something else may be going on here.

Opinions?

 

Check to see if the buzz is coming from the speakers. An improper ground in a head unit or amp causes this alot.

Not much help but it might take one thing off "the list"

gl

 

The S4 Alternator is known for being insufficient for stock.

 

I have the same problem. Is there another alternator I can use to fix this?

 

You can do the S5 alt conversion.

 

I have this problem as well, and mine is an S5 vert. My thought is maybe the brake light connections are corroded since the seals for them aren't in the best of shape. I have just been too lazy to check it though, I am too busy tracking down cold solder joints.

 

try tightening the belt more, if that doesnt work, it looks like you will need an alternator in the very near future. 12.8 is a little low, look into a replacement

 

Thanks for the responses everybody.

I may just get a new alternator if I don't find another culprit soon.

Ill let everyone know how it turns out.

 

Look into upgrading. Look into the Taurus Alternator or the Tempo Alternator, there are write ups on here.

 

normally the sound coming from the speakers is caused by a ground problem or a bad component. This problem can also be caused by a battery that is old and has lost it's ability to filter. This would also cause your other problems. I would take the battery to an auto parts store and have it load tested. It is normally free to have them do it.

 

Is it only doing it when you're braking at idle or does it happen as well when you're braking at speed with the gear engaged?

 

The noise is not coming from the speakers. I turned of the stereo immediately when I started to hear the sound with the volume down.

I have load tested the battery, that is why I posted results in the orginal post.

 

what you did is not a true load test. Yes you tested the cars electrical system while running under some load. You have not load tested just the battery. And if the sound (buzzing) isn't coming from the speakers where is it coming from? Give some details and it will help this process out.

 

When I have my headlights on, and brake at idle. My voltage is fine with headlights on until I hit the brakes, then it sags a bit and the idle goes down slightly. Never happens when I am moving.

 

My 91 FC does that from time to time. It's just that your alternator isn't providing enough voltage when the engine speed is that low. As long as you're not doing too much stop and go traffic at night, you should be fine. It's normal, but annoying, for some alternators.

12.8V is the minimum your voltage should be for charging the battery. You can dip below it for a little bit at a time, but you should make sure that the majority of your time is spent above 12.8V.

 

Yes i did do a proper load test. I hooked up a battery tester at work and load tested with various conditions. First using the CCA rating with the engine NOT running. The tester will put a load on the batery such as a start up and measure its current CCA rating and voltage. I then proceeded to do a startup test to check the cranking voltage and followed up with a charging system test.

The buzzing sound is coming from the rear of the vehicle. I cannot pin point the location. I have no rear speakers and it is not the fuel pump.