What exactly does the BAC do? ive tried searching...anyone got any info? thanks

 

It increases the idle when there is a load on the engine (mostly the A/C)

 

Does what Rico said and more. It gets inputs from the Fog light switch, headlight switch, rear defroster switch, a/c switch, p/s pressure switch, cas, tps, water thermo sensor, clutch switch, neutral switch and intake air thermosensor. I got this information from page F1-40 of the 89 factory service manual. It uses these inputs to maintain the idle when loads are put on the engine, like turning the a/c on or the headlights. Also see page F1-76 of the 89 manual and you will see a *Relationship Chart* that talleys with the above statement.
Its been unclear to me in the past exactly what the inputs were to the BAC, because I have a 87 service manual and it does not have the info I refered to in the 89 manual i.e. pages F1-76 and F1-40
I'll attach the jpg of those pages when I get home tonite. If you monitor the bac you'll see approx 8v at the bac, and when you turn the a/c on it will drop down in the mid to lower 7v range. Pop the headlights on and you'll see a futher reduction of the voltage. Working harder.Its not a just open and shut valve. It works at approx 125hz in general.

 

HAILERS,

This is very good info.

I was under the impression the headlights being put on had no effect on BAC from what I was reading in the FSM88 manual.

hugues -

 

HUGUES......Until I saw the 89 manual and pages F1-40 and F1-76, which I'll jpg this evening, I couldn't say for sure. I could look at voltage at the bac and flip switches and see the voltage drop but my 87 manual didn't seem to address the inputs to the bac. A fellow named Six Rotors answered one of the posts on this the other day and said the headlights do cause the bac to react. That guy is dead reliable on emissions, bac etc. So I got looking at the 89manual that I have on my hard drive at work and saw F1-40 and F1-76 that confirmed it.

 

HAILERS,

I have just had a look at FSM89 manual page 41,
which shows the inputs to ECU that would activate BAC,
and there clearly is an Electrical Load module inside ECU that controls the BAC, with inputs from the various elec switches including head lights.

It seems to me the FSM89 is better written than the FSM88. You get more info out of it, IMO.

thanks HAILERS (and Six Rotors) for pointing this stuff out

Hugues -

 

I am confused now.

I have just looked at FSM88 and there is no PIN for any 'headlight switch' on the ECU harness. So, how can the ECU know the headlights are on and activate the BAC.
Looking at www.fc3s.org and the ECU checklist for the 89, there is pin 3L that gets input from headlight switch.

That would mean the headlights activate BAC on 89 but not on 88 ??

Hugues -

 

That table can't be reliably applied to the 86-88 cars. The S5 ECU performs many functions that the S4 one doesn't. I've had a good look through the S4 wring diagrams and can't find any connection between the headlights and the ECU, just like Hugues. I don't think the S4 ECU uses the BAC valve for much except the A/C (and maybe power steering?). Checking the duty cycle (not voltage) is the only way to be sure. I don't get any increase in rpm from turning on my lights or rear demister, but I do for the A/C and PS.
It's a shame the S4 FSM doesn't have as much info as the S5 one. It has so much more detail, clearer explanations and things like that relationship chart.

 

NZ CONVERTIBLE......I bought a Fluke 88 automotive meter with the ability to to measure duty cycle and pulse width. The S4 does indeed adjust the duty cycle to the bac when the headlights are turned on. I posted what the changes were on another thread. I invite any doubting Thomas's to put a meter on the bac and tell me where I might have gone wrong.

 

wellwhat about if u cut ur a/c on and ur car stalls?
i know it has to do witht the bac but how could i figure out if its bad or how kan i fix u know?

jack

 

And how do you test/ Replace the BAC valve, mine needs replacing. When i turn the a/c on the engine revs decrease

 

If you suspect BAC problems, go to this site: http://www.teamfc3s.org/info/articles/idle.html Down near the bottom he talks about the bac. Don't worry if you don't have a turbo. A bac is a bac. Works the same in the n/a.