when i turn on my ac at idle my cars idle drops and sometimes stalls out or make it go arratic. im going to geuss its my bac isnt working right and is in need of being cleaned? im sure this is somewhat a common question i did search but didnt really find much.

Thanks,
Russ

 

just an added load on the engine, i can tell a significant difference in the power when it's off/on. how's your idle? maybe just raise your idle up a bit, unless it's already around 750-800

 

Your bac is most likely bad.Check as per fsm.

 

alright ill check/clean my bac...my idle bounces around 750-850rpms so i dont really wanna adjust my idle its good were its at.

 

I have the same problem, just took my BAC off this weekend and sprayed carb cleaner into the 3 openings with carb cleaner, did not help. I did not test it electrically, don't have a meter.

 

When you have the BAC off, blow through the metal air supply tube and see if the inner seal leaks. If it leaks, replace the BAC. Cleaning will only have a very short term effect. It will function properly during the electrical test, but that leaking inner seal is a sure killer.

 

I second the BAC valve being inoperable.

-Markus

 

Mine turned out to be TPS sensor.

 

I have that problem too, I think I'm going to fix it by ripping it out...

 

DO YOU OWN A DIGITAL METER? IF YES, THERE IS A WAY TO TELL IF IT'S FUNCTIONAL OR NOT BY OBSERVING ITS VOLTAGE WHILE THE ENGINE IS RUNNING. IF YOU DO, ANSWER IN THE AFFIRMITIVE. IF NOT.....I WON'T WASTE YOUR TIME.

 

SOMETHING I SHOULD HAVE ASKED. WHEN YOU TURN YOUR HEADLIGHTS ON AT IDLE....DO THE RPMS DROP DOWN? OR JUST HOW DOES YOUR RPMS REACT TO THE HEADLIGHTS BEING TURNED ON????

 

i have the same problem. rpms also drop down when headlights turned on. What is my problem,

 

bump me to! i have emissions removed. vack leak?

 

Where can we get BAC valves? My idle is 750 rpm with no electrical load, if I use AC it stalls, lights, power brakes and power steering all make the idle drop.

 

It's possible the bac is just fine.....but the output from the ECU to the bac is corrupt. If you read this article http://www.teamfc3s.org/info/articles/idle.html down near the bottom of the article it explains how to repair a busted ECU output to the bac.

I've done it in the past and it works. Cost of about a buck fifty.

You could test the bac to see if its getting an output in several ways. If you have a meter, you can get on the output to the bac by backprobing the ECU and observing the voltage there. While looking at the output to the bac, turn the a/c on or the headlights or step on the brakes. You should see a voltage drop....and no, Charlotte, it ain't the battery voltage that's a droppin.

Or you could remove the bac. Place a temp blockoff plate over the hole in the manifold...put the bac elec plug on the bac..... start the car and look at the piston in the bac. It should be vibrating. If not vibrating....the output from the ECU ain't thar.

Or......remove the bac....put its electrical plug on....turn the key to ON, NOT START.. I *think* the bac will vibrate. Remind me to check this out. I'm about 95 percent that will be the case. If not vibrating.....no ECU output. And as I write this, I've just become 200 percent it works like that. I'll go put a spare bac on the plug on a car and see if it vibrates with the key to ON. Been there done that in the past. It'll work.

So do the paragraph above and see if the bac vibrates. Come to the right conclusion. It could save you money by not buying another bac when the one you have is good and its just that buck fifty transistor in the ECU that is poop.

BAC's have a duty cycle......they open and closes all the time. They are not just full open or full close items. Read article above.

 

And in fact that last paragraph or two is a great way to see if your bac is working and you don't have a meter.

Pull the bac off, it's just two small nuts and a hose. Put the elec plug on it. Turn the key to ON. Now touch the bac's piston with your finger. It vibrates. Now touch the piston and reach over there and pull your throttle wide opne and let it return to close. The bac should make a sudden surge when the throttle is closed. If there is no vibration the ECU signal is not there or one of the two sockets in the bac plug are pushed back not making contact, a common bac problem.

 

HAILERS is not wrong ...

What follows for s4 NA .. turbo and s5 tad different but similar.

The AC load is different from the other loads since the ECU relies on the AC switch to bump the idle (same thing when you turn the wheels and you have power steering ... there's a power steering switch ... I think turbo is different though since there's a separate valve).

So you need to tell us if the rpm drops when you turn the headlights on (I think s5 have a switch for that) or turn the ventilation to max (I think s5 have a switch for that) [no AC].

Check this page for more info on the BAC (that's pretty well written):

http://www.geocities.com/huguesdc1/idlecontrol.html

As HAILERS said, monitoring the voltage at the BAC itself or at the ECU pin will tell you everything you need to know ... We're talking DC voltage here .. You can even post what you got and I can compare with what I have since I can monitor the voltage at pretty much all ecu pins when I drive around (HAILERS can do it too if you ask nicely).

I have just noticed you car is an s5 ... then it's a little bit different ... the idle system steup was improved on the s5 ... there are more switches that tell the ECU to bump up the idle without having to rely on indirect rpm information from the CAS (headlight on switch, ventilation switch, etc) ... You'll have to look at s5 fsm to see what inputs to the ECU are w/r to idle control ...

Hugues -

 

my friend's car does the opposite thing. the idle goes up when a/c is on. normally, it stays at 700rpm. then 1300 when a/c is on.

is it normal? his car is s5.

 

thanks, i'll check it out.