I seem to have lost my way on the operation of the BAC. I read a post today with the clear implication that the bac does not react to the headlights being turned on. That goes counter to what I have observed while monitoring the output of the BAC.
With the a/c on and the fan full blast the bac has a voltage reading of approx 7.2volts on my car. If I turn the headlights on, the voltage drops to appox 6.4 volts, which leads me to think the bac is reacting to the load of the headlights. The voltage drop being the extra work being done by the bac.
Also when the headlights are turned on, the rpm drops a tad and recovers.
So why should I not believe the bac reacts to the headlights being turned on?

 

i dont know.. but i wished my bac worked lol

 

I don't think you should.The BAC will react to any change in engine speed to try to maintain the target idle speed.When you throw a big electrical load on ,the alternator tends to work harder slowing the motor a little and the BAC reacts.As you are aware loads for A/C and PS have specific settings(A/C through the BAC and P/S through the air supply valve) in the ecu.

 

as far as I know the bac has nothing to do with electrical. but then again I didn't think it did anything with A/C either? I thought that was the air surply. but I've never looked. that was info I've read in the past.

the voltage drop is probably just because the whole car's voltage drops when you turn the lights on? my volt meter goes down a volt or 2 when I turn mine on.

also doesn't the BAC just open and close? there is no inbetween is there? so voltage wouldn't mean anything in that case.

 

The reason youre seeing a drop in BAC voltage is because, obviously at that point in time the BAC only sees a percentage of overall voltage...example 7vdcx not 13vdc. So when you turn your lights on, or any other big load, the *whole system's* voltage drops, and thus the percentage that the BAC sees. I think you have uncovered a relationship, but not the type you're after.

FOr instance, I have no BAC, or anything else but an afm and 4 injectors on my t2...but if I turn on my headlights ALL the car's voltage drops almost a volt(due to shitty alternator). That's not because the computer is compensating, or anything else is, just that power is going somewhere else. My 0.02...

 

lots of good advice and points. i agree with HYP!

 

Team FC3S has a good article on it's page:

http://www.teamfc3s.org/info/articles/idle.html

Very helpful to me...

Greg O.

 

13B2QUICKLY....Read this, http://www.teamfc3s.org/info/articles/idle.html especially towards the bottom where P.Stoakes talkes about repairing bac circuits. I've done this repair on a spare ECU and its not difficult and Radio Shack has the transistor if it comes to that.
Due to the above posts, I'm back on track with the bac. By reading the thread earlier that indicated the bac was only for A/C etc I was starting to doubt what I thought I had learned. Especially when I know that the voltage for the injectors drops when the head lights are turned on. That made me think the voltage drop on the bac that I was seeing might be just an overall voltage drop, not more work being done by the bac. Wrong.
I forgot the voltage to the bac is directly from the ECU, which has a voltage regulator of its own to prevent erratic operation of the ECU due to voltage drops like the lights being turned on. I observed five other outputs from the ECU to tps, temp sensor, air flow meter etc and you don't see a voltage drop when the lights are turned on. But there is a large drop at the bac, because its been told by the ECU to work harder, to keep the idle at 750. And Scott, the bac works at somewhere around 125hz. ;Sort of a vibrator, opening and shutting at a rapid rate. See the above article.
The voltage does drop at the fuel injectors when the lights are turned on, because that 12v does not come from the ECU. Only the grounding pulse comes from the ECU for the fuel injectors.

 

Are you just trying to figure out how it works (boredom), or are you trying to fix an idle problem?

 

SAMPS....My cars do not have idle, hesitation or high rpm problems. I was fairly sure I knew how the bac, idle circuit, acv, tps etc worked. Just sometimes I'll read something that is not in line with what I think, from someone who is a fairly reliable source, and that will throw me into some self doubts. Like that article from http://www.teamfc3s.org/info/articles/idle.html that deals with the bac and other items. While its somewhat general in nature, you get the idea that the bac only reacts to a/c inputs. It does not deal with other reasons for it to react. The only idle problem I've noticed recently is when I took the wife to a doctors office that had valet parking. When the valet brought the car back, and he got out, the darn thing sat there and did that yoyo idle for about five cycles. Never ever does that to me. I don't do valet parking anymore. Guys a jink that does the parking. The car does not like strangers. P.S Thank you Six Rotors for the input. Your close to bullet proof on emissions and seemingly on bac operation too.

 

My car always has the damn yo-yo idle; I just live with it. With no ACV, no solenoids, the throtle body mod, and some porting, I'm lucky it idles at all! I can relate to the "just wanting to know" idea. I'll take something apart just to see how it went together, and usualy break it in the process. I have put in a lot of wrench time on my 7, and I feel comfortable doing anything now.

 

My understanding is that the BAC works like a fuel injector, varying its duty percentage (percent open per pulse) to vary the amount of air flowing through it. This is also the same way EBC’s work, by pulsing a solenoid open at varying duties to control boost. So what you really need to measure is the BAC’s duty, not voltage. There are some DMM’s that can do this, or if you can get one, an oscilloscope would be ideal.

 

I'm straightened out on the bac. I downloaded the 89 manual and it details the six or seven things that make the bac react. Headlights are included. See https://www.rx7club.com/forum/showth...threadid=85062 for details.

 

HAILERS

on FSM88, there's no PIN for headlight switch
there is 1 on 89

Check my latest post on thread
https://www.rx7club.com/forum/showth...threadid=85062

 

OK. 86-88 no on lights. 88-on yes on lights......for now anyway. Still a wee amount of doubt but its hard to dismiss the lack of pin input.

 

Hey HAILERS! Ever considered that maybe your BAC is faulty? If you have read over every piece of info available and still cannot find a logical explaination, then maybe yours is just on its way out. Just a thought.

 

According to Six Rotors,

the ECU will activate the BAC whenever the engine speed does not match the throttle opening (as indicated by TPS)

At idle, if you apply brakes or switch on ur headlight (on a S4), since the rpm drops while the throttle plate stay at same angle, the ECU activates the BAC not because of a direct input but because of a change in engine speed without TP variation.

What Six Rotors says makes a lot of sense otherwise how would you be able to keep your idle when you do the above. More intake air has to come in but since the throttle plates are not moving, the BAC has to bypass it (there is no other way for more air to get in, right?).

HAILERS,
this would explain why u notice BAC activation with lights turning on even though there is no pin for the headlight switch on a S4.

Hugues -

 

SAMPS....Nope on bad bac. Don't have a bad bac or any idle problems. I've three of these cars and don't have idle problems. Just got on this subject because I've seen two views on the relationship of the bac to the headlights. Just trying to reconcile the two views. I know what I see voltage wise on the bac when a load is put on the engine by the a/c and when its put on by the headlights. Both voltage drops are similar, but not as sharp with the headlights being turned on. NZ//My brud has a 'O'scope. Maybe I'll take a look, maybe not.Gotta go take a look at what Six Rotors wrote. Thanks for the info HUGUES....I'm a little embarrased that I did not look a the schematics for the 87 and notice the lack of an input from the headlights. There's still a large part of my being that believes the bac reacts to the headlights being turned on.

 

HAILERS:

This is what I think is happening (this would reconcile the 2 views somewhat)

on S4,
I believe the BAC reacts to headlights on because the ECU sees an engine speed that does not match the throttle position (due to the drop in rpms). I am pretty the ECU must have a predefined map of engine speed vs throttle position.

on S5,
I believe the BAC reacts to headlights on because there is an explicit signal from the headlight switch (kinda like P/S or AC switch inputs)

So,
it is a different kinda of reaction,
but a reaction nonetheless.

What I am saying is best of my knowledge. I certainly could be wrong.

hugues -

 

How about when I step on the brakes and the idle drops? (Actually begins to bounce a bit too)

I would think that even on a S5 the ECU would have to have a way to compensate for engine loading. Think about those guys with the killer sound systems...

 

Asherwood......we did determine that on the series five, the bac will respond to a load put on the engine by the headlight and stop lights. There is not doubt whatsoever about the bac on a series five responding to a load. Its the series four that there is some mild doubt about whether or not the turning on of the headlights or stop lights will cause the bac to change its duty cycle and compensate for the extra load. Gonna put a o'scope on the bac sometime soon to prove it beyond a shadow of a doubt. Thank you for your input HUGUES.

 

I am starting to better understand the BAC, but still not enough to fix my problem. I wish I had a better understanding of the relationship between ECU and BAC on the S4.

maybe you can help me on this,
since I am at this point out of ideas

my S4 idles fine with no load on at 750
but as soon as I

1) tap on brakes enough to just turn on the stop lights, or

2) turn on vent, or

3) turn on headlights

my rpm drops significantly to almost stalling and then goes back to 750 with some bouncing.

I am pretty convinced now (although not 100% sure) that the ECU should increase duty cycle of BAC when the above happens.

I have switched alternator just to make sure it's not a bad alt.

I have checked TPS, it's ok.

I have checked the BAC and its solenoid. It's OK according to FSM checking procedure. I am 99% sure the BAC is good.

The ECU to BAC seems to be ok.
The rpm drops when I unplug it.
Also, when I short the PS switch (I used to have PS), the rpm increases.

If my assumption about the duty cycle increasing is correct, then I would have to think that something is wrong with the ECU then. I just don't see what else to blame.

Any ideas ?

Thanks,
hugues -

 

Maybe..That part about how the idle dropping down and having to recover to 750 and bouncing for a spell I've come to the following possibility. The idle has not been set close enough to 750 when the idle was set with the initial set jumper. The idea here is that if it was set too low, then when the initial set jumper is disconnected and things are back in a normal configuration, and then you put a load on the engine, the engine speed falls lower than it would if the idle had been set originally to 750, causing the BAC to have to hunt to establish the 750 idle by overshooting and undershooting the target of 750. If the idle had been set right at 750 when the initial set jumper was inserted, then the bac(ECU) wouldn't have to work or hunt so much to establish the target of 750.

Hey! You said any ideas. You didn't specify good or bad! So anyway I suggest you revisit the idle setting with a fully warm engine and through in a check of the tps again for grins. Oh! One other thing is to check the dashpot setting. Heck I'd do that first since I'm sure you've done the other things several times already. A dashpot not cushioning the return of the throttle might be a part of the problem.

 

I guess you don't know if an idea is good or bad until you try it.

Nice try with the dashpot but it's fine

HAILERS, I haven't thought about re-adjusting the idle speed.

What does shorting the initial set connector do ?

Does it turn off the BAC ?

The BAC receives a light duty cycle when at idle when left alone (no load), 25% I believe, right ?

Is shorting the initial set connector same as unplugging BAC ?

So many questions,
hugues -

 

The initial set coupler? I don't have the 89 manual available right now but in there I definitly remember reading a statement that said it disabled outside influences to the bac that would cause it to go to a different duty cycle. They used better words than those. Ever since I read that I've wanted to go out and install the initial set coupler and turn the a/c on to see what happens. I'll do that someday, not right now. I don't think its exactly the same thing (installing the initial set coupler) as disconnecting the bac. I'll find that passage in the 89 manual tomorrow that describes the initial set coupler.